Hybridogenetic reproduction of Pelophylax water frogs from different hemiclonal population systems from Eastern Ukraine: selective mortality, and clonal and ploidy diversity

European water frogs from the Pelophylax esculentus complex include two sexual species, Pelophylax ridibundus (genome RR) and Pelophylax lessonae (genome LL), and their hybrids (genome LR), which usually clonally transmit one of the parental species’ genomes. This reproductive strategy, known as hybridogenesis, allows hybrids to reproduce with parental species, creating diverse population systems. Unlike most well-studied populations in Europe, in the Siverskyi Donets River basin (eastern Ukraine), P. lessonae is absent, while diploid and polyploid hybrids coexist with P. ridibundus (R-E system). To reveal diverse system compositions, genetic divergence, and tadpole selective mortality, we combined novel data from over a decade of observations with previous research on population systems in the Siverskyi Donets River. Two key population types of the R-E system were identified: those with diploid hybrids in northern localities and those with both diploid and triploid hybrids, extending from the mainstream of the Siverskyi Donets River to its tributaries. Both sexes were found among both P. ridibundus and hybrids. Additionally, we found higher genetic diversity in R-genomes compared to L-genomes and discuss the cause of variation differences between the genomes. We highlighted the importance of continuous monitoring to unravel water frog population dynamics and complexity.