Help Me Conceptualize a "Third Gender"

[KRSOradio]

OK we have men and women, (male and female) pretty easy so far.

I was reading in Wikipedia...


What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, of more than 1.5 million living species,[18] recorded up to about the year 2000, "no third sex cell — and so no third sex — has appeared in multicellular animals.


OK I guess I am trying to visualize a "third sex" and having problems with that concept. I never really thought about it before.

So if we had a third sex, what could it possibly be like?

[Bootis]

I'm fascinated by species that have a sex that is completely different in form than the other. It would be cool if we had a 3rd sex that was like a giant caterpillar or mollusk.

[lissener]

I'm fascinated by species that have a sex that is completely different in form than the other. It would be cool if we had a 3rd sex that was like a giant caterpillar or mollusk.

Some jokes write themselves.

[jayjay]

In Piers Anthony's (I KNOW, okay? But his non-Xanth stuff is occasionally pretty good) Kirlian series, the Spicans were a triple-sexed species. The third sex was a sort of catalyst whose presence was necessary along with the "father" and "mother" genders to actually produce an offspring.

Also, Asimov's The Gods Themselves (http://en.wikipedia.org/wiki/The_Gods_Themselves) has aliens known as the Hard Ones and the Soft Ones. The Soft Ones have a three genders:

His aliens consist of the "hard ones" and the amorphous "soft ones". The soft ones have three sexes with fixed roles for each sex:-

* Rationals - Called "lefts", rationals are the logical and scientific sex. Rationals are identified with masculine pronouns and produce a form of sperm.
* Emotionals - Called "mids", emotionals are the intuitive sex. Emotionals are identified with the feminine pronouns and provide the energy needed for reproduction.
* Parentals - Called "rights", parentals bear and raise the offspring. Parentals are identified with masculine pronouns.

All three 'genders' are embedded in sexual and social norms of expected and acceptable behavior.

[Chronos]

There's somewhat of a mistake in that Wikipedia article: The existence of a third sex would not necessarily imply the existence of a third type of sex cell. There are fungi with over a dozen different sexes, but you still only need two at a time to mate (any two of different sex can produce offspring).

[sunstone]

As I recall, some fungi exist in several genetic forms..quite a few more than two different forms. Only certain genetic forms will fuse with others (think union of sperm and eggs), so you might call these different sexes. Sex can be considered the union of genetic materials from two (or more) different individuals.

Additionally, there are any number of fungi that have several different forms in a life cycle. Wheat rust is an example as I recall. Each stage looks and acts differently to reproduce.

Sorry, but it has been 40+ years since I studied this, so I can't be specific in the details, but it can be found by googling fungi life cycles. As an example take a look at http://www.lclark.edu/~seavey/bio210/wheat_rust_l.c.html

Some of the so-called algae have even more complicated life cycles.

[tygerbryght]

...

Also, Asimov's The Gods Themselves (http://en.wikipedia.org/wiki/The_Gods_Themselves) has aliens known as the Hard Ones and the Soft Ones. The Soft Ones have a three genders:
Actually, bees, ants and other colony insects - have the best example available to us on our own planet of species with three genders. Of course, one of them is neuter. Not, I think, what the OP had in mind.

The Gods Themselves is the best example I can think of - and it won the Hugo (http://www.thehugoawards.org/) - SF fandom's highest award - the year it was published. But there are very interesting treatments of other methods (http://en.wikipedia.org/wiki/The_Left_Hand_of_Darkness) of reproduction by Ursula K. LeGuin (also a Hugo winner) and James Tiptree, Jr. (a nom de plume) wrote "Your Haploid Heart" about a different reproductive alteration in humans from that hypothesized by LeGuin. It was collected in Star Songs of an Old Primate (http://www.amazon.com/Star-Songs-Primate-James-Tiptree/dp/0345254171/ref=sr_1_15?ie=UTF8&s=books&qid=1221287053&sr=1-15) While this story didn't win any awards (more's the pity, IMO; I think it's a great story), Tiptree was another winner of many awards and honors. IIRC, another story of hers which did win a Hugo was the first prominent story by a good (i.e., respected by their peers) writer which featured cloning as reproduction; it was "Houston, Houston, Do You Read?", and was first collected in Aurora: Beyond Equality (http://www.amazon.com/Aurora-Equality-Vonda-editor-McIntyre/dp/B000S9JRPM/ref=sr_1_1?ie=UTF8&s=books&qid=1221287675&sr=1-1)

Larry Niven's Pierson's Puppeteers have a "third gender", but they're extremely reticent about their method of reproduction. I can't recall in which story it is revealed, but a Puppeteer reveals that the third gender is non-sentient. The protagonist in that story gets the impression that it's actually a host animal in which the puppeteer embryos grow and develop in much the same way as some earthly insects (and arachnids? having trouble recalling) parasitize other species by laying their eggs in them. Ringworld, the first novel in which he included a puppeteer as a character, was yet another Hugo winner, by the way ...

<snark>I'm not recommending anybody of Piers Anthony's caliber, you see.</snark> :)

Perhaps yet another SF fan will be able to think of other examples. I don't think Octavia Butler's work qualifies, as it features inter-species reproduction. It is, however, highly regarded, and she, too, won awards.

[Risha]

Perhaps yet another SF fan will be able to think of other examples. I don't think Octavia Butler's work qualifies, as it features inter-species reproduction. It is, however, highly regarded, and she, too, won awards.Actually, she was the one I first thought of when I read the OP. If I recall correctly, the inter-species reproduction was actually an adaption of the aliens' three sex reproduction. In that species, there was a male, female, and a third sex whose name escapes me. The male and the female (who never touched, even casually) only had sex with the third sex, who mixed the genetic material and implanted it into the female. Once the inter-species part came in, they added an additional human male and female to the family, who also only mated with the third sex.

[runner pat]

On Alien Nation (http://en.wikipedia.org/wiki/Alien_Nation_(TV_series)), two different forms of male were needed to fertilize and catalyze the female for conception to occur. The third gender, known as a binnaum, comprised a very small percentage of the population.

Tenctonese. (http://en.wikipedia.org/wiki/Tenctonese)Scroll down to "Reproduction"

[as_u_wish]

I think Frank Herbert's Bureau of Sabotage works have a third gender. But it may simply be a third life stage of some sort--it's been a long time. Given my uncertainty, I probably shouldn't post--but I think the "The Tactful Sabatour" may be useful in dealing with how a three gender species might interact with a two gender species.

[Lumpy]

The Wikipedia article is talking about different sexes at the gamete level. I doubt there's much you can do to elaborate on the "sessile ovum, motile sperm" arrangement. So genetically, I don't think there's any way to interchange genes between more than two creatures at a time, at least not beyond the fungi/algae level. As has been pointed out, there's a tremendous amount of possible variety at the organism level, but that's more in the line of reproductive strategy, not a true third genetic sex.

