Mode of hybridogenesis and habitat preferences influence population composition of water frogs (Pelophylax esculentus complex, Anura: Ranidae) in a region of sympatric occurrence (western Slovakia)

Nuclear gene introgressions in hybrid populations of water frog Pelophylax esculentus complex: geographical analysis of the phenomenon and its interpretation

Reproduction of water frog hybrids Pelophylax esculentus (Pelophylax ridibundus x Pelophylax lessonae) is associated with hemiclonal reproduction and backcrossing. The hemiclonal mode of reproduction occurs within P. esculentus allodiploids. In this case, the unrecombined genome of one parental species is transmitted to the offspring after premeiotic elimination of the chromosome set of the second parental species. Usually, the chromosome set of P. lessonae is eliminated, and the altered genome of P. ridibundus is passed on to the progeny. The hemiclonal inheritance within diploid Pelophylax esculentus hybrids may be accompanied by certain aberrations of premeiotic elimination. As a result, the formation of P. ridibundus specimens with introgressions of the P. lessonae genetic material, or the formation of recombinant hybrids occurs, depending on which of the parental species backcrossing takes place. The aim of our study is to describe the aberration of premeiotic elimination within the water frog P. esculentus complex detected by the nuclear gene Ldh-B inheritance, with an attempt to find out the causes of this phenomenon. It has been established that aberrations of premeiotic elimination are widespread, but only within populations of water frog from the river system of Upper Dnieper within Ukraine. The highest level of introgression takes place in the water frog populations within Kiev metropolis under conditions of expressed anthropogenization, while the maximum frequency of recombinants was detected within populations from the basin of Desna River, that has preserved native ecosystems. It was demonstrated that the frequency of premeiotic aberrations does not correlate with the intensity of interspecific water frog hybridization. Populations with introgressions are more common than populations with recombinants, however, within the latter, the frequency of recombination events is higher. The primary factor of gametogenesis aberrations, most likely, is the genetic characteristics of the local populations of parental species, since unambiguous explanations of this phenomenon based on the action of environmental stress (pollution of water systems) are not obvious.

Sperm-dependent asexual species and their role in ecology and evolution

Sexual reproduction is the primary mode of reproduction in eukaryotes, but some organisms have evolved deviations from classical sex and switched to asexuality. These asexual lineages have sometimes been viewed as evolutionary dead ends, but recent research has revealed their importance in many areas of general biology. Our review explores the understudied, yet important mechanisms by which sperm-dependent asexuals that produce non-recombined gametes but rely on their fertilization, can have a significant impact on the evolution of coexisting sexual species and ecosystems. These impacts are concentrated around three major fields. Firstly, sperm-dependent asexuals can potentially impact the gene pool of coexisting sexual species by either restricting their population sizes or by providing bridges for interspecific gene flow whose type and consequences substantially differ from gene flow mechanisms expected under sexual reproduction. Secondly, they may impact on sexuals' diversification rates either directly, by serving as stepping-stones in speciation, or indirectly, by promoting the formation of pre- and postzygotic reproduction barriers among nascent species. Thirdly, they can potentially impact on spatial distribution of species, via direct or indirect (apparent) types of competition and Allee effects. For each such mechanism, we provide empirical examples of how natural sperm-dependent asexuals impact the evolution of their sexual counterparts. In particular, we highlight that these broad effects may last beyond the tenure of the individual asexual lineages causing them, which challenges the traditional perception that asexual lineages are short-lived evolutionary dead ends and minor sideshows. Our review also proposes new research directions to incorporate the aforementioned impacts of sperm-dependent asexuals. These research directions will ultimately enhance our understanding of the evolution of genomes and biological interactions in general.

Genomic basis for an informed conservation management of Pelophylax water frogs in Luxembourg

Genetic identification methods have become increasingly important for species that are difficult to identify in the field. A case in point is Pelophylax water frogs. While their morphological determination is highly complex, they include species protected under EU law and some that are classified as invasive. Additionally, genetic data can provide insights into their complex breeding systems, which may or may not involve the reproductive dependency of one species on another. Here, we generate baseline data for water frog monitoring in Luxembourg. We applied a countrywide sampling approach and used SNPs generated by ddRAD sequencing to identify individuals and infer the breeding systems present in the country. We found Pelophylax lessonae and P. kl. esculentus throughout Luxembourg, mostly living in syntopy. In general, a reproductive dependency of P. kl. esculentus on P. lessonae (L‐E system) was revealed. Besides this general system, we detected triploid P. kl. esculentus in six ponds. This indicates a modified L‐E system with reproductive dependency of the triploids on the diploid P. kl. esculentus. The invasive P. cf. bedriagae was detected in three ponds in southern Luxembourg, with evidence for hybridization with native water frogs. In addition to the ddRAD data, we tested a simple genetic method for future monitoring based on the MND1 marker. It showed in almost all cases, an identical species identification as the ddRAD data and was successfully applied to DNA extracts from mouth swabs. Combining this method with our baseline data will enable informed choices for the protection of native water frog species in Luxembourg.

