Hybrid dysfunction in fire-bellied toads (Bombina)

Reproductive isolation between two taxa may be due to endogenous selection, which is generated by incompatibilities between the respective genomes, to exogenous selection, which is generated by differential adaptations to alt ernative environments, or to both. The continuing debate over the relative importance of either mode of selection has highlighted the need for unambig uous data on the fitness of hybrid genotypes. The hybrid zone between the f ire-bellied toad (Bombina bombina) and the yellow-bellied toad (B. variegat a) in central Europe involves adaptation to different environments, but evi dence of hybrid dysfunction is equivocal. In this study, we followed the de velopment under laboratory conditions of naturally laid eggs collected from a transect across the Bombina hybrid zone in Croatia. Fitness was signific antly reduced in hybrid populations: Egg batches from the center of the hyb rid zone showed significantly higher embryonic and larval mortality and hig her frequencies of morphological abnormalities relative to either parental type. Overall mortality from day of egg collection to three weeks after hat ching reached 20% in central hybrid populations, compared to 2% in pure pop ulations. There was no significant difference in fitness between two parent al types. Within hybrid populations, there was considerable variation in fi tness, with some genotypes showing no evidence of reduced viability. We dis cuss the implications of these findings for our understanding of barriers t o gene flow between species.