[BrotherCadfael]

In The Star Beast, Heinlein (speaking through the mouth of one of the main characters, as is his wont) notes that the Hroshii come in six assorted sexes -- but doesn't get into any detail beyond that.

Lummox was female, more or less

[Mangetout]

Larry Niven's Puppeteers have three genders - a female and two types of male - both of which are required to impregnate the female. It turns out though that the female is an entirely different species and serves as host to the parasitic embryo - a parasitic (or perhaps symbiotic) relationship.

[tygerbryght]

In The Star Beast, Heinlein (speaking through the mouth of one of the main characters, as is his wont) notes that the Hroshii come in six assorted sexes -- but doesn't get into any detail beyond that.


:smack: :smack:

Ohgawd, you're right. It's been too long since I reread that, but it's behind a whole stack of boxes I can't move. Of course, she was more important than anybody knew, but ... I won't add any real spoilers.

You consider the alien translator a main character? I wouldn't have. None of the humans had a clue about that species. I thought the aging bureaucrat was priceless, and loved how he managed (almost) everybody.

Y'know, I've always wondered how they'd be able to make the changes in the title character that they were planning at the end of the book. Oh, well, they were aliens - very strange ones, at that. I really miss that old man, even if his writing did get kinda weird those last few years. And I'd absolutely love to know what his reaction would have been to Cordelia Naismith Vorkosigan.

[The Tao's Revenge]

Wouldn't a third reproductive gender to be evolutionary viable need some way to get it's chromosomes in the action too?

Which would mean a three gender species would likely produce young with 3 sets of chromosomes.

What form that 3rd chromosome set would be delivered I can't hazard a guess.

[as_u_wish]

Wouldn't a third reproductive gender to be evolutionary viable need some way to get it's chromosomes in the action too?

Which would mean a three gender species would likely produce young with 3 sets of chromosomes.

What form that 3rd chromosome set would be delivered I can't hazard a guess.

Well, the way I envision it is as follows for genders M, F & T:

There are only two chromosomes involved in determining gender. These chromosomes match up as:

MM, MF, MT, FF, FT, and TT.

Homozygous individuals clearly have only one possible gender, that is MM = M, FF = F, TT = T. The gender of heterozygous individuals depends on in-utero hormonal triggers. Thus an MF individual can have either the M or F gender depending on the pre-birth environment. (Alternatively, heterozygous individuals have distinguishing sexual traits--color, hair patterns--that indicate they can switch gender at will. This makes them highly desirable and there is much discrimination against the homozygous--see HoZy Anti-Defamation League.)

In "nature," viable offspring can only be produced by the mating of individuals of two different genders. If an MF (gender M) mates with a TF (gender T), they could have an FF (gender F) child.

Of course, it might be different. Perhaps T is always dominant, but there is a smaller frequency of this allele. Perhaps any coupling can produce viable offspring, but there is a cultural aversion to same gender pairings. I can even imagine scientific break throughs such as invitro fertilization for same gender couples resulting in viable offspring and much social unrest.


Or am I just being silly?

[BrotherCadfael]

:smack: :smack:

Ohgawd, you're right. It's been too long since I reread that, but it's behind a whole stack of boxes I can't move. Of course, she was more important than anybody knew, but ... I won't add any real spoilers.

You consider the alien translator a main character? I wouldn't have. None of the humans had a clue about that species. I thought the aging bureaucrat was priceless, and loved how he managed (almost) everybody.I believe Permanent Undersecretary Kiku so described the Hroshii when speaking to Ambassador Greenberg, but I could be wrong.

Mr. Kiku is one of my favorite Heinlein characters/mouthpieces. Certainly the characterization belies the frequent charge that all of Heinlein's heroes are alike - if Heinlein can make a government bureaucrat not only heroic but likable...

[dtilque]

The Azad in Banks' The Player of Games had three sexes. The apex sex was dominant, with the males and females subordinate. This was in stark contrast to the Culture which had only two sexes and was totally egalitarian. In fact, the inhabitants of the culture could change sex and sometimes did.

It should be noted that if we ever do meet aliens, it's virtually certain they'll have two sexes. Having two sexes has a disadvantage (you need two of different sexes to reproduce -- if there's none of the other sex around, you're out of luck) but its advantage (gene mixing) outweighs it. With three sexes, the disadvantage (needing three different sexes) outweighs the advantage (more gene mixing).

[Malacandra]

The Kurii, occasionally-necessary aliens in John Norman's Gor books, had three sexes (possibly four depending on how they're counted... bear with me a moment). The sex that ovulated wasn't the sex that gestated - males impregnated egg-carriers, who then passed the zygote on to blood-nursers, and the offspring could be any of these. Within the males there was a distinction between "dominants" and "nondominants", a differentiation that apparently existed from birth, and only the former were allowed to breed; but in rare cases it seemed that a nondominant could be pushed around once too often and become a dominant. All this according to General Zarendargar, "Half-Ear", who himself stated that even the Kurii disagree on whether this counts as three sexes or four, and expounded in Beasts of Gor.

[Fly Pusher]

Wouldn't a third reproductive gender to be evolutionary viable need some way to get it's chromosomes in the action too?


I was actually thinking of the same thing. One could theoretically envision a triploid organism, with similar biochemistry as life on earth, only with 3 sets of chromosomes instead of 2. Cell division would involve triplication of each chromosome, and dividing into 3 daughter cells. Meiosis could proceed in triplicate as well, eventually ending up with 9 monopoid gametes for every cell you start with. There would be 3 types of gametes, say, egg, sperm-I and sperm-II, and fusion of all three would be required to produce a viable zygote. Ensuring that you only get one of each type of sperm would be more complicated than what we have to go through, but not outside the realm of possibility.

[Fly Pusher]

Oops, now that I think about it, there's no need for triplication, which only complicates things. Duplicating every chromosome and dividing in 2 is the way to go, unless we also imagine a genetic material that is 3 stranded instead of 2 like DNA.

ETA: the key would still be in meiosis, where the second division goes from 2 3n cells to 6 1n cells.

[Annie-Xmas]

Conversation on 3rd Rock after Sally discovers homosexuals:

Sally: You know how we said that having only two sexes must be boring?
Dick: Yeah.
Sally: I think some of them have found a loophole.

[Tastes of Chocolate]

How about a 2 strand DNA, where AA, BB and AB are all different and viable genders. There could be a set up where AB are female and both AA and BB are male (or even flip flop that). So AA and AB could reproduct, as could BB and AB. But AA and BB couldn't directly have offspring.

