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-   -   Will we ever encounter a multisexed sentient species? (https://boards.straightdope.com/sdmb/showthread.php?t=643208)

BrainGlutton 02-24-2012 07:46 PM

Will we ever encounter a multisexed sentient species?
 
It's a staple, or at least an idea occasionally played with, in SF that an alien race might have more than two sexes. But, assuming there are ETs, and assuming most of them are Starfish Aliens, I think they still would be two-sexed, because two (or one, with hermaphrodite species) is the minimum number necessary to reshuffle the genetic deck each generation, and requiring more than two individuals to be involved in reproduction would make it that much harder to arrange, and multi-sexed organisms would get outbred-to-extinction by two-sexed organisms. Anyone see any reason to expect otherwise?

njtt 02-24-2012 08:00 PM

Quote:

Originally Posted by BrainGlutton (Post 14808430)
I think they still would be two-sexed, because two (or one, with hermaphrodite species) is the minimum number necessary to reshuffle the genetic deck each generation

Who says aliens would have chromosomes, or, more to the point, ones with crossing over and recombination, like ours?

You may be right. Two sexes is plenty complicated enough for me; I hate to imagine having to cope with more. But it is space; it is very big. Who knows?

BrainGlutton 02-24-2012 08:11 PM

Quote:

Originally Posted by njtt (Post 14808464)
Who says aliens would have chromosomes, or, more to the point, ones with crossing over and recombination, like ours?

Well, some functional equivalent to that would be necessary to produce a species we could talk to. It speeds up the evolutionary process immensely; see the relative complexity of Earth lifeforms that reproduce sexually (in a genetic sense) compared to those that do not.

BrainGlutton 02-24-2012 08:14 PM

Quote:

Originally Posted by BrainGlutton (Post 14808487)
Well, some functional equivalent to that would be necessary to produce a species we could talk to. It speeds up the evolutionary process immensely; see the relative complexity of Earth lifeforms that reproduce sexually (in a genetic sense) compared to those that do not.

See Asimov's 1961 story "What Is This Thing called Love?" aka "Playboy and the Slime God."

Der Trihs 02-25-2012 02:10 AM

Quote:

Originally Posted by BrainGlutton (Post 14808430)
It's a staple, or at least an idea occasionally played with, in SF that an alien race might have more than two sexes. But, assuming there are ETs, and assuming most of them are Starfish Aliens, I think they still would be two-sexed, because two (or one, with hermaphrodite species) is the minimum number necessary to reshuffle the genetic deck each generation, and requiring more than two individuals to be involved in reproduction would make it that much harder to arrange, and multi-sexed organisms would get outbred-to-extinction by two-sexed organisms. Anyone see any reason to expect otherwise?

There's other ways to do it; I understand that some slime molds have 5000 sexes or so and do fine. There's just a biological hierarchy (for lack of a better term) with each successive gender being "male" for those lower down the totem pole; Sex 3 is "female" for Sex 2 and "male" for sexes 4-5000 and so on. "Female" and "male" being respectively the gender that does and does not pass down its mitochondria in this case; mitochondrial DNA not being part of nuclear DNA appear to be a major factor in the two-sex system since dumping the mitochondria from two genders in one egg tends to produce destructive competition among the separate breeds.

And there are other possibilities. A species with 3 strand DNA could evolve 3 genders, one for each. Or the creature might be a chimera, a combination of two or more species that have become symbiotes to the degree that they've essentially merged and still have separate reproductive methods.

Northern Piper 02-25-2012 04:15 AM

Quote:

Originally Posted by BrainGlutton (Post 14808497)
See Asimov's 1961 story "What Is This Thing called Love?" aka "Playboy and the Slime God."

For the theme in the OP, see also Asimov's novel, The Gods Themselves.

BrainGlutton 02-25-2012 11:17 AM

Quote:

Originally Posted by Northern Piper (Post 14809232)
For the theme in the OP, see also Asimov's novel, The Gods Themselves.

Like I said, SF staple. But there's nothing in the book to suggest the evolutionary-advantage superiority of three sexes over two -- unlike "WITTCL?" which is all about the evolutionary-advantage superiority of two sexes over asexual reproduction.

BrainGlutton 02-25-2012 11:18 AM

Quote:

Originally Posted by Der Trihs (Post 14809128)
There's other ways to do it; I understand that some slime molds have 5000 sexes or so and do fine. There's just a biological hierarchy (for lack of a better term) with each successive gender being "male" for those lower down the totem pole; Sex 3 is "female" for Sex 2 and "male" for sexes 4-5000 and so on. "Female" and "male" being respectively the gender that does and does not pass down its mitochondria in this case; mitochondrial DNA not being part of nuclear DNA appear to be a major factor in the two-sex system since dumping the mitochondria from two genders in one egg tends to produce destructive competition among the separate breeds.

Now imagine large animals trying to work a system that complicated.

