Gamete production patterns and mating systems in water frogs of the hybridogenetic Pelophylax esculentus complex in north-eastern Ukraine

Abstract

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.

Taxonomic composition and ploidy level among European water frogs (Anura: Ranidae:Pelophylax) in eastern Hungary

“…; Biriuk et al. ), western and eastern Poland (Rybacki and Berger ), Germany (Günther ), France (Pagano et al. ) and Slovakia (Mikulíček et al.…”

“…, ; Biriuk et al. ) remains an open question that can only be fully answered by crossing experiments. Alternatively, Hungarian hybridogenetic lineages can produce R gametes (as in the vastly distributed LE system), be reproductively dependent on P. lessonae individuals, which, however, should reach extremely low densities in studied populations.…”

“…Using population genetics to infer breeding system The RE-system with syntopic occurence of hybrids with P. ridubundus without P. lessonae tends to be uncommon in Europe; isolated populations observed in eastern Ukraine (Borkin et al 2004;Biriuk et al 2016), western and eastern Poland (Rybacki and Berger 2001), Germany (G€ unther 1990), France (Pagano et al 1997) and Slovakia . Whether hybrids in Hungarian RE populations eliminate the R genome and form L gametes, as in some German and Polish populations (Uzzell and Berger 1975;Berger 1977;Uzzell et al 1980) or the simultaneous production of the L and R gametes, as was observed in populations in Ukraine (Vinogradov et al 1990(Vinogradov et al , 1991Biriuk et al 2016) remains an open question that can only be fully answered by crossing experiments. Alternatively, Hungarian hybridogenetic lineages can produce R gametes (as in the vastly distributed LE system), be reproductively dependent on P. lessonae individuals, which, however, should reach extremely low densities in studied populations.…”

“…; Biriuk et al. ). Besides hybrid forms living and mating with one or the other parental species, there are P. esculentus populations that are reproductively independent of the sexual species.…”

“…Most P. esculentus in central Europe eliminate the L genome during gametogenesis, clonally transmit the R genome and backcross with syntopically living P. lessonae (LE system). Hybrid forms that eliminate the R genome form lessonae gametes and backcross with P. ridibundus (RE system) are less common and have been found only in eastern Germany, Poland and eastern Ukraine (Uzzell and Berger 1975;Berger 1977;Uzzell et al 1980;Borkin et al 2004;Biriuk et al 2016). Besides hybrid forms living and mating with one or the other parental species, there are P. esculentus populations that are reproductively independent of the sexual species.…”