The Effect of Assortative Mating on the Coexistence of a Hybridogenetic Waterfrog and Its Sexual Host.

In central Europe, the hybridogenetic waterfrog Rana esculenta, a hybrid between Rana ridibunda and Rana lessonae, lives in sympatry with one of its parental species, the poolfrog Rana lessonae. As R. esculenta has to backcross constantly with R. lessonae in order to produce viable offspring, this coexistence is obligatory for R. esculenta. Since R. esculenta has a higher primary fitness than R. lessonae, a mechanism is required that prevents the hybrid from driving the parental species, and hence itself, to extinction. Here, we present an analytical model and a computer simulation that investigate whether assortative mating can operate as a such a control mechanism. Our results show that assortative mating is very effective in regulating coexistence in such a hybrid-host system. This is particularly true when choice is affected by the proportion of the two male types in the population. Furthermore, we could show that even if the species composition in a mixed hybrid-host population may be largely influenced by differences in life-history parameters, assortative mating still plays a very important role by stabilizing coexistence. Thus, mating behavior turns out to be more important for the populations dynamics of hybridogenetic waterfrog systems than previously assumed.