Body size variation in hybrids among populations of European water frogs (Pelophylax esculentus complex) with different breeding systems

In some populations, hybrids reproduce with a parental species by eliminating the genome of this species from their own germline and produce gametes that only contain the genome of the other parental species (sexual host). This mode of reproduction, known as hybridogenesis, leads to a conflict of interest between the two parties because the sexual host should avoid mating with the hybrid to prevent a reduction in reproductive success, whereas the hybrid depends on such matings for survival. We investigated European water frogs (Pelophylax esculentus complex), including hybrids (P. esculentus, genotype LR) and two sexual host species (P. lessonae, LL and P. ridibundus, RR). We hypothesized that to maximize fitness, hybrid males should be morphologically more similar to the sexual host that is preferred by females for successful reproduction. To test this hypothesis, we compared hybrid males in two different population types, L-E (hybrids coexist with LL) and L-E-R (hybrids coexist with both LL and RR). The latter was described in terms of genome composition, sex ratio, and mate choice preferences; the sex ratio of hybrids was significantly male-biased. We found that LR males from the L-E-R populations were significantly larger than those from the L-E, which makes them more similar to P. ridibundus, the largest species within the P. esculentus complex. We suggest that a larger body size of hybrid males may provide a reproductive advantage in the L-E-R population type, where the most common type of pair caught in the breeding season was LR males × RR females.