Sex is a deeply puzzling thing for evolutionary biologists. The problem is that biparental sex requires and produces males, who do not partake in the costs of childbearing, but do munch at the limited food supply of reproductive females. Indeed, if males were out of the picture, a population of purely asexual females would increase in number at twice the rate of its male-burdened counterparts. Why then do most animals and plants go to such a bother? Why suffer males?
One of the oldest answers to this question is that sex accelerates population adaptation in the face of change. Building on this idea, the Red Queen hypothesis proposes that sex allows plants and animals to stay one step ahead in their endless arms race against co-evolving parasites. New research by Levi Morran and colleagues at the University of Indiana in the USA provides the strongest experimental support yet for this idea, showing that parasites can tip the sexual balance, making males worth their weight.
Using experimental evolution, Morran and colleagues pitted the nematode worm C. elegans against its lethal natural parasite, the bacterium Serratia marcescens. Most C. elegans are hermaphrodites that reproduce by self-fertilization; however, about 20% of the time hermaphrodite worms reproduce by mating with rare males. When the researchers forced wild-type worms to co-evolve with their parasites for 30 worm generations the rate of biparental breeding in the population increased to nearly 90%. By contrast, when parasites infected mutant worms that were unable to mate with males, they drove the worms extinct within 10 generations. Thus sex, and lots of it, kept worms alive, while its absence led to their doom.
Now what about the parasites? Because bacteria and worms can be frozen and later reanimated, the team revived the ancestral parasite and host and compared them with their evolved descendants. Infecting the ancestral worms with the evolved parasites, Morran and his colleagues saw dramatic results; the evolved bacteria killed worms 2- to 3-fold more effectively than their ancestors. However, when the evolved bacteria infected co-evolved worms, they were no more effective at killing them than the ancestral parasites were at killing the original worms. That is, while parasites evolved to become nastier, the worms simultaneously evolved to resist them, both staying essentially where they were to begin with. This outcome is exactly what the Red Queen hypothesis predicts; both parasite and host have to keep running to stay in exactly the same place.
Lethal parasites exert very strong natural selection on their hosts, because hosts must either resist infection or die. This study shows that sex, or more specifically the presence of males, helps hosts run at a slightly faster speed than their co-evolving parasites. This advantage just about makes the cost of males bearable.