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 system, represented by one parental species (Rana lessonae) and a hybrid form (A. esculenta). Lacking one parent (R. ridibunda), the hybrid is a reproductive parasite on the coexisting parent: progeny from matings between hybrids are inviable; the hybrid reproduces hybridogenetically by first eliminating the R. lessonae genome from the germ-line cells prior to meiosis, reduplicating the R. ridibunda sec, and then transmitting the R. ridibunda genome hemiclonally to gametes. Hybridity is restored in each generation, since R. esculenta gametes are fertilised by gametes from R. lessonae. In order to ascertain whether there are any differences in susceptibility to environmental disturbances between the parental species and the hybrid form, the green frogs living in Lake Trasimeno (central Italy) were analysed. In the absence of reliable morphological systematic characteristics, hybrids and nonhybrids were distinguished by means of a species-specific centromeric satellite DNA, and the percentages of R. lessonae and R. esculenta specimens at each sampling station were calculated. The results indicate that the parental host is more susceptible to negative environmental factors, which could threaten the whole local hybridogenetic system.