[CalMeacham]

Besides the SF examples linked to above, there are at least two examples I know of where multiple genders (more than 3) have to hook up for sex. it was treated as a comedic look at human sexual politics in a story in Playboy circa 15-20 years ago, and as a cartoon strip (that ran for several pages) in National Lampoon.


I don't know of any examples that really try to treat the mechanics of such science fictiion three-way sex, except where the third partner is a sort of catalyst or facilitator (as in Alien Nation, or in John Varley's Titan series, during the time when Rocky has to catalyze the centaur eggs before they'll impregnate. Otherwise, nobody wants to try and figure anything out. In asmov's the Gods Themselves the beings are sort of ill-defined and fuzzy. The most cases, they simply describe the participants as crowding together in a multisexual orgey.

[The Tao's Revenge]

I think people are getting DNA and chromosomes mixed up.

You don't get half DNA strands from each parent, you get whole chromosomes from each parent. A chromosome is a clump of DNA. You have IIRC 46 chromosomes. 23 of which are redundent. They have DNA for the same things as the other 23. You get 23 from each parent, and when you have kids your kids will each get half your DNA because they will get copies of half your chromsomes. Some might get the chromosome for blue eyes, and some might get the chromosome that has black eyes, but of the two chromosomes with eye color genes they will get one.

IANA biologist so someone correct me if I'm wrong.

In DNA an A segment on one side of the helix always connects to a T on the other. G connects to C etc. When the DNA strand is copied it's split down the center, and this pairing is used to rebuild the missing side on each strand. This has about as much to do with sexual reproduction as it does with any tissue growth.

[Lemur866]

The trouble with a lot of these setups is that they don't make any evolutionary sense. If you need a second type of male to "catalyze" reproduction, what's in it for the catalyst male? And how did this setup evolve? In the case of "Alien Nation" the explanation is that the newcomers have been genetically engineered, and the third sex could be an artificially created method of limiting reproduction.

The other common setup, where you have a male who provides sperm and a female who provides eggs and a third gender that gestates the offspring suffers from the problem of what's in it for the gestator. Why would an organism go to the trouble of gestating a baby that it isn't the parent of? In the puppeteer example the puppeteers parasitize the third "gender", so it isn't an example of a third sex any more than a caterpillar infested with wasp larvae is a third gender of wasp.

For all the organisms that we know two genders is probably too many. Rather than two genders most species would be better off with one and a half. Sex is advantageous, but you only need a little bit to get the advantages of sexual reproduction. And so we see countless organsims that have both sexual and asexual methods or reproduction, but none that have three sexes.

[Fly Pusher]

The other common setup, where you have a male who provides sperm and a female who provides eggs and a third gender that gestates the offspring suffers from the problem of what's in it for the gestator. Why would an organism go to the trouble of gestating a baby that it isn't the parent of? In the puppeteer example the puppeteers parasitize the third "gender", so it isn't an example of a third sex any more than a caterpillar infested with wasp larvae is a third gender of wasp.

I don't see this as much stranger than say, worker bees not having the ability to reproduce. I agree that wasps laying eggs in caterpillars is not an example of a third gender, but what if the wasp lays it's egg in another member of its species that has specialized to become a gestation machine? You have your male and female wasp, and a third gender that looks like a large caterpillar which munches on leaves and builds up a large reserve of nutrients which is used to feed the hatching larvae. Seems like a plausible setup to me.

[Lemur866]

But what does the third gender get out of the setup? I suppose it could work with some sort of haplodiploidy like sterile haploid hymenopterans. But it seems to me a mistake to call the gestator a third sex. It's an infertile neuter, not a third sex.

[Annie-Xmas]

The other common setup, where you have a male who provides sperm and a female who provides eggs and a third gender that gestates the offspring suffers from the problem of what's in it for the gestator. Why would an organism go to the trouble of gestating a baby that it isn't the parent of?

Human females are becoming "gestational surrogates," using their wombs to give birth to a couple's child, or even a child for a gay couple using donor sperm and/or eggs. There are medical, financial, altruistic and psychological reasons. Some women like being pregnant,but don't want another child and don't want to give up a genetic child.

[The Tao's Revenge]

But what does the third gender get out of the setup?

That's why I postulate a third gender would be viable in a species with 3 sets chromosomes, or structures different then nuclear DNA/chromosomes.

Although what about a species with alot more emphases on mitochondrial DNA? Say instead of it being 5% it's 40% or 30%. The first two genders pass on nuclear DNA, the third gets to pass on it's mitochondria.

[Fly Pusher]

But it seems to me a mistake to call the gestator a third sex. It's an infertile neuter, not a third sex.
I think that's debatable, depending on the exact mechanisms. I can imagine instances where the gestator would affect the offspring, either through environmental conditions or contribution of some of its cells and/or genetic material. I think that should qualify.


The first two genders pass on nuclear DNA, the third gets to pass on it's mitochondria.
That's an interesting idea too. Just goes to show even dealing with familiar biochemistry, there's a lot of options. Not necessarily practical or even likely, but it's not like evolution only results in practical features :)

[Blake]

I think that's debatable, depending on the exact mechanisms. I can imagine instances where the gestator would affect the offspring, either through environmental conditions or contribution of some of its cells and/or genetic material. I think that should qualify.

But in that case it is no longer a gestator any more than a female mammal is a gestator. And at that point we are back to the original question: how could this evolve? Why would a gestator evolve to contribute some (let's say 10%) of its genetic materials when it could remain conservative and contribute half of the genetic material? What's in it for the gestator that outweighs losing 40% of its reproductive potential? For such a setup to evolve a proto-gestator would have to be able to produce at least 41% more offspring than standard sexual reproductives.


That's an interesting idea too. Just goes to show even dealing with familiar biochemistry, there's a lot of options. Not necessarily practical or even likely, but it's not like evolution only results in practical features :)

But we are still left with the question: how could this possibly evolve? Why would any individual forego the ability to contribute 100% of nuclear and 100% of mitochondrial DNA in order to just contribute mitochondrial? And here we have another level of complexity:how could the mitochondrial donor possibly evolve even if it somehow exists? It has no ability to control the phenotype of its own offspring since that is controlled entirely by the nuclear DNA. It;s an evolutionary dead end and will rapidly be outcompeted by organisms that simply reproduce the old fashioned way.

For a third sex to evolve it would need to offer at least a 18% reproductive advantage to all the individuals involved since all individuals will lose 17% of their reproductive capacity by entering into the arangement. IOW if a standard coupling produces two offspring a triple sex partnership would need too produce four to be evolutionarily viable (technically it only need to produce 3.1, but births are quantum events so four becomes the minimum).