Musicat 02-25-2012 11:29 AM

Quote:

Originally Posted by njtt (Post 14808464)
You may be right. Two sexes is plenty complicated enough for me; I hate to imagine having to cope with more.

National Lampoon once had a cartoon story illustrating date night on a planet with 23 (?) sexes. To get laid, you had to visit a lot of bars to pick up one of each before heading back to your (big) bachelor pad.

Der Trihs 02-25-2012 11:45 AM

Quote:

Originally Posted by BrainGlutton (Post 14809854)
Now imagine large animals trying to work a system that complicated.

It's not really all that complicated, especially if the species uses some scent based gender identification. <sniff> isn't complex.

Half Man Half Wit 02-25-2012 11:50 AM

Quote:

Originally Posted by BrainGlutton (Post 14809854)
Now imagine large animals trying to work a system that complicated.

Actually, species with such a system would have more potential mating partners than species with a two-gender system (where about half of all individuals are potential partners). So it should be easier to find a viable mate. (See this blog post.)

Lemur866 02-25-2012 06:07 PM

The optimum number of sexes is lower than two but more than one. There are plenty of species that reproduce asexually, some that reproduce both sexually and asexually, and some that only reproduce sexually. Of course, if you reproduce sexually that doesn't mean you must have two genders--it's perfectly possible for one individual to produce both types of gametes.

There are some species that are isogamous--they reproduce sexually but their gametes aren't differentiated into eggs and sperm--http://en.wikipedia.org/wiki/Isogamy. And the aforementioned "thousands of sexes" fungi are isogamous so these are more properly "mating types" rather than sexes, since the species that operate this way are isogamous. Which means that it's misleading to talk of lots of different sexes, because most mating types are able to breed with most other mating types. You don't need one of each, you just need two of two different types.

jackdavinci 02-25-2012 10:59 PM

I think more than two sexes as we think of them would require pretty exotic chemistry, because our genetics is pretty hard wired to be dualistic due to the double helix.

Assuming our genetics, to get more sexes you'd have to maybe do something chromosomal, or else have it be role based such as having two mate and then implant in a third who didn't contribute genetically but did impregnate. I suppose we already have this with IVF and surrogates.

Perhaps something could be done with methylation.

The other option I can think of would be similar to our mitochondria, which are inherited in a deprecate way. So maybe genders could based on highly independent organelles.

Lemur866 02-26-2012 03:46 PM

We don't have two genders because of the structure of DNA. Each gender contributes a distinct set of chromosomes, one from each parent. This is because many species are diploid--each chromosome is paired. It's possible to have haploid species where chromosomes are not paired, or polyploid species where chromosomes aren't just doubled but are tripled or quadrupled and so on.

It is simply very common for species to reproduce asexually, so the requirement to have two sexes isn't a consequence of the genetic structure of life on Earth. And species that do reproduce sexually are often hermaphroditic, able to produce both eggs and sperm, although it is also very common for hermaphroditic species to have mechanisms to make self-fertillization difficult.

There just aren't any species on Earth that require three sexes to complete the mating process, even though there are plenty of species that have more genders than "male" and "female". But there aren't any species that require three gametes for reproduction, and it's pretty easy to see why.

When you engage in sexual reproduction you're already throwing away half your genes. That's a pretty high burden, and you only do that if there's a substantial advantage to sexual reproduction. But with a three gamete mating system you'd be throwing away 2/3rds of your genes. But to what advantage? You already get all the benefits of sexual reproduction in a two gamete system, what's the benefit of a more complicated system?

Frylock 02-26-2012 05:43 PM

Anyone else heard of the thousand-sexed creature mentioned upthread?

foolsguinea 02-26-2012 06:46 PM

Quote:

Originally Posted by jackdavinci (Post 14811529)
I think more than two sexes as we think of them would require pretty exotic chemistry, because our genetics is pretty hard wired to be dualistic due to the double helix.

The "double helix" term is misleading. Yes, DNA has two strands, but one is a mirror of the other, and the two strands break apart for mitosis (molecular duplication, in effect). That is an entirely different mechanism from the diploid business of having two of each chromosome.

BrotherCadfael 02-26-2012 06:55 PM

Quote:

Originally Posted by BrainGlutton (Post 14808497)
See Asimov's 1961 story "What Is This Thing called Love?" aka "Playboy and the Slime God."

Asimov got the name wrong. It should have been, "What Is This Thing Called, Love?"

(Unashamedly stolen from Benny Hill!)

Lemur866 02-26-2012 10:02 PM

Quote:

Originally Posted by Frylock (Post 14813304)
Anyone else heard of the thousand-sexed creature mentioned upthread?

Read the link Half Man Half Wit posted: http://blog.mycology.cornell.edu/?p=1060

It's really misleading to think of fungal mating types as "sexes". These fungi are isogamous, which means they don't have eggs or sperm, just one type of gamete that can fuse with any other gamete. However, there are subtle differences between these gamete types that makes some compatible with some types and others with others.


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