Hybridization and barriers to introgression between full-fledged species. The case of the butterflies Parnassius apollo and P. phoebus in the Alps.

Two butterfly species living in the Alps, Parnassius apollo and P.phoebus, frequently hybridize in certain localities of this region. The features of this phenomenon have been previously studied by biometry and starch gel ele ctrophoresis, but some points remained obscure. We present them in a study combining results from cellulose acetate electrophoresis and wing pattern b iometry with a determination of the mitochondrial haplotype by a PCR-RFLP a nalysis in a sample of butterflies from the southern French Alps. It was al ready known that the male hybrids are fecund and thus that interspecific ge ne exchange could take place via backcrosses with the parent species. In th e present case, combining the identification of mtDNA with the analysis of nuclear genotypes allows us to demonstrate that hybridization can involve b oth sexes of both species, Moreover, it suggests that at least some female hybrids are not sterile. The impact of Haldane's rule is therefore not very strong in the present case. However, although the prerequisites for introg ression between the concerned species are fulfilled, at the level of both n uclear and mitochondrial genomes, no indication of such a phenomenon could be gathered in the studied sample. (C) 2000 Academie des sciences/ Editions scientifiques er medicales Elsevier SAS.