While most of Western society became increasingly comfortable with sex during the 1970s, evolutionary biologists started to find the subject very embarrassing indeed. In two seminal books, George C. Williams and John Maynard Smith pointed out that the prevalence of sexual reproduction in nature poses a major problem for evolutionary theory. The argument rests on the deceptively simple observation that males cannot produce eggs. In theory, a population could grow twice as rapidly if investment in male offspring were eliminated and those resources were used instead to produce clonal females, each capable of independent reproduction via unfertilized eggs. Such obligately asexual lineages, which arise relatively often, ought to invade and rapidly replace populations that are still burdened by the cost of making males. However, in stubborn defiance of this logic, almost all animals and plants retain sex and males in some form, and obligately asexual lineages are typically short-lived in evolutionary time. Evidently, sexual reproduction has selective benefits that not only outweigh its immediate costs, but are so fundamental as to make it practically indispensable in the long term. The precise nature of these benefits remains unresolved despite forty years of theoretical and empirical inquiry, and is a perennial target of inquiry for curious evolutionary biologists.
I study a group of animals that provides an unusual perspective on sex. Bdelloid rotifers are microscopic invertebrates, less than half a millimetre long, which can be found by the thousands in almost every freshwater habitat in the world. Remarkably, every individual examined is female; bdelloids seem to have abandoned males and sexual reproduction tens of millions of years ago, and now reproduce exclusively via unfertilized eggs that hatch into clones of their mother. In contrast to the prevailing pattern, these obligate asexuals have not gone extinct; indeed, they have diversified into four families, over a dozen genera and many hundreds of species. Apparently, some peculiarity of bdelloids has allowed them to avoid or greatly postpone the typical fate of asexual lineages, whatever that may be. Indeed, the exceptional success of the bdelloids may furnish important clues about the maintenance of sex more generally: a hypothesis would be especially satisfying if it not only predicted a very general benefit to sex, but specifically accommodated the ancient asexuality of this group as a special case.
My present work focuses on an idea known as the Red Queen hypothesis, which suggests that sex functions to break down genotypes that have become vulnerable to parasites and pathogens. These ubiquitous natural enemies adapt relentlessly and rapidly to better infect their hosts. They are therefore expected to cause considerable difficulties for asexual lineages, which are slow to generate genetic novelty to respond to changing selective pressures. In principle, infections could suppress clonal populations so strongly that sex has a net selective advantage despite its considerable costs. If this hypothesis is correct, it would imply that the celibate bdelloid rotifers have some unusual alternative means of dealing with coevolving parasites. To test this prediction, I am investigating their ecological and genetic interactions with lethal endoparasitic fungi.
Why asexual organisms are on their last legs, The New York Times, January 2010
How to survive without sex for 50 million years, National Geographic, February 2010
(Deutsch): Selbstverteidigungstrick: Rädertierchen flüchten per Salto, Der Spiegel, January 2010
(Português): Animal revela como sobrevive sem sexo, Folha de Sao Paulo, February 2010
(Español): Un escándalo evolutivo, BioUnalm, January 2010
(Hebrew): מי צריך סקס כשאפשר להתייבש?
(video): The mystery of the celibate rotifer, Science Friday, January 2010
(Video courtesy of National Public Radio’s Science Friday [full story])
Wilson, C. G. & Sherman, P. W. (2013) Spatial and temporal escape from fungal parasites in natural communities of anciently asexual bdelloid rotifers. Proceedings of the Royal Society B 280: 20131255. [link to paper]
Wilson, C. G. (2011). Desiccation-tolerance in bdelloid rotifers facilitates spatiotemporal escape from multiple species of parasitic fungi. Biological Journal of the Linnean Society 104: 564-574. [link to paper].
Wilson, C. G. & Sherman, P.W. (2010). Anciently asexual bdelloid rotifers escape lethal fungal parasites by drying up and blowing away. Science 327: 574-576. [link to paper]
Wilson, C. G. (2008). Male genital mutilation: an adaptation to sexual conflict. Evolution and Human Behavior 29: 149-164. [link to paper]
– “Cutting The Competition“, The Economist, June 2008