If the third sex doesn't produce such an advantage then it is either a parasite or is being parisitised. The situation is then no different from the numerous viruses that contribute small amounts of genetic material to all organisms. And nobody calls a virus a third sex.

It's really hard to imagine a scenario in which the third sex could possibly produce such a massive evolutionary advantage.

[The Tao's Revenge]

But we are still left with the question: how could this possibly evolve? Why would any individual forego the ability to contribute 100% of nuclear and 100% of mitochondrial DNA in order to just contribute mitochondrial? And here we have another level of complexity:how could the mitochondrial donor possibly evolve even if it somehow exists? It has no ability to control the phenotype of its own offspring since that is controlled entirely by the nuclear DNA. It;s an evolutionary dead end and will rapidly be outcompeted by organisms that simply reproduce the old fashioned way.


Well males pass on absolutely no mitochondria. Just ain't room in the sperm for it.

A species with 40% of it's DNA in it's mitochondria would either be chock full of junk DNA or performing alot of otherwise nucleic functions in it's mitochondria.

These functions would be time tested since more rapidly changing things would be better suited to chromosomal DNA which can be mixed and matched, and would also possibly indicate an environment where rapid reaction was crucial. Simply put it'd be the many small hands of the mitochondria floating near the cell membrane able to react sooner and in multiple ways vs a big central nucleous. A larger critter would be able to maintain some kind of homeostasis and shield it's internals from the environment, but microscopic animals couldn't.

Also it wouldn't necessarily be one gender. Each organism could do all three gender rolls.

Plus a selfish gene setup might play out when an organism will develop into the 3rd gender to help it's relatives reproduce, and indirectly pass on it's own genes.

[Blake]

Well males pass on absolutely no mitochondria. Just ain't room in the sperm for it.

Firstly sperm are packed with mitochondria. How else did you think they move? It is the ovum that has very low mitochondrial density because it is entirely passive

Secondly sperm do pass on mitochondria, but only under conditions in which the ovum is mitochondrially derelict, either no mitochondria, defective mitochondria or mitochondria that can't reproduce. We still aren't sure how common paternal mitochondrial inheritance is but it seems to be a standard fall back defence against defective maternal mitochondria.

Which highlights the problem of a mitochondrial gender: mitochondrial inheritance is so worthless form an evolutionary POV that males only pass on that genetic material when there is no other choice. In fact it is now believed that this is because paternal mitochondria have a higher chance of being incompatible with the gemetes and may even trigger autoimmune responses.

A species with 40% of it's DNA in it's mitochondria would either be chock full of junk DNA or performing alot of otherwise nucleic functions in it's mitochondria.

Precisely, yet it would need ~40% of its phenotypic genetics in the genetic material committed by the third sex for the third sex to have any chance of evolving

These functions would be time tested since more rapidly changing things would be better suited to chromosomal DNA which can be mixed and matched, and would also possibly indicate an environment where rapid reaction was crucial.

Is that based on anything? I can;t see any logical reason for such a claim.

Simply put it'd be the many small hands of the mitochondria floating near the cell membrane able to react sooner and in multiple ways vs a big central nucleous.

Quite simply cells are so small, the speeds of diffusion and active transport so relatively fast, the transmission trigger molecules across the organelle membrane so precise and the processes of transcription so exacting that a few extra microns couldn't possibly make any difference to the rate of gene expression.

A larger critter would be able to maintain some kind of homeostasis and shield it's internals from the environment, but microscopic animals couldn't.

Exactly. A single celled organism always has all perts of the cell, whether nucleus or mitochondria, in close contact with the environment. The idea that the extra millisecond it takes to diffuse a hundred microns could have any effect on the rate of gene expression seems to contradict everything we know about the process.

Also it wouldn't necessarily be one gender. Each organism could do all three gender rolls.

I fail to see how this changes anything. You still need to decrease your genetic input into any offspring by 18%, it doesn't matter whether you are a hermaphrodite or not. Why would any organism forfeit 17% of its reproductive fitness for no apparent advantage?


Plus a selfish gene setup might play out when an organism will develop into the 3rd gender to help it's relatives reproduce, and indirectly pass on it's own genes.

And the question that has been asked about 10 times in this thread is: How could this possibly happen? We all agree that it needs to happen for a third sex to evolve, but how could it possibly happen?As others have noted their are plenty of organisms form bees to birds to rats that have non-reproductive helpers in their colonies, but non ehave ever evolved athird sex. By definition such individuals must be non-reproductive. If they are successfully reproductive at all then they must rapidly evolve to become perfect reproductives simply because perfect reproductives will produce more offspring.

Have a look at the life cycle of a typical bumble bee colony to see what happens if helpers become reproductives.

Can you explain how any organism can be reproductive yet remain 17% below optimal reproductively? Isn't such an organism going to rapidly lose out, either to any sibling that is 17% more efficient at reproducing or to those groups of organisms where helpers devote no energy to personal reproduction?

[The Tao's Revenge]

Well this way out of my league so I'll leave it at that. I do agree that a third gender is very unlikely with earth biology as we know it for most things, save for the fungus mentioned up thread.

[levdrakon]

What about ferns? IANABiologist, but as I understand it, a mature fern produces spores, neither male nor female, nor eggs or sperm, and the spores germinate into a form with both male and female parts, which must then reproduce sexually in order to produce another mature fern.

So, I could see the third gender being the mature spore producing gender who gives birth to both males and females who then have to mate in order to produce another mature spore producer.

[Blake]

So, I could see the third gender being the mature spore producing gender who gives birth to both males and females who then have to mate in order to produce another mature spore producer.

But this isn't a third gender, it's just an alternation of generations. All plants and most algae have that life cycle. Even a few animals alternate freely between sexual and asexual generations.

[tygerbryght]

... I don't know of any examples that really try to treat the mechanics of such science fictiion three-way sex, except where the third partner is a sort of catalyst or facilitator (as in Alien Nation, or in John Varley's Titan series, during the time when Rocky has to catalyze the centaur eggs before they'll impregnate. Otherwise, nobody wants to try and figure anything out.

Egads, I completely forgot about the Titan trilogy.

But it was because the alien being who altered the centauroids' reproductive system to require Rocky's saliva to activate their fertilized ova - and the burden of the responsibility drove her fairly insane - and definitely addicted (don't blame her, but it was a logical substitute for warfare as population control, and she's the one who wanted an end to the warfare between the centauroids and the "angels"). But in book 3, they discovered that (Robin's ?) "miraculous" baby also possessed the ability.

But talk about complicated reproductive strategies! That was more than complex enough, I'd think, to enthrall the OP!

