CAN THE FIRE-BELLIED TOADS BOMBINA BOMBINA AND BOMBINA VARIEGATA MAINTAIN THEIR DISTINCTNESS IN THE FACE OF HYBRIDIZATION?

ABSTRACT

Bombino bombirla and Bombina variegata meet in a narrow hybrid zone which runs for several thousand km across Central Europe at a transition between mountains and lowlands. Allozymes and morphology (as well as mtDNA and mating calls studied only near Cracow) change in parallel over 5–10 km. The central step, in which associations among parental alleles of unlinked allozyme loci are highest, is flanked by long tails of introgressing alleles. Selection against hybrids acts on correlated blocks of genes at the center and maintains steep clines. Two transects in southern Poland show striking similarities and provide a rare opportunity to quantify predictions of the strength of selection against hybrids, the number of genes under selection, and the extent of gene flow between the two species. Lowered fitness of hybrids and contrasting habitats at either side of the zone which favor alternative sets of adaptations confine introgression to a narrow zone.