- adding voice to chorus *
I can’t see why that would be the case, but I’m no geneticist.
Don’t know the answer to this, but note that in lion/tiger hybrids the size of the offspring is determined by the species/gender combo. The liger (male lion x female tiger) lacks a certain growth inhibiting gene, and can become enormous. Much bigger than either parent. A tigon (male tiger x female lion) is not thusly afflicted.
From that same link, horses have 64 chromosomes, and zebras 44. I would think that the number of chromosomes wouldn’t matter much in terms of survival of the fittest, apart from having the same number throughout the gene pool. That in turn would seem to lead to the number mostly not changing. So why such a large difference between two species related closely enough that they can still breed?
As I think was stated somewhere upthread, there’s this belief going around right now that greatly overemphasizes the importance of chromosome number, to the point that I’m seeing people who believe that the number of chromosomes you have DEFINE what species you are, and are confused that there are nonhuman species with 46 chromosomes.
What’s important for viability is that you have the correct number of all the correct genes, not how they’re organized into strands*. What’s important for fertility is the ability for homologous chromosomes to pair up correctly - again, some difference in number is tolerable as long as all the genes line up in the correct order. For instance, in a hypothetical human/chimp hybrid, the human chromosome 2 could pair up just fine with both the chimp 2A and 2B. Differences in chromosome number really aren’t all that big a deal, as long as the species are closely related enough that the chromosomes still look very similar.
As to the male/female more/fewer chromosomes thing, I’ve never heard of such a rule, and I can’t think of why it would happen. There is a “rule” (more of a general rule of thumb with lots of exceptions) that when you have a population of hybrid animals, the heterogametic sex (the one with XY rather than XX sex chromosomes) is more likely to be sterile or have more problems with fertility. But I don’t see any obvious connection to what’s being discussed.
Whoops - I left a hanging asterix. I hate that. What I meant to add was this:
*I’m oversimplifying here. There are some organizational issues that have to be considered, like position from heterochromatin and the action of cis-acting enhancers and repressors.
The accepted theory many years ago is that chromosomal multilplication is caused when populations repeatedly separate then rejoin. During periods of separation the chromosomes can either join or split in any population, and both probably occur with equal frequency. But when the populations rejoin, the smaller chromosomes are favoured, and the population as a whole settles on the genome with the maximum number of chromosomes. So if population A had one a fused chromosome 1 and 2 and a split 3a and 3b, and they joined with Population B that had a fused chromosome 6 and 7 and a split 8a and 8b, the genome would select for the maximum number and end up with a single 1 and 2, 3a and 3b, a single 6 and 7 and 8a and 8b
That means that while each individual population had the same number of chromosomes, the fused population will end up with two extra pairs more than either parent population.
As for why the smaller chromosomes are favoured, that’s probably down to a combination of diversity and gene competition.
If you have one big chromosome, its likely to contain a mix of detrimental and beneficial genes, and any organism has to take the good with the bad. If you have two small chromosomes, you only have half the chance of inheriting the bad. Once crossing over has transferred any novel beneficial genes off the larger chromosome, it’s doomed to die out.
But of course, there must also be some competing mechanism to favor lower chromosomal count, or you’d expect that after all these billions of years, we’d have a lot more chromosomes. Any idea on what that mechanism would be?
Just off the top of my head, if you had too many chromosomes, you’d begin to overload the spindle mechanisms and increase the chance of nondisjunction during both mitosis and meiosis. Making sure that each and every centromere is properly oriented and attached to the correct spindle pole is nontrivial.
That at least is fairly uncontroversial. It’s down to the effects of gene proximity and regulation. Basically the physical relationship of two genes on the DNA strand can control their ability to be expressed at the same time. Sometimes that means that a stimulus that expresses one gene represses the other, and sometimes expressing one gene expresses both.
In the right circumstances that gives a big evolutionary advantage since it allows an organism to initiate shortcuts. So, for example, an organism that can link a gene expressed when coming out of hibernation to a gene for sperm production may have an advantage when mating season rolls around, because it started to produce sperm weeks before its competitors, that only started producing sperm when the females went into season.
As a result, there will be advantages to fusing chromosomes because it increases the number of possible gene linkages.
The mechanism is counterbalanced by inability to select against specific detrimental genes I described above, but in a stable population that effect is smaller. With a stable population, any novel mutation will only arise once, and it will spread relatively slowly so it has plenty of time to move to the correct site in the genome.
When two separated population re-merge, the linkage system gets screwed up severely because the way that Population A evolved to deal with a problem isn’t the way that Population B did. All of a sudden you have a flood of novel genes that often don’t work at all within the existing linking system. In that situation, gene competition is going to override organism competition. IOW genes are going to be selected for more than organism with good combinations of genes. The pressure for chromosomal merging drops dramatically
The genotype of a stable population is like a good sports team. The team works well because the individuals/genes work well together. Individual strengths are less important than how well you contribute to the team. When the population gets merged with another, you merged two team from different towns, assigning players from each of the old teams to one of two new teams at random.
Because nobody knows how to work with anyone else, suddenly individual performance is *all *that matters. Jack and Bob may have been a brilliant Quarterback/Fullback combo, but mediocre individual players. While the team was held together that didn’t matter, But once the teams are split and Bob has to play with another fullback, he kinda sucks. Similarly, Ted and Joe may be just a good Quarterback and good Fullback, nothing brilliant. But because they don’t rely on teamwork they will still be good on any team. They won’t suck like Jack and Bob. So teams with the good players are going to win more games than teams with the brilliant combo.
So a stable population is like a table sports team. It favors the ability to keep the best combinations together. If you have to let Jack’s halfwit cousin play Guard to keep the Jack/Bob combo. it’s worth that price. the more winning combos you can put together the better the team will play. IOW the bigger the chromosome, the better your odds.
A merged population is like a merged team. It favours individual performance and the ability to reject the poorest performers. If Jack isn’t going to be playing with Bob in most games, then his contribution to the team is going to be minimal in most games. You’re better off getting rid of him and replacing him with a competent player who can play with anybody. IOW smaller chromosomes with more selective ability are favoured.
Of course after a while the genome becomes thoroughly mixed, everyone learns who can play with who, and the fusing starts all over again.