So I'm extremely dubious about whether that really counts as three genders; three participants, more like, since Rocky was contributing enzymes, not genetic material. But, KRSOradio, I think you might really enjoy that series. There are aliens in it who have an incredibly complex reproductive process, which they can vary in a startling number of different ways, in terms of the number of needed participants. Now that was ingenuity, creating that system. It seems almost like something that might have been dreamed up on LSD.


In Asimov's the Gods Themselves the beings are sort of ill-defined and fuzzy. The most cases, they simply describe the participants as crowding together in a multisexual orgey.

I fixed the spelling of the Good Doctor's name. :)

Ummm. It was only the one, facilitating, gender that could be described as "ill-defined", I think, and only because it had to sort of merge partly with each of the two others, in order for sex to take place. It's been a very long time since I read that, but I'm pretty sure that each of the other two genders were pretty well defined - it was just that their bodies were sort of ovoid. Wasn't it (another book I can't get to)?

It has always seemed to me that book was more of an experiment. Wasn't it written after he married Janet, and finally had a happier home life? Or merely after he met her? I mean, he'd never really done emotions before, that I can recall. Prior to that, (excluding some of his positronic robots), the only human character he ever wrote that seemed to me to have a real personality was the protagonist in Pebble in the Sky (the guy who gets hit by the beam of radiation at the beginning of the book, the one who loved the Robert Browning poem).

[groman]

a) I don't understand why many posters are insisting on some sort of genetic contribution by the third individual to be considered a separate sex (or even evolve)? You could have two haploid producing sexes that need some sort of an incubator individual to produce offspring. The incubator does not need to pass on their genes to evolve -- nor do they have to have any genetic markers for being an incubator sex. Plenty of species have sterile offspring that fulfill various important tasks -- remember, as long as giving birth to some offspring of the third gender gives your genes no disadvantage, the mutation stays.


b) Life on earth just happens to tend to have genetic differences between the two sexes but I don't see how that's a pre-requisite for having sexual reproduction.

Imagine a species with two sexes but nothing equivalent to X/Y chromosomes -- rather the mother's (or incubator's) body arbitrarily (or not) picks the sex by supplying different hormones during different stages of development. Organs develop differently, produce different sexual characteristics, and you still need a male and a female to reproduce, but there is nothing about either parent's haploid that would affect which sex the child will be.

c) It's not hard to imagine a species that requires three or four genetically contributing genders. Reproduction and life cycles don't have to be efficient or logical to evolve. One can even imagine something seemingly bizarre like a haploid individual stage, where a larval individual develops from a haploid, then mates with an adult, undergoes metamorphosis to become another adult.

[Blake]

a) I don't understand why many posters are insisting on some sort of genetic contribution by the third individual to be considered a separate sex (or even evolve)? You could have two haploid producing sexes that need some sort of an incubator individual to produce offspring.

Because that isn't in any way unusual, it's as common as muck. Mosquitoes or tapeworms fit your example perfectly. That doesn't make you the third mosquito sex. It simply makes you a host for a parasite.

Plenty of species have sterile offspring that fulfill various important tasks -- remember, as long as giving birth to some offspring of the third gender gives your genes no disadvantage, the mutation stays.

But it doesn't make it a sex, it makes it a host. You may wish to look up a dictionary. An oak is not and never has been considered the third sex of a moss no matter how closely entwined the commensal relationship may be, no matter how dependent the parasite is upon its host nor how benign the parasite may be.

You are arguing that any host is the third sex of any obligately parasitic or commensal organism upon it. Since this is GQ the time has come for you to provide a reference for this assertion. Does anyone in the entire world agree that an oak tree is the third sex of a moss?

Life on earth just happens to tend to have genetic differences between the two sexes but I don't see how that's a pre-requisite for having sexual reproduction.

1) Life doesn't have that tendency at all. the majority of organsism and individuals are completely asexual.
2)Nobody is arguing that there is any pre-requisite for sexual reproduction. We are arguing that where sexual reproduction occurs there is no advantage in having a third individual contributing genetic material.

Imagine a species with two sexes but nothing equivalent to X/Y chromosomes -- rather the mother's (or incubator's) body arbitrarily (or not) picks the sex by supplying different hormones during different stages of development. Organs develop differently, produce different sexual characteristics, and you still need a male and a female to reproduce, but there is nothing about either parent's haploid that would affect which sex the child will be.

You mean exactly like crocodiles, or turtles, or many fish, or any of a plethora of other animal and plant species?

There's no need to imagine this situation, it is an astoundingly common solution in the world all around you.

It's not hard to imagine a species that requires three or four genetically contributing genders.


Since oyu seem to be able to be able to imagine this can you please elaborate on your imaginings. Because I can;t imagine how this could possibly be achieved for the reasons outlined above.

What is the advantage for any pair of individuals in allowing a third individual to contribute genetic material to this union? It lowers the reproductive potential of both the original couple by 17%, how do they recoup that loss?


Reproduction and life cycles don't have to be efficient or logical to evolve.

Yes, they do. By definition they have to be more efficient than the ancestral condition, otherwise they would never compete with that condition. And they need to be logical insofar as they have to m comply with the laws of mathematics and physics.

But how can any organism be more efficient by reducing its reproductive fitness by 17%? Why would this organism not be rapidly outcompeted by its ancestral type that is 17% more efficient? What' sin it for the very first tri-sex individual that will allow it to compete against its own di-sex siblings?


One can even imagine something seemingly bizarre like a haploid individual stage, where a larval individual develops from a haploid, then mates with an adult, undergoes metamorphosis to become another adult.

What do you mean by "mates with an adult"?

With a diploid adult? But again, how would this be advantageous to either party? The haploid only gets to contribute 33% of the offspring's genetic material, compared to 50% if it was diploid, so will rapidly be outcompeted by its own diploid siblings. The diploid adult only gets 33% increased genetic diversity material for its offspring as opposed to 50% if it mates with a diploid, so once it will rapidly evolve to select against this new lifestage, making it even less viable evolutionarily.


With a haploid adult? That is no different to the alternation of generations seen in plants. Indeed it is no different to the production of gametes seen in most animals: two haploid forms fuse to form a diploid. Nobody says that sperm and ova are third and fourth sexes. They are gametes are at best alternate generational forms.

I still can't see any possible way in which this situation has any advantages over the ancestral condition.

[groman]

You are arguing that any host is the third sex of any obligately parasitic or commensal organism upon it. Since this is GQ the time has come for you to provide a reference for this assertion. Does anyone in the entire world agree that an oak tree is the third sex of a moss?


No I am not. I am arguing that any organism that is capable of giving birth to three types of individuals genetically related to it with all three serving distinct and required reproductive functions would be considered to have three sexes. Honeybees almost fit, however strictly speaking worker bees are not required for reproduction. Personally I would consider honeybees to have three sexes -- drones, queens and workers.


