HYBRID ZONES BETWEEN 2 GENETICALLY DIFFERENTIATED FORMS OF THE POND FROG RANA-LESSONAE IN SOUTHERN ITALY

Two distinct population groups of the pond frog liana lessonae were de tected in peninsular Italy and Sicily by multilocus electrophoresis: o ne group inhabits the peninsula down to northern Calabria, the second occurs in southern Calabria and on Sicily. Fixed alternative alleles d istinguish the two groups at 5 of the 25 loci examined; marked allele frequency differences were observed at two additional loci. On average , the two groups differ by a Nei's standard genetic distance of 0.4. A wide hybrid zone (about 120 km) occurs between the two groups, with h igh genotypic diversity and absence of pure parental genotypes in cent ral Calabria. Patterns of allozyme variation suggest that at least two distinct contact and hybridization events occurred, one in the Catanz aro, the other in the Crati-Sibari plains, about 70 km to the north. G eological evidence indicates that these areas correspond to two main m arine-flooded grabens that would have repeatedly interrupted or reduce d genetic exchange during Plio-Pleistocene times. The finding of a fix ed difference at the Mdhp-1 locus between Sicilian and Calabrian R. le ssonae witnesses their continuing differentiation following their last separation by definitive opening of the Strait of Messina, about 50 0 00 years ago. The wide hybrid zone, the diversity of genotypes and the agreement with Hardy-Weinberg expectations suggest complete hybrid fe rtility. Different patterns of introgression were observed at the vari ous loci. The pattern of allelic variation at loci in R. lessonae is p aralleled by the pattern of variation in lessonae genomes of the sympa tric hemiclonal hybrid Rana esculenta, into which lessonae genomes are introduced each generation as a result of hybridogenesis.