Distribution, Population Systems, and Peculiarities of Hybrid Gametogenesis in Water Frogs (Pelophylax esculentus complex) in the Dniester River Valley (Moldova)

Hybridogenesis is well-known attributes of water frogs of the genus Pelophylax. The P. esculentus complex consists of two parental species, P. lessonae and P. ridibundus, as well as their hemiclonal (sometimes meroclonal) hybrid, P. esculentus. DNA flow cytometry, genetic and morphometric analyses were used to examine water frog compositions in 27 localities (n = 212) throughout the Dniester River valley in Moldova. Two species were revealed in the studied region (42% individuals were P. esculentus and 58% P. ridibundus). All frogs proved to be diploid. We registered the first species in 74% localities and the second in 78%. Populations, where we found only P. ridibundus occurred in 26% localities and only P. esculentus in 22%, while mixed population systems in 52%. Both species were usually observed in open water bodies and rivers that flow down through agricultural land and urban areas. In populations of P. esculentus the number of males strongly exceeded (90%) the number of females. The half of studied hybrid males was sterile, and most of fertile hybrid males produced sperm with the genome of P. lessonae. Several hybrid males gave the mixture of sperm with genomes of P. lessonae and P. ridibundus (amphispermy) and only one male with the P. ridibundus genome. Both species had the mitochondrial DNA of P. ridibundus. Based on nuclear markers, in P. esculentus and P. ridibundus we revealed the presence of alleles of the closely related Anatolian species P. cf. bedriagae. This might affect the successful reproduction of hybridogenetic P. esculentus. However, comparison of percentage of sterile males and alleles of P. cf. bedriagae among hybrids showed no correlation.