Nobody is arguing that there is any pre-requisite for sexual reproduction. We are arguing that where sexual reproduction occurs there is no advantage in having a third individual contributing genetic material.

There doesn't have to be a direct advantage. Just insufficient disadvantages to lose any particular mutation. Life has no goals, and evolution has no design guidelines. Most of the processes that go on in your body are not there because they are currently the most efficient solutions to some well defined problems -- they are there because they mutated at some point and were not selected against sufficiently to be eliminated.


You mean exactly like crocodiles, or turtles, or many fish, or any of a plethora of other animal and plant species?


I was addressing the OPs problem with conceptualizing a third gender as well as the poster above that mentioned problems/solutions using specific sex chromosomes. I am aware that there is a plethora of species without such differentiation.




Since oyu seem to be able to be able to imagine this can you please elaborate on your imaginings. Because I can;t imagine how this could possibly be achieved for the reasons outlined above.


Just off the top of my head I can imagine a mutation on a Y chromosome that creates some bizarre protein that

Makes a normal offspring of a male carrying mutation A not viable (or infertile)
Creates a much greater likelihood of a viable chimera offspring between an egg with mutation A and an egg fertilized by a male that does not carry A
Gives some advantageous immunity to an individual who carries A


That sort of thing, however unlikely, would begin one type of split into three genetic contributors. Males carrying A have advantages but need non-A's to mate with the same female to pass on that advantage.


What is the advantage for any pair of individuals in allowing a third individual to contribute genetic material to this union? It lowers the reproductive potential of both the original couple by 17%, how do they recoup that loss?


Sexual reproduction is disadvantageous to the individual's contribution in general. That does not mean it could not evolve, as it clearly did. If the advantages of every mutation on the way to having three genetic parents outweigh the disadvantages it can evolve -- even if none of the advantages have anything to do with having three genetic parents.

Honeybee queens give birth to lots and lots of children that will not get the chance to pass on the queens genes. Certainly this ability evolved not through a descent line of a sterile worker bee but rather through her sibling's comparative advantages.


What do you mean by "mates with an adult"?

With a diploid adult? But again, how would this be advantageous to either party? The haploid only gets to contribute 33% of the offspring's genetic material, compared to 50% if it was diploid, so will rapidly be outcompeted by its own diploid siblings. The diploid adult only gets 33% increased genetic diversity material for its offspring as opposed to 50% if it mates with a diploid, so once it will rapidly evolve to select against this new lifestage, making it even less viable evolutionarily.


I know that in haploid-diploid insects, the haploid individuals are usually males. However, imagine a species where the haploid larva is female and carries an egg, when the egg is fertilized, the individual enters the pupal stage, the haploid parts are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs that develop into haploid female larva, and the diploid males only mate with the larva. There you go, three sexes, stupid and unnecessarily complex life cycle (like most of insect life cycles), and a good foundation for a really creepy social insect. :)

[Blake]

No I am not. I am arguing that any organism that is capable of giving birth to three types of individuals genetically related to it with all three serving distinct and required reproductive functions would be considered to have three sexes.

No, that is not what you are arguing . You specifically said "I don't understand why many posters are insisting on some sort of genetic contribution by the third individual. Ii even quoted that for you. So don't now claim that you were referring to a situation where all three individuals make a genetic contribution. You very specifically said that was not the case.

Personally I would consider honeybees to have three sexes -- drones, queens and workers.

Well this is GQ, so can we have a reference for this belief of yours?


There doesn't have to be a direct advantage. Just insufficient disadvantages to lose any particular mutation.

As we have pointed out ad nauseum there will alwyas be a direct disadvantage: a loss of 17% of genetic input. that's a massive evolutionary disadvantage.

So we can scrap that idea.


. I am aware that there is a plethora of species without such differentiation. [

Then you should also be aware that they are never considered to be different sexes.



Just off the top of my head I can imagine a mutation on a Y chromosome that creates some bizarre protein that

Makes a normal offspring of a male carrying mutation A not viable (or infertile)
Creates a much greater likelihood of a viable chimera offspring between an egg with mutation A and an egg fertilized by a male that does not carry A
Gives some advantageous immunity to an individual who carries A


That sort of thing, however unlikely, would begin one type of split into three genetic contributors. Males carrying A have advantages but need non-A's to mate with the same female to pass on that advantage.

Nope, doesn't address the problem at all. Whatever this mutation confers immunity to must be lethal to over 50% of offspring just to make it evolutionarily viable. But only one of these mutations exists in the whole world when the catastrophe strikes. With 50% mortality the survivors are going to be selecting for their own survival genes far faster than you mutation can disseminate in the population.

Not only that but any female that actively resists mating with such a male (or simply refuses to mate more than once) will be giving here male offspring as big an advantage as the mutation gives them. Your mutation increases their survival by 50%, but it reduces the number of offspring they can have by an equal amount.

So simple sexual selection will eliminate such a mutation in short order.

And this is the point we've been hammering home. The mutation can give no benefit, it's just parasitic. There must be an equal number of non-carriers for it to "infect" just to survive. There can never be more of these mutant males in the breeding pool than there are naturally resistant males that they can cuckold. As such it can never confer any more survival benefit than the mutant males already have. Even if it confers a thousand fold increase in survivability then it will simply have a thousand fold fewer hosts and will have to reduce its own population down to that of the host.

In short the parasite can't produce actually more offspring than the number of non-infected males. All it can do is reduce the number of offspring the non-infected males have. In very short order sexual selection and plain old fashioned natural selection will eliminate it completely.

I can trace this out for you generation for generation if you wish.

Sexual reproduction is disadvantageous to the individual's contribution in general.

Yes, but only to a degree that is compensated by increased diversity. OTOH a third sex of the type you propose a never compensate for the decreased diversity because it is entirely dependent on the existence of equal number of non-carriers.

Look, all sexually produced offspring has double the survival odds of an asexual one because they have double the gene pool and thus can very rapidly dominate the population. In short order the entire species can become sexual reproductives carrying sexual reproduction genes.

You tri-sex hypothetical requires that it always be in a minority. As soon as there are fewer standard males than the mutants the mutant population must decline to that level. It is a predator-prey or host-parasite relationship. When the host population crashes so must the parasite's. It can never be more successful than its host because there is a direct 1:1 relationship to the host built in. The population can never, ever consist of more than ~30% of individuals.

In contrast any individuals carrying resistance genes without the parasite is 50% or 30% more succesful reproductively, and they can make up the entire population. IOW your third sex is at the mercy of the host population, but the host can and will try to eliminate the parasite.

