Thursday, November 10, 2005

(a)Sex scandal   posted by Razib @ 11/10/2005 04:19:00 PM

Bdelloid rotifers have been called an asexual scandal because they are long-lived (30-40 million years) lineage which does not make recourse to sex. The fact is that asexuality and clonal lineages litter the tree of life, but they are invariably lineages with little time depth. In other words, asexuality is often a recently derived character (from an ancestrally sexual state) across a taxon where there are a mix of sexual and asexual lineages. One could hypothesize that asexuality is a recent fad. This seems implausible. Rather, the conventional explanation is that asexuality in complex organisms results in a tradeoff, in exchange for a short term benefit there is a long term increase in the risk of lineage extinction.

Imagine a group of organisms which exhibit a variation in fitness. Even if a trait is 100% heritable, and perfect assortative mating results in trait matching, there is a strong possibility the favorable genetic combinations of the parents will be destroyed by Mendelian segregation (as alternative perfect combinations are broken up and scrambled together in the next generation in a more disharmonious fashion). On a population genetic level alleles which are coinherited together because of physical linkage on a particular chromosome also tend to be dispersed (toward linkage equilibrium) by recombination. Now imagine that one of these hyperfit individuals mutates and develops the ability to reproduce asexually (one assumes this would be a "female"). Voila, no more dilution of its advantageous genotype. Assume that the mutation that flipped the organism to asexuality was on one particular locus, call it Sex-. Because Sex- has packaged itself into a nice suite of genes and traits, it will start to outcompete Sex+ (i.e., the form of the gene which confers sexuality). Additionally, all the genes within the asexual individual are now being passed at a 100% rate to the next generation, instead of a 50% rate. Remember, your children are 1/2 of you genetically, clones are 100% of you. A mutation that confers clonality should spread like wildfire.

All is well. But the problem is that fitness is quite often not some intrinsic tendency, in other words, it is not necessarily an essential characteristic. A mutation that screams "abort me" and autodestructs is obviously intrinsically unfit, but we are talking about more subtle differences. Fitness is often a relative state, that is, the difference between an individual organism (or locus) and the mean population. It can vary a great deal as a function of time, environmental context and the dynamical state of the population within which the individual is embedded (both intra and interspecies). The problem with the Sex- locus is that it has fixed itself into a particular stable strategy that it won't be able to snap out of, barring mutation. In the short run there might be riches, but in the long run it will likely be overtaken by the changes in the world around it. And I haven't even gotten into explicitly genetic issues like Muller's Ratchet.