Systematics and Hybridization in European Green Frogs of Rana esculenta Complex

Abstract

European green frogs include the phenotypically distinct, largely sympatric forms Rana lessonae Camerano 1882, R. esculenta Linnaeus 1758, and R. ridibunda Pallas 1771, the relationships of which is still unsufficiently known. Recent investigations on Polish green frog populations indicate that R. lessonae and R. ridibunda are taxa at species rank, whereas R. esculenta is a hybrid resulting from interspecific crosses between R. lessonae and R. ridibunda. All three forms of green frogs are easily crossable indicating close relationship, and lack of genetic isolation. Ecological isolating mechanisms were probably established during the Pleistocene. In spite of very frequent natural hybridization, very high number of F1 hybrids (= esculanta) in nature, and successful backcrosses especially with lessonae, introgression between lessonae and ridibunda seems not to occur owing to an incompletely understood hereditary mechanism.