Negative environmental factors and biodiversity: the case of the hybridogenetic green frog system from Lake Trasimeno

South of the Po Valley in Italy, green frogs constitute a hybridogenetic sy stem, represented by one parental species (Rana lessonae) and a hybrid form (A. esculenta). Lacking one parent (R. ridibunda), the hybrid is a reprodu ctive parasite on the coexisting parent: progeny from matings between hybri ds are inviable; the hybrid reproduces hybridogenetically by first eliminat ing the R. lessonae genome from the germ-line cells prior to meiosis, redup licating the R. ridibunda sec, and then transmitting the R. ridibunda genom e hemiclonally to gametes. Hybridity is restored in each generation, since R. esculenta gametes are fertilised by gametes from R. lessonae. In order t o ascertain whether there are any differences in susceptibility to environm ental disturbances between the parental species and the hybrid form, the gr een frogs living in Lake Trasimeno (central Italy) were analysed. In the ab sence of reliable morphological systematic characteristics, hybrids and non hybrids were distinguished by means of a species-specific centromeric satel lite DNA, and the percentages of R. lessonae and R. esculenta specimens at each sampling station were calculated. The results indicate that the parent al host is more susceptible to negative environmental factors, which could threaten the whole local hybridogenetic system.