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.
Jacek M. Szymura
NEI'S genetic distance estimate (D) between Polish Bombina bombina and B. variegata based on electrophoretic comparisions of 29 proteins encoded at 39 presumptive gene loci, is 0.487±0.127. This estimate suggests Pliocene separation of the species, in agreement with the fossil record. Remnants of B. bombina-like and B. variegata - like animals from Upper Pliocene have been found in Czechoslovakia, and B. bombina- like animals from the same period have also been found in Poland. Separation of European Bombina was probably connected with an ecological shift of ancestors of B. bombina, the most derived species of the genus, to lowland environment. The Pliocene divergence proposed here is considerably earlier than that postulated by MERTENS (1928) who linked Bombina speciation events to Pleistocene glaciations. Comparisons ofgenetic divergence measures and the fossil record for three other species pairs of European amphibians (Triturus cristatus/marmoratus, Bufo calamita/viridis and Rana lessonae/ridibunda) provide additional evidence that pre-Pleistocene speciations in the extant European amphibian fauna were more important than is traditionally believed.
Linda R. Maxson and Jacek M. Szymura
Relationships among frogs of the genera Alytes, Bombina, Discoglossus, and Baleaphryne were studied using the quantitative immunological micro-complement fixation technique. Albumins were compared among these genera as well as among several species of each genus. Our results indicate that the closest genetic relative of Baleaphryne muletensis is Alytes obstetricians. Moreover, the albumins of Baleaphryne and A. obstetricians are as similar or more similar to each other than the albumins of all four species of Bom bina, both species of Discoglossus, or the two species of Alytes available for study are to each other. We conclude that Baleaphryne is congeneric with Alytes. The distances measured among Alytes, Bombina, and Discoglossus suggest a Cretaceous (85-92 MYBP) divergence of Alytes from the lineage that later gave rise to Bombina and Discoglossus. These latter two genera appear to have diverged in the late Cretaceous, roughly 69-75 MYBP. Extant species of Bombina began diverging in the mid- to late-Miocene, as did Alytes obstetricans and A. cistemasii.
Discoglossus pictus and D. sardus in Europe last shared a common gene pool in the Pliocene. Albumins of individuals of D. pictus from Spain compared with northwestern African D. pictus suggest that these populations have been separated for some 9-10 million years, and are probably not conspecific. Our findings do not support Lanza et al.’s (1975, 1976) suggestions that Bombina and Alytes belong in the separate family Bombinidae.