One of the most talked about ideas in genetic studies of asexual reproduction has been that of extreme Allelic Sequence Divergence (ASD), often called the “Meselson effect” after Matthew Meselson who is usually credited with this idea. In obligatory asexuals (apomicts), which never have the opportunity to recombine during meiosis, the once homologous chromosome pairs are now independent of each other. Their gene copies (previously normal alleles) can accumulate independent mutations, be subject to different types and intensities of selection, and diverge in structure and function without recombination bringing them back together again. Since homologous chromosomes contain the maternal and paternal alleles in meiotic (sexual) species these will in effect become independent genetic loci when meiosis is abandoned, much like gene duplicates elsewhere in the genome. Although they will start out as very similar, once meiosis stops they will start to diverge along independent trajectories and the prediction is that in ancient asexuals they will become very divergent indeed. There are some unusual phylogenetic relationships between apomict “alleles” too.Just like with gene duplicates there will be three broad classes of outcome.
- One copy may accumulate debilitating mutations and become a pseudogene
- Purifying selection may prevent the accumulation of many non-synonymous substitutions and the copies may continue to fulfill the same functional role.
- They may diverge in function and become members of a gene family.
An interesting question is whether these apomicts can suffer from haploinsufficiency effects. Polyploid apomicts are more common, probably due to hybridization being a common way to originate apomixis, but do diploids apomicts suffer when option 1 or 3 (above) occur? Is 2 just a method of maintaining the di-allelic buffer that normal sexual diploids have?
I was reading MJD White (1945) “Animal Cytology and Evolution” last year and was struck by the similarity there to current ideas of extreme ASD.
“If we suppose an ameiotic form evolving for a very long period of time we might imagine its two chromosome sets becoming completely unlike, so that it could no longer be considered as a diploid either in a genetical or cytological sense.” (p283)
The divergence of chromosomes as described by White 65 years ago is, just like gene sequence divergence, continuous. I like this quote as a different perspective on modern ideas of extreme ASD in ameiotic species (Meselson effect). Did these ideas originate with White? Some of them it seems, but as always, its probably more complex than that. I would like to see a real review of the history of studies of asexual reproduction. I sometimes joke that if there is any doubt evolutionary ideas should just be assumed to originate with Haldane until there is evidence to the contrary.
The figure is taken from: Lunt, D.H. (2008) BMC Evolutionary Biology 2008, 8:194