Chromosomes and Evolution

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I have just learned something new. Or, rather, become newly aware of the implications of some things that have been rattling around in my mind for a while. Greg Cochran linked (indirectly) to this quote:

The loci in question are so tightly linked that rare recombinants practically never arise – this explains why the different multi-locus genotypes appear, when crossed, to segregate like single locus genotypes. A set of genes so tightly linked that they behave like a single locus has been termed a supergene.

This was a eureka moment for me. I have sometimes wondered about the evolutionary implications of chromosomes. I’m sure that there’s a molecular reason for them – certainly, it would be hard to imagine a diploid genetic architecture, necessary for sexual reproduction, without them! But having said that, it would seem that chromosomes only get in the way of sexual reproduction: If sexual reproduction is about facilitating genetic recombination, then more would certainly be better than less, and we know that many other species have many more than our 23 pairs: horses have 32, dogs have 49, ferns have 630! So why haven’t we evolved the maximum possible number of chromosomes? It’s certainly possible to have a lot more chromosomes than we have.

Clearly, it seems to me, the answer is that sexual reproduction is not always a good thing. Rescrambling our genes every generation has the effect of breaking up favorable combinations of genes, so it must be that a small number of chromosomes is an adaptive response to this. Genes on the same chromosome get rescrambled not every generation, but once out of many generations, with genes closer together getting rescrambled less often than genes farther apart. The infrequency of the rescrambling makes time for selection to weed out unfavorable linkages as they arise.

Some predictions:

1. Linkage disequilibrium is not necessarily a sign of recent positive selection – it could also be a sign of coadapted gene complexes.

2. Coadapted gene complexes that involve genes on different chromosomes would have to be much more advantageous than those involving genes on the same chromosome, in order to be maintained.

3. The advantage necessary to maintain coadapted gene complexes varies according to the physical distance on the chromosome of the genes involved. (I know it’s a bit more complicated than that, but roughly.)

4. A reduction in the number of chromosomes could be adaptive if it locks-in a favorable gene complex.

5. A coadapted gene complex could also explain this, as these genes don’t recombine.

6. This could be part of the answer why we have sex so often (i.e. more often than models would predict)!

PS: This is another example of the importance of tradition.

(Cross-posted at Rishon Rishon.)

9 Comments

  1. In various wild species of Drosophila (fruit flies), there are a substantial number of stable inversion polymorphisms. Most of these are paracentric (not containing the centromere) inversions. Because of some peculiarities in Drosophila biology, crossovers within the inversion region do not make it into mature eggs and are not produced in males. Thus, there is no reproductive cost of being heterozygous for the inversions. 
     
    A consequence of all of this is that alleles of genes within these inversions behave as if they are completely tied to each other. These inversions easily encompass 300 or more genes. 
     
    That the inversions are stable in the population would seem to indicate either heterozygous advantage or perhaps different environments in which each prospers. 
     
    This is an extreme version of the situation David posits.

  2. one thing, i believe lit. surveys show that non-sexish behavior crops up in ‘complex’ organisms now and then, but almost all these tend to be young lineages (bdelloid rotifers are the major exception). the implication is that they don’t last over the long haul.

  3. The beauty of chromosomes is that they enable one organism to have a whole spectrum of gene-gene relationships, from totally sexual to (almost) totally asexual. And this can change over long time periods.

  4. I suspect that models assume that we have sex terribly infrequently, given how much sexual energy many of us put into thinking about their sex lives instead.

  5. “It’s certainly possible to have a lot more chromosomes than we have.” 
     
    What does this mean ? How could we have more chromosomes than we have ? Or is this just a little word salad ?

  6. Hello. I just found your site and I am curious about the research side of this science… what kind of things/variables that could occur in an animal Facility/research facility might affect ‘gene expression’ in the research animal that could cause an affect in the end result? This is a question I pose to those of you who are researchers. Does temperature and humidity variations and/or long term/short term exposure to cleaning chemicals cause an adverse effect?  
     
    Has anyone looked into this? 
     
    Can anyone direct me to where I might find the answers? 
     
    Reason for asking… Background: I am just barely allergic to wheat and chicken and do not appear to be reactive. I started having seizures at work for no apparent reason. They sent me to a specialist where I was placed in a study where I had electrodes placed on my head for 48hrs. Even though I told them not to give me anything to eat that was either chicken and/or wheat… they didn’t listen. I was hungry so I ate the meal anyway. I normally do not react outwardly. I was overblown when I realized that I reacting at a cellular level… meaning that my brain waves were kicking only when I was consuming the wheat and/or chicken and not the other foods at that meal. And yet, I was not having a noticable reaction… it made me wonder.  
     
    It made me wonder what else can affect these research animals that yoou use to pull the gene expressions from. 
     
    Dee

  7. David, I think you have a serious misconception about this. I posted a long comment about it on the copy of this post you put on your own site. 
     
    The bottom line: genes on the same chromosome which are not physically near one another (say, at opposite ends) have no more than a 50:50 chance of being inherited together.

  8. Steven, Thanks for pointing that out. Although it would mean that the specific number of chromosomes not very important (though if 50:50 is correct it would still have a noticeable statistical impact), it doesn’t invalidate anything else I said. Specifically, that arranging genes in chromosomes results in a spectrum of gene-gene relationships from totally sexual to almost totally asexual (for really close genes). And it doesn’t invalidate any of my predictions (some of which have been confirmed by commenters).

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