The high diversity of gametogenic pathways in amphispermic water frog hybrids from Eastern Ukraine

Interspecific hybridization can disrupt canonical gametogenic pathways, leading to the emergence of clonal and hemiclonal organisms. Such gametogenic alterations usually include genome endoreplication and/or premeiotic elimination of one of the parental genomes. The hybrid frog Pelophylax esculentus exploits genome endoreplication and genome elimination to produce haploid gametes with chromosomes of only one parental species. To reproduce, hybrids coexist with one of the parental species and form specific population systems. Here, we investigated the mechanism of spermatogenesis in diploid P. esculentus from sympatric populations of P. ridibundus using fluorescent in situ hybridization. We found that the genome composition and ploidy of germ cells, meiotic cells, and spermatids vary among P. esculentus individuals. The spermatogenic patterns observed in various hybrid males suggest the occurrence of at least six diverse germ cell populations, each with a specific premeiotic genome elimination and endoreplication pathway. Besides co-occurring aberrant cells detected during meiosis and gamete aneuploidy, alterations in genome duplication and endoreplication have led to either haploid or diploid sperm production. Diploid P. esculentus males from mixed populations of P. ridibundus rarely follow classical hybridogenesis. Instead, hybrid males simultaneously produce gametes with different genome compositions and ploidy levels. The persistence of the studied mixed populations highly relies on gametes containing a genome of the other parental species, P. lessonae.

Stability of spermatogenesis and displays of natural selection at the cellular and individual level in immature representatives of the Pelophylax esculentus complex

Pelophylax esculentus is an interspecific hybrid which reproduces hemiclonally by crossing with the parental species (P. ridibundus or P. lessonae). The structure of hemiclonal population systems is of great interest. The majority of investigations into populations of water frogs deal with samples of mature animals, while subadults are less studied. We collected a random sample of 73 small water frogs from three localities in the Siverskyi Donets River floodplain. All frogs were measured, injected with colchicines and killed after anesthesia. We determined the species specifity, sex, age and ploidy of every individual. In addition, we measured testis length and studied the germ cells of all males by means of karyological analysis. We calculated the portion of triploids in the largest subsample from the vicinity of the Biological Station of V.N. Karazin Kharkiv National University and carried out a meta-analysis of previous publications dealing with composition of green frog samples from this locality. The proportion of triploids in our sample appeared to be approximately the same as 12 years ago. However, this proportion in our sample differed significantly from that obtained in recent samples of green frogs belonging to other age groups (adults and metamorphs) from the same locality. The share of triploids of P. esculentus is the lowest in the sample of metamorphs. This proportion increases in froglets because of early death of representatives of parental species which originated from hybrid-hybrid crossing. Then the number of triploids among adult P. esculentus individuals declines again probably because of their lower viability compared to diploids. We have revealed that subadult diploids have erythrocytes smaller than 28 microns, while triploids’ erythrocytes exceed 27 microns. Therefore, in borderline cases true ploidy could be determined only by the karyological technique. The average ratio between testis length and body length appeared to be larger in the parental species than in both diploid and triploid hybrids. Karyological analysis has revealed that P. esculentus had significantly lower portions of spermatocytes I with normal karyotype (13 bivalents) in the testis in comparison with P. ridibundus, but this value increases in adult hybrids. We suggest that increasing of spermatogenesis stability in adult frogs as opposed to subadults might be the consequence of both selection of germ cell lines in the testis and more frequent survival of individuals with stable gametogenesis.