Honeybee queens give birth to lots and lots of children that will not get the chance to pass on the queens genes. Certainly this ability evolved not through a descent line of a sterile worker bee but rather through her sibling's comparative advantages.

Exacatly, and the entire honeybee population is now made up of such individuals.

But the entire population can never be made up of your third sex individuals or carriers thereofo, because by definition they need equal numbers of non-carriers to parasitise. In contrast no honeybee needs any honeybees that don't cary the colony genes. they are entirely self sufficient and able to outcompete such individuals.

Simple questions:

What is the maximum percentage of the bee population that could be made up of colonial bees? Is it possible to have a landamass where 100% of bees carry the colonial genes?

What is the maximum percentage of the population that could be made up of your third sex? Is it possible to have a landmass where 100% of individuals carry the tri-sex genes? If so then what exactly are they mating with, since the males and female are mutually sterile?

If you can answer those questions you should see why such a trait, even if it could evolve, will be eliminated in short order and offers no reproductive advantage.


I know that in haploid-diploid insects, the haploid individuals are usually males. However, imagine a species where the haploid larva is female and carries an egg, when the egg is fertilized, the individual enters the pupal stage, the haploid parts are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs that develop into haploid female larva, and the diploid males only mate with the larva. There you go, three sexes, stupid and unnecessarily complex life cycle (like most of insect life cycles), and a good foundation for a really creepy social insect. :)

Nope, makes no sense. If a diploid male mates with a haploid larva the result will be triploid, which you have neglected to include. I assume when you say "mate" you mean use haploid sperm to fertilise. If that is so then all you've done is describe the normal human lifecycle using non-standard terms.

It's not three sexes.There's nothing creepy about this, it's where we all come from.

The haploid larva (read polar body) is female and carries an egg (remember the ovum doesn't complete mieosis until after fertilisation, so the unicellular polar body carries the ovum as a separate cell). When the egg is fertilised the individual enters the pupal stage, the haploid parts (polar bodies) are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs that develop into haploid female larva, and the diploid males only mate with the larva.

And there you go. This is not three sexes. It is the standard human life cycle with the standard alternation of generations common to almost all multicellular organisms.

If you want to we can use ferns (http://www.esu.edu/~milewski/intro_biol_two/lab_2_moss_ferns/Fern_life_cycle.html) instead.

The haploid larva (gametophyte) is female and carries an egg, when the egg is fertilized, the individual enters the pupal stage (sporophyte), the haploid parts are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs (spores) that develop into haploid female larva (gametophytes), and the diploid males only mate with the larva.

There is nothing in any way unusual about the situation you describe. I can't think of any muticellular organism that doesn't exhibit this lifecycle. But nobody considers that we have more than two sexes.

And once again, where this differs from your hypothetical above is that there is no competition between the "sexes". Producing a polar body or gametophyte doesn't reduce the fitness of the female or the male, and the entire population can be (and is) made up of individuals with genes coding for this lifecycle. Nobody depends on individuals with a third distinct genetic makeup.


I think the trouble is that you are thinking that alternation of generations somehow equates to a different sex. It doesn't. We alternate between three. Some organism alternate between six different generational forms. Nobody describes the generations as sexes.

[Fly Pusher]

But we are still left with the question: how could this possibly evolve?

That's the tricky part, I'll have to agree. I'll concede that it's extremely unlikely that anything resembling terrestrial animals will ever evolve three sexes from two.

However, that doesn't preclude something like it evolving from the same conditions that sexual reproduction evolved on Earth in the first place. Instead of 2 cells fusing to form a diploid organism, you could just have 3 instead. Over time these 3 cells might have differentiated into different roles, just as our sex cells have. Would you accept this setup as three genders?

[Lemur866]

Sure, but we have to remember that three gametes doesn't mean three phenotypes. You could have an organism capable of producing all three types of gametes.

But the fatal flaw for any three gamete systems is that the optimum number of genders is probably lower than two. There are obvious advantages to asexual reproduction...all your offspring are 100% genetically identical. And with two genders, your offspring are only 50% genetically identical. And we see over and over again in the natural world that organisms that have sexual reproduction secondarily evolve asexuality, or preserve asexuality.

Sex is advantageous for several reasons. But you don't need very much sexual reproduction to reap the benefits of sexuality. The only reason organisms frequently have two genders is that two genders is the minimum number of genders you can have and still have sexual reproduction. Adding more complexity than that is counterproductive. If there was a physically possible way to have sexual reproduction with fewer than two mating types I'm certain that we'd have fewer than two. So a three gendered species is logically possible, but any mutation that eliminated one of the genders would be so evolutionarily successful that it would rapidly spread through the species until only two mating types were left.

Thats why we don't see three genders. It has nothing to do with physical impossibility, or logical impossibility. It is because mating systems with only two types will rapidly outcompete systems with three types, since mutants that support two types are 17% more successful (50% vs 33%).

[Tastes of Chocolate]

As we have pointed out ad nauseum there will alwyas be a direct disadvantage: a loss of 17% of genetic input. that's a massive evolutionary disadvantage.

So we can scrap that idea.

About a system requiring a third gender acting as a host.

Just because a % of the offspring don't reproduct, doesn't make it an evolutionary disadvantage. What % of bees or ants ever reproduct? How about the wolves, where even though all animals CAN reproduct, mostly the alpha pair do? If that % of non-reproducting offspring increase the survival rate of the rest of the offspring, where is the disadvantage?

Let's look at a male/female/host set up. What if the host was able to carry the fertilied eggs of more then one female. Let's randomly pick 3. Offspring ratio - 3 male, 3 female, 1 host. You could claim a 14% loss. Or you could claim about a 1/3 gain, if it freed up the females to reproduct more frequently. On top of that, the females now no longer have to have the equipment to incubate the egg. So more energy and resources are available to her to product the next egg.

Or how about a set up where the host never leaves the hive, where it is well protected, thus reducing the loss of host and young. Kind of like a queen bee.

Maybe a host who is able to eat a different set of food. You could have light weight flying males and females, and a non-flying host. Put them on different foods, and you've expanded the resources available to the species.

Heck, how about different intelligence levels. Male and female gender are ephemeral, seasonal creatures. Little intelligence. They are gonads with legs. Maybe like a mayfly. The hosts are sturdier. Larger bodies, bigger brains, longer lives. They could carry the fertilied eggs through the non-optimal season, making longer gestation possible, as well as increased protection.

[Fly Pusher]

So a three gendered species is logically possible, but any mutation that eliminated one of the genders would be so evolutionarily successful that it would rapidly spread through the species until only two mating types were left.