Genome and Karyotype Reorganization after Whole Genome Duplication in Free-Living Flatworms of the Genus Macrostomum

The genus Macrostomum represents a diverse group of rhabditophoran flatworms with >200 species occurring around the world. Earlier we uncovered karyotype instability linked to hidden polyploidy in both M. lignano (2n = 8) and its sibling species M. janickei (2n = 10), prompting interest in the karyotype organization of close relatives. In this study, we investigated chromosome organization in two recently described and closely related Macrostomum species, M. mirumnovem and M. cliftonensis, and explored karyotype instability in laboratory lines and cultures of M. lignano (DV1/10, 2n = 10) and M. janickei in more detail. We revealed that three of the four studied species are characterized by karyotype instability, while M. cliftonensis showed a stable 2n = 6 karyotype. Next, we performed comparative cytogenetics of these species using fluorescent in situ hybridization (FISH) with a set of DNA probes (including microdissected DNA probes generated from M. lignano chromosomes, rDNA, and telomeric DNA). To explore the chromosome organization of the unusual 2n = 9 karyotype discovered in M. mirumnovem, we then generated chromosome-specific DNA probes for all chromosomes of this species. Similar to M. lignano and M. janickei, our findings suggest that M. mirumnovem arose via whole genome duplication (WGD) followed by considerable chromosome reshuffling. We discuss possible evolutionary scenarios for the emergence and reorganization of the karyotypes of these Macrostomum species and consider their suitability as promising animal models for studying the mechanisms and regularities of karyotype and genome evolution after a recent WGD.

Genetic diversity and distribution patterns of diploid and polyploid hybrid water frog populations (Pelophylax esculentus complex) across Europe

Polyploidization is a rare yet sometimes successful way for animals to rapidly create geno- and phenotypes that may colonize new habitats and quickly adapt to environmental changes. In this study, we use water frogs of the Pelophylax esculentus complex, comprising two species (Pelophylax lessonae, genotype LL; Pelophylax ridibundus, RR) and various diploid (LR) and triploid (LLR, LRR) hybrid forms, summarized as P. esculentus, as a model for studying recent hybridization and polyploidization in the context of speciation. Specifically, we compared the geographic distribution and genetic diversity of diploid and triploid hybrids across Europe to understand their origin, maintenance and potential role in hybrid speciation. We found that different hybrid and parental genotypes are not evenly distributed across Europe. Rather, their genetic diversity is structured by latitude and longitude and the presence/absence of parental species but not of triploids. Highest genetic diversity was observed in central and eastern Europe, the lowest in the northwestern parts of Europe. This gradient can be explained by the decrease in genetic diversity during postglacial expansion from southeastern glacial refuge areas. Genealogical relationships calculated on the basis of microsatellite data clearly indicate that hybrids are of multiple origin and include a huge variety of parental genomes. Water frogs in mixed-ploidy populations without any parental species (i.e. all-hybrid populations) can be viewed as evolutionary units that may be on their way towards hybrid speciation. Maintenance of such all-hybrid populations requires a continuous exchange of genomes between diploids and triploids, but scenarios for alternative evolutionary trajectories are discussed.

Contrasting reproductive strategies of triploid hybrid males in vertebrate mating systems

The scarcity of parthenogenetic vertebrates is often attributed to their 'inferior' mode of clonal reproduction, which restricts them to self-reproduce their own genotype lineage and leaves little evolutionary potential with regard to speciation and evolution of sexual reproduction. Here, we show that for some taxa, such uniformity does not hold. Using hybridogenetic water frogs (Pelophylax esculentus) as a model system, we demonstrate that triploid hybrid males from two geographic regions exhibit very different reproductive modes. With an integrative data set combining field studies, crossing experiments, flow cytometry and microsatellite analyses, we found that triploid hybrids from Central Europe are rare, occur in male sex only and form diploid gametes of a single clonal lineage. In contrast, triploid hybrids from north-western Europe are widespread, occur in both sexes and produce recombined haploid gametes. These differences translate into contrasting reproductive roles between regions. In Central Europe, triploid hybrid males sexually parasitize diploid hybrids and just perpetuate their own genotype--which is the usual pattern in parthenogens. In north-western Europe, on the other hand, the triploid males are gamete donors for diploid hybrids, thereby stