May I borrow your genome?

By any standard, the sex life of the edible frog, Rana esculenta, is a strange affair. Common in Europe, the frog is a hybrid of R. lessonae and R. ridibunda. Put a male and female R. esculenta together, though, and their offspring rarely survive. So how does an edible frog avoid becoming the end of the genetic line? It broadens its horizons.

Before meiosis (the process of cell division that leads to the formation of sperm or eggs), R. esculenta dumps one parental half of its genome--usually the R. lessonae half--then goes in search of a mate from the species whose genome is now missing from its germ cells. So, in effect, R. esculenta is re-created from scratch with every new generation. Without enough R. lessonae to mate with, however, edible frogs couldn't reproduce. There's also the danger that R. esculenta numbers could explode, increasing the ratio of R. esculenta to R. lessonae. Shouldn't this system collapse? Heinz-Ulrich Reyer, a zoologist at the University of Zurich in Switzerland, and his colleagues have discovered why it doesn't.

The team compared R. esculenta and R. lessonae females throughout the year, noting the number of eggs in their ovaries, the concentration of sex hormones in their blood, their reproductive activities, and other relevant attributes. Their finding: the key to the stability of the system is that R. lessonae females can breed almost twice as often as their hybrid counterparts. ("Low proportions of reproducing hemiclonal females increase the stability of a sexual parasite-host system (Rana esculenta, R. lessonae)," Journal of Animal Ecology 73: 1089-1101, 2004)

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