Programmed Genome Elimination Is Evolutionarily Conserved Across Pelophylax Hybrids – as Evidenced by P. grafi Hybridogenetic Reproduction

Gametogenesis is a fundamental biological process that ensures both genetic recombination and the continuity of successive generations. Interspecific hybrids can reproduce through modified mechanisms, such as hybridogenesis, by transmitting clonal, unrecombined genomes of only one of the parental species via their gametes. Pelophylax grafi (PR) is a natural hybrid frog composed of mixed genomes (subgenomes) of two related species, Pelophylax perezi (P) and Pelophylax ridibundus (R), and coexists in populations with P. perezi. This study tested the involvement of programmed genome elimination in gamete production of P. grafi, providing new insight into reproductive mechanisms of hybrid vertebrates. Using comparative genomic hybridization (CGH) and fluorescent in situ hybridization (FISH), we examined the genomic constitution of germline cells in tadpoles and adult male and female P. grafi. Controlled crosses between P. perezi and P. grafi produced F1 hybrid tadpoles, whose genotypes confirmed that P. grafi parents transmitted the R subgenome through their gametes. In the early germline cells (gonocytes) of these tadpoles, P chromosomes were selectively eliminated via micronuclei formation during interphase. The occasional presence of the R genome and mixed R/P genome micronuclei, suggests variability and imperfect fidelity in the elimination process. In adult hybrids, the majority of diplotene oocytes, spermatogonial stem cells (SSC) and spermatocytes carried R subgenomes. We demonstrated that programmed genome rearrangement in Pelophylax hybrids is an evolutionarily conserved mechanism underlying this unique reproductive strategy.