That's quite a compelling argument. I don't dispute that the advantages of 2 genders over 3, from both a genetic and a logistical standpoint. That being said, I think there are a few advantages in having 3 types of gametes in a triploid organism. The main advantage would be more possible combinations to increase variation. There may also be more redundancy against harmful mutations, but I suppose you can have that by the polypoidy alone. There might also be something in the fact that an organism can contribute more than 1 gamete to a given mating. With 3 gametes, one organism can theoretically provide 66% of the genetic materials. There might be some competitive advantages in that, though I'm not quite certain if my logic works. All in all. while I doubt that these are enough to overcome the disadvantages, it's there.

Any existence of tri-gendered organisms is sure to be the exception rather than the rule. Nevertheless, Evolution is a chance driven process, and however unlikely a feature is on paper, given the wide varieties of biochemistries possible, and given some combination of environmental factors, I think it's still possible for tri-gendered species to exist.

[Yllaria]

Whether it would be advantageous or not, the snakes in Vonda N. McIntyre's novel, Dreamsnake, had three sexes, one of which contributed extra-neuclear genetic material.

[Lemur866]

There might also be something in the fact that an organism can contribute more than 1 gamete to a given mating. With 3 gametes, one organism can theoretically provide 66% of the genetic materials. There might be some competitive advantages in that, though I'm not quite certain if my logic works. All in all. while I doubt that these are enough to overcome the disadvantages, it's there.
Sure, there's a big advantage in being a double parent in a tri-gamete system. Which is why just about every organism in a tri-gamete system is going to try to be a double parent and prevent everyone else from being a double parent. The evolutionarily stable outcome for this battle is a two gamete system.

[groman]

No, that is not what you are arguing . You specifically said "I don't understand why many posters are insisting on some sort of genetic contribution by the third individual. Ii even quoted that for you. So don't now claim that you were referring to a situation where all three individuals make a genetic contribution. You very specifically said that was not the case.


Do you have some sort of reading deficiency?


I am arguing that any organism that is capable of giving birth to three types of individuals genetically related to it with all three serving distinct and required reproductive functions would be considered to have three sexes.


Where am I "now referring to a situation where all three individuals make a genetic contribution"? Three types of individuals that are serving distinct and required reproductive functions does not mean they all pass on their genes. The ability to give birth to three sexes does not require the third sex to pass it on. The ability can be encoded in all three sexes, and as long as it gives no disadvantage.



Well this is GQ, so can we have a reference for this belief of yours?


I think I'm a fairly good reference for statements I believe are true, but I will bite:

Since we are discussing the concept of a 'third sex', we are left with a definition of 'sex' that does not rely on defining it as the differentiation between males and females. As such, we are left with something like:

noun: the properties that distinguish organisms on the basis of their reproductive roles from quick definition at OneLook.

The reproductive roles of worker bees, queens and drones are distinctly different. They are also organisms. What do you find difficult about such a belief?







As we have pointed out ad nauseum there will alwyas be a direct disadvantage: a loss of 17% of genetic input. that's a massive evolutionary disadvantage.

So we can scrap that idea.



As I have implied, sexual reproduction using two sexes results in 50% loss of genetic input, so why don't you go scrap THAT idea instead?






Nope, doesn't address the problem at all. Whatever this mutation confers immunity to must be lethal to over 50% of offspring just to make it evolutionarily viable.


I'm going to ask for a cite. That is a ridiculous statement.

Yes, but only to a degree that is compensated by increased diversity. OTOH a third sex of the type you propose a never compensate for the decreased diversity because it is entirely dependent on the existence of equal number of non-carriers.


Look, all sexually produced offspring has double the survival odds of an asexual one because they have double the gene pool and thus can very rapidly dominate the population. In short order the entire species can become sexual reproductives carrying sexual reproduction genes.


No, they do not. On average over time the genetic contribution % of asexual reproduction are slightly lower. That tells you nothing about the survival odds of all sexually produced offspring, much less any sexually produced offspring. Any parent that outlives their child due to the child developing some genetic disorder for which the parent was only a carrier disproves that the offspring has double the survival odds. If the offspring was a clone of the parent they would be a lot less likely to die before the parent, no?



You tri-sex hypothetical requires that it always be in a minority. As soon as there are fewer standard males than the mutants the mutant population must decline to that level. It is a predator-prey or host-parasite relationship. When the host population crashes so must the parasite's. It can never be more successful than its host because there is a direct 1:1 relationship to the host built in. The population can never, ever consist of more than ~30% of individuals.


So? That's a red herring. There's a lot less queens than any other type of bees and they are a distinct minority. So what? Are they not a sex now?


But the entire population can never be made up of your third sex individuals or carriers thereofo, because by definition they need equal numbers of non-carriers to parasitise. In contrast no honeybee needs any honeybees that don't cary the colony genes. they are entirely self sufficient and able to outcompete such individuals.


Just like the entire population cannot be made up of males or carriers of the Y chromosome. Doesn't stop us from having two sexes.



What is the maximum percentage of the population that could be made up of your third sex? Is it possible to have a landmass where 100% of individuals carry the tri-sex genes? If so then what exactly are they mating with, since the males and female are mutually sterile?


Good questions. Have nothing to do with feasibility.



If you can answer those questions you should see why such a trait, even if it could evolve, will be eliminated in short order and offers no reproductive advantage.


No, it wouldn't. Using your logic widespread genetic diseases would not exist, and every single feature we have offers a reproductive advantage.


Nope, makes no sense. If a diploid male mates with a haploid larva the result will be triploid, which you have neglected to include. I assume when you say "mate" you mean use haploid sperm to fertilise. If that is so then all you've done is describe the normal human lifecycle using non-standard terms.


That's what one usually means when they say "mate", yes. But no, I did not describe normal human life-cycle using non-standard terms. Neither sperm nor eggs are ever external individual organisms for humans. A haploid larva that eats, moves around and is clearly a distinct organism is a sex. If it pupates and turns into a diploid it is different genetically, a completely different individual, you cannot consider that just a 'life stage' like in case of normal metamorphosis.




I think the trouble is that you are thinking that alternation of generations somehow equates to a different sex. It doesn't. We alternate between three. Some organism alternate between six different generational forms. Nobody describes the generations as sexes.

I think the trouble is that you are begging the question. You want to define sex using male and female, and then reject any third sex because it obviously does not fit the definition. Physiological differentiation between different organisms of the same species that results in a distinct and different reproductive role is a sex.

You also keep implying that evolution will always find some better solution. Evolution has no goals and no reasoning. Everything that is not a sufficient detriment to be eliminated will stay. Nothing that doesn't mutate by chance will evolve. Evolution is not a process that yields 'most efficient' or 'most fit'. It always yields 'most sufficiently fit' from the set of 'randomly mutated' -- which is very unlikely to be any sort of efficient.