When a clonal genome finds its way back to a sexual species: evidence from ongoing but rare introgression in the hybridogenetic water frog complex

Abstract

Besides several exceptions, asexual metazoans are usually viewed as ephemeral sinks for genomes, which become 'frozen' in clonal lineages after their emergence from ancestral sexual species. Here, we investigated whether and at what rate the asexuals are able to introgress their genomes back into the parental sexual population, thus more or less importantly affecting the gene pools of sexual species. We focused on hybridogenetic hybrids of western Palaearctic water frogs (Pelophylax esculentus), which originate through hybridization between P. ridibundus and P. lessonae, but transmit only clonal ridibundus genome into their gametes. Although usually mating with P. lessonae, P. esculentus may upon mating with P. ridibundus or another hybrid produce sexually reproducing P. ridibundus offspring with the introgressed ex-clonal genome. We compared the rate of nuclear amplified fragment length polymorphism (AFLP) and mitochondrial introgression in two types of populations, that is, those where P. ridibundus occurs in isolation and those where it lives with the hybridogens. Although significant differentiation (Φpt) between sexual and clonal ridibundus genomes suggested limited gene flow between sexuals and hybridogens, a non-negligible (~5%) proportion of P. ridibundus bore introgressed mtDNA and AFLP markers. Whereas transfer of mtDNA was exclusively unidirectional, introgression of nuclear markers was bidirectional. The proportion of introgressed P. ridibundus was highest in syntopic populations with P. esculentus, proving an ongoing and site-specific interspecific genetic transfer mediated by hybridogenetic hybrids. It turns out that asexual hybrids are not just a sink for genes of sexual species, but may significantly influence the genetic architecture of their sexual counterparts.

Contrasting reproductive strategies of triploid hybrid males in vertebrate mating systems

“…In contrast, triploid LLR hybrids in the four populations of types 3 and 4 occurred as males only; LLR females were neither caught during this study (Table ) nor found during previous samplings performed by Mikulíček et al . ().…”

Bidirectional mitochondrial introgression between Korean cobitid fish mediated by hybridogenetic hybrids

“…Similarly, hybridogenesis of water frogs ( Pelophylax esculentus ) has been suggested to be a genetic sink for sexual species. However, a recent study showed evidence that the water frog hybrids were able to mediate mtDNA transfer to its parental species ( P. ridibundus ) (Mikulïček, Kautman, Demovič, & Janko, ). Interestingly with our studying species, Ko () described how triploid hybrid females could transmit genetic material to the parental species by backcrossing (Figure ).…”

Population genomics and geographical parthenogenesis in Japanese harvestmen (Opiliones, Sclerosomatidae, Leiobunum)

“…Additionally, we might consider the fit of pseudogamous scenarios, as in gynogenetic species (Choleva et al., ), where male stimulation of embryogenesis is necessary in order to catalyze reproductive processes (Schlupp, ). It is hypothesized that this form of asexuality also develops from rare sexual reproduction (Mikulíček, Kautman, Demovič, & Janko, ), and while Tsurusaki disproved the necessity of males for parthenogenesis in L. manubriatum (Tsurusaki, ), the possibility remains for variation in reproductive mode in isolated populations of the species. Such reproductive mechanisms have never been described in Opiliones, but could explain a portion of the discordance between the nuclear and mtDNA data in these species.…”