Formation of hemiclonal reproduction and hybridogenesis inPelophylax waterfrogs studied with species-specific cytogenomic probes

Abstract

Meiosis is a conservative process in all sexual organisms which ensures fertility and is central for producing genetic diversity by recombination and random segregation of parental chromosomes. Yet unexplored mechanisms may disrupt it and cause ‘loss of sex’ followed by the emergence of clonal modes of reproduction. Interspecific hybridization is the primary trigger for this process, but mechanistic basis of the transition to asexuality remains still unknown for most vertebrate animals. To study these processes in water frogs, we performed reciprocal mating between two sexual species,Pelophylax ridibundusandP. lessonae, and produced vital F1 progeny (P. esculentus). The RepeatExplorer2 analysis of low-coverage genomic data of the two parental species identified theP. lessonae-specific minisatellite markerPlesSat01-48(44 bp), which hybridized to (peri)centromeric regions of two chromosome pairs inP. lessonae– the acrocentric chromosome 8 and the chromosome 10 (a carrier of nucleolar organizer region; NOR). Chromosomal mapping combining the novel hybridization probe with the previously designed marker forP. ridibundus-specific centromeric satellite DNA showed that theP. esculentusprogeny do not reproduce sexually. Instead, the F1 generation ofP. esculentusinstantly modified its gametogenesis and established asexual reproduction via hybridogenesis. Gametogenic modifications included premeiotic elimination of one of the parental genomes and clonal propagation of the remaining genome via endoreplication followed by standard meiotic division. The origin of DNA elimination and hybridogenesis in laboratory-produced hybrids supports a hypothesis thatP. esculentusarises recurrently in nature whenever parental species come into reproductive contact. Based on the observed pattern of DNA elimination in the F1 progeny we discuss the origin and evolution of population systems in water frogs and the applicability of a newly designed chromosomal probe for otherPelophylaxtaxa.