Sex is a real drag. It costs a huge amount of energy, reduces the efficiency of gene transfer and spreads disease. Theoretically, it would be a lot more efficient just to reproduce asexually, but less than 1% of animal species make do without sex. The ‘Red Queen’ (‘It takes all the running you can do, just to stay in the same place!’ Lewis Carroll, Through the Looking Glass) hypothesis posits that organisms must constantly evolve just to maintain their fitness relative to other co-evolving systems. In this hypothesis, the main reason for the evolutionary success of sex is that it helps create genetic variability. This helps animals quickly develop adaptations against relentlessly co-evolving parasites and pathogens. Bdelloid rotifers (a class of small freshwater invertebrates) are the big exception to this rule. They have reproduced asexually for tens of millions of years and diversified into over 450 species. Bdelloid rotifers seem to be impervious to the Red Queen effect as asexuals and manage to stay ahead in the evolutionary arms race, but how do they cope with co-evolved enemies? Chris Wilson and Paul Sherman at Cornell University recently tested the idea that rotifers ditch co-adapted parasites simply by drying up and dispersing on the wind, and published their results in a recent edition of Science.
Wilson and Sherman started by measuring how long a population of Bdelloid rotifer clones (Habrotrocha elusa) can survive when infected by a fungal parasite (Rotiferophthora angustispora). When infected, the rotifers were wiped out after 13 days in water. Next, the team infected hydrated rotifer populations with fungi, dried the cultures out for several weeks and looked at how the rotifer and fungal populations recovered when re-hydrated. The fungi were able to recover and destroy rotifer populations when dried out for up to 3 weeks. However, after dry spells longer than 3 weeks, the rotifers recovered but the fungi were almost non-existent. These results suggest that rotifers can evade parasites by simply outlasting them in dry conditions.
Knowing that rotifers are small enough to be carried as spores (tuns) by wind when dried out, the researchers wanted to find out whether rotifers could use this ability to evade fungal parasites in space. To test this, they grew cultures of rotifers and fungi, dried them out for a week and then put the dried out cultures in a wind tunnel. Collecting the rotifer tuns downwind, the team then rehydrated them. Over half of the rotifer populations tested were uninfected and started new fungi-free populations. When cultures containing rotifers and fungi were dried out for a week and rehydrated, the fungi quickly regained the upper hand in all cultures, so wind dispersal clearly increased the chances of rotifers shedding their fungal freeriders.
Wilson and Sherman provide strong evidence that spatiotemporal escape can help protect asexual animals from co-evolved enemies. The experiments are exceedingly simple, but very elegant; they will almost certainly evoke a ‘Why didn't I think of doing that?’ response from a large host of biologists. Most importantly, the work of Wilson and Sherman cuts right through the heart of a long-standing knot in evolutionary biology. The authors have discovered one way asexual rotifers can outrun the Red Queen.
