Losing sex

A forthcoming article in Trends in Ecology & Evolution (a.k.a. TREE) looks at the problem of detecting whether an organism reproduces sexually or asexually. This might seem to be a trivial problem, but it isn’t. For one, many organisms are poorly studied. And we are starting to discover that some organisms can switch between sexual and asexual reproduction.

Komodo dragons provide a good example of the latter. Because they are “charismatic megafauna” (they’re the biggest lizards in the world, and we love biggest and smallest things), there’s no shortage of people looking at them and maintaining them in zoos. But it was only a few years ago that it was found that females could also reproduce asexually (Watts et al. 2006).

In the TREE article, Schurko and collegues list four ways that an asexually reproducing species, like Marmorkrebs, can originate.

• Hybridization: Many asexual species appear to be the result of interbreeding between two different sexual species. Just by virtue of how many parthenogens are thought to be hybrids, this might be the number one contender for the origin of Marmorkrebs.


• Spontaneously: You get an unlucky (or lucky, depending on your point of view) genetic change, either through a mutation or anomalous gamete formation.


• Infectious: Some bacteria, most famously Wolbachia, can influence the reproduction of hosts they infect, and can dramatically skew sex ratios.


• Contagious: I’m not 100% clear how this one works, but involves the spread of “asexual genes.” The reference given is a crustacean case, Daphnia pulex (Innes & Hebert 1988).

It’s a completely open question at this point which of these four possibilities led to the formation of Marmorkrebs. But knowing the range of possibilities is important in figuring out ways of testing those hypotheses.

References

Innes DJ, Hebert PDN. 1988. The origin and genetic basis of obligate parthenogenesis in Daphnia pulex. Evolution 42(5): 1024-1035

Schurko AM, Neiman M, Logsdon JM Jr. 2009. Signs of sex: what we know and how we know it. Trends in Ecology & Evolution: in press. http://dx.doi.org/10.1016/j.tree.2008.11.010

Watts PC, Buley KR, Sanderson S, Boardman W, Ciofi C, Gibson R. 2006. Parthenogenesis in Komodo dragons. Nature 444(7122): 1021-1022. http://dx.doi.org/10.1038/4441021a