Who needs sex when you have gene conversion?

By Razib Khan | July 22, 2013 4:49 am

Credit: Gross, Liza. “Who Needs Sex (or Males) Anyway?.” PLoS Biology 5.4 (2007): e99.

Bdelloid rotifiers get a fair amount of attention because they seem to be a lineage of obligate asexual metazoans (Richard Dawkins discusses them extensively in The Ancestor’s Tale). The fact that they don’t have sex isn’t that big of a deal. Bacteria do not have sex, and they’re quite successful. Rather, the issue is that they don’t have sex, they are complex, and, they are successful as a lineage. These do not usually go together. One of the posited explanations is that complex organisms are subject to phenomena such as Muller’s Ratchet, where they begin accumulating a load of deleterious alleles. Sex, with genetic recombination, is a way to evade this process, by mixing and matching alleles. By producing offspring with more than the expected payload of deleterious alleles the lineage can slough off unfavorable mutations which might otherwise fix. Without the ability to offload bad mutations over time they build up, and eventually one presumes the lineage would be unviable. This is just one of the myriad reasons biologists give for the long term lack of success of parthenogenetic metazoan lineages. Sex is ubiquitous among metazoans despite its two-fold cost. That is an overwhelming fact against which stands the example of the bdelloids.

A new letter in Nature proposes to answer the enigma of bdelloid rotifers by an analysis of their genomes. Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. From what I can gather the authors found that the genome of a bdelloid rotifer (or this species) is a total mess. The chaotic arrangement of individual copies of genes is such that proper pairing which is necessary for meoisis, the conventional precursor to sexual reproduction, can not occur. By this, I mean that there seems to be a structural barrier to sex. This goes to show that evolutionary process is not just a chemistry blind fitness optimization process. The very structure of the substrate of genetic code seems to mitigate against the reemergence of sexuality, closing off possible paths toward adaptation and fitness optimization. The adaptive landscape is most certainly not flat.

So how do these rotifers remain then, knowing what we know about asexual multicellular lineages (that they are not long for this world)? The two primary answers put forth in this paper seem to be gene conversion and horizontal gene transfer (e.g., from bacteria via viruses). In the gene conversion process presumably deleterious alleles are simply reverted back to the wild state. Meanwhile, horizontal gene transfer (e.g. from bacteria) likely results in a persistent infusion of new variation into the species, which is otherwise a proliferation of clonal lineages.

The two above phenomena seem to be almost banal and obvious ways in which rotifers might avoid the pitfalls of their structurally enforced asexuality. This prompts me to ask why more organisms don’t exhibit this profile? Many lineages have parthenogenetic branches. But few of them become as wildly successful as bdelloid rotifers. It would be curious if other sexual metazoans were to be found with the genomic features of bdelloids. Perhaps there are lineages which manage to enter into meiosis despite the hurdles which bdelloids have circumvented. Of course for that we’ll need many more “genome papers.”

CATEGORIZED UNDER: Evolution, Evolutionary Genetics
MORE ABOUT: Evolution

Comments (3)

  1. razibkhan

    probably only feasible at a few loci. though might be only immune related ones you need. probably more realistic to just cross isolated populations in the short term.

  2. DK

    Bacteria do not have sex, and they’re quite successful

    Lots of bacteria do of course have sex. It’s called conjugation and a whole complex apparatus was evolved for it. Conversely, lots of fungi lost sexual stage and are quite successful without it.

    • zmil

      Conjugation is very, very different from what we typically think of as sex, and is not an essential part of any bacterial species’ life style, that I am aware of. In my opinion, it’s better to think of conjugation as a method of replication of the plasmid or other mobile element that generally codes for the conjugation machinery. Transfer of genes other than the conjugation element is basically a side effect, and a fairly uncommon one at that.

      OTOH, the problem that sex is supposed to solve, at least according to the argument Razib mentions here, is ameliorated in bacteria by conjugation, natural competence (uptake of extracellular DNA), transducing phages, and other forms of horizontal gene transfer.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com


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