Mate choice and fitness in a hybrid frog: Rana esculenta females prefer Rana lessonae males over their own

Summary The evolution and maintenance of female choice based on purely genetic differences is still a controversial issue, not only for theoretical reasons, but also because of the practical difficulty of demonstrating the fitness consequences of preferences and heritability of and genetic variability in the chosen traits. We argue that hybrid systems (broadly defined) offer suitable models for studying mate choice according to genetic differences. We present such a study for European water-frogs of the hybridogenetic Rana lessonae/Rana esculenta complex (L/E complex). R. esculenta, originally a hybrid between R. lessonae and R. ridibunda, eliminates the L genome premeiotically and only produces eggs and sperm containing only the R. ridibunda (R) genome. Consequently, the hybrid will only persist when it lives and mates with R. lessonae in mixed populations where it can regain the lost L genome. In such mixed populations, there is strong selection against E x E matings. because these will produce no viable offspring. We tested whether females of the hybrid R. esculenta do indeed avoid their own R. exculenta males and choose males of the parental species R. lessonae instead. Eleven E females were offered a simultaenous choice between one L and one E male. Females exhibited a significant preference for L males that was determined by the type of male, rather than by its size or activity. This choice is in the direction predicted from genetics. The question of why L males agree to mate with E females, but L females only rarely mate with E males, is answered by a sexual asymmetry in the cost/benefit ratios of mating with the wrong type and the right size. Our results are consistent with the mating pattern found in natural populations, but further studies are needed to show that female choice really causes this pattern.Correspondence to: H.-U. Reyer