A. V. Korshunov’s research while affiliated with V. N. Karazin Kharkiv National University and other places

Hybridization and polyploidy play an important role in animal speciation. European water frogs of the Pelophylax esculentus complex demonstrate unusual genetic phenomena associated with hybridization, clonality and polyploidy which presumably indicate an initial stage of reticulate speciation. The Seversky Donets River drainage in north-eastern Ukraine is inhabited by both sexes of the diploid and triploid hybrid P. esculentus and only one parental species Pelophylax ridibundus. Based on the presence of various types of hybrids, all populations studied can be divided into three geographical groups: I) P. ridibundus-P. esculentus without triploids; II) P. ridibundus-P. esculentus without diploid hybrids; and III) P. ridibundus-P. esculentus with a mixture of diploids and triploids. A study of gametogenesis revealed that diploid P. esculentus in populations of the first type usually produced haploid gametes of P. ridibundus and a mixture of haploid gametes that carried one or another parental genome (hybrid amphispermy). In populations of the second type, hybrids are derived from crosses of P. ridibundus males with triploid hybrid females producing haploid eggs with a genome of P. lessonae. Therefore, we suggest that clonal genome duplication in these eggs might be the result of suppression of second polar body formation or extra precleavage endoreduplication. In populations of the third type, some diploid females can produce diploid gametes. Fertilization of these eggs with haploid sperm can result in triploid hybrids. Other hybrids here produce haploid gametes with one or another parental genome or their mixture giving rise to new diploid hybrids.

In this study, quantitative analysis of paternal genome inheritance by a hybrid form Rana “esculenta” (= Rana esculenta L., 1758 × Rana ridibunda Pall., 1881) (Amphibia, Ranidae) was examined. The hybrid form examined was characterized by a polymodal mode of inheritance (genome of any of the parental species can be inherited). The absence of correlation between the proportion of normal gametes and either sex or ploidity of the producer was demonstrated. The gametes produced could be both haploid and diploid (hybrid or homozygous). The mechanism of alloploid reproduction is discussed.

Based on DNA flow cytometry, we identified green frog localities with hybrid Rana esculenta. We found or confirmed some southernmost localities of the species situated in Moldavia and Ukraine. According to our data, the southern distributional border of R. esculenta is demarcated as follows from the west to the east: Vilkovo (Danube River Delta), Vulcanesti (Moldova), Doibani, Tiraspol’, and Kitscani (Dniestr River), Beryozovo (Tiligul River), Golaya Pristan District (Dnieper River), Russky Orchik (Kharkov Province, Ukraine) as well as along the middle part of Seversky Donets River in northeastern Ukraine and Rostov Province in adjacent Russia. The majority of samples contained diploid hybrid frogs from the RE kind of population systems. The REL system was identified in Russky Orchik. The records of R. esculenta in Kherson Province of Ukraine are geographically far isolated from populations in northern and central parts of Ukraine. However, this large gap seems to be resulted from inadequate frog sampling and species identification rather than from relic nature of the southernmost records.

Mass triploidy in hybrid Rana esculenta was revealed by means of DNA flow cytometry in 19 localities of eastern Ukraine (Kharkov, Donetsk, and Lugansk provinces). Triploids consist of two groups in terms of parental species genomes: LLR and LRR. The both groups were recorded in population systems of the E and R–E types. Two tetraploids were found as well. All polyploids were distributed along the middle part of Seversky Donets River basin (above 450 km). The nearest localities with mass occurrence of triploidy were known at distance of 1000 km (Poland) or 1500 km (Hungary).

In eastern Ukraine, the Rana esculenta complex consists of three species: R. lessonae, R. ridibunda, and hybrid R. esculenta. The first one was rare, whereas two latter frog taxa were very common. Based on DNA flow cyto-metry, mass occurrence of the triploidy in Rana esculenta has been revealed in 14 localities of Kharkov, Do-netsk, and Lugansk Provinces. One hybrid specimen from Kharkov Province was tetraploid. All polyploids were recorded along the middle part of Seversky Donets River (above 450 km). Triploids comprised two groups with different genome composition (LLR and LRR), and were found in three types of population systems (E, R–E, and L–E–R). Geographic distribution of polyploidy in European green frogs is briefly outlined. Different meth-ods of ploidy level identification are discussed. The chromosome count and nuclear DNA cytometry provide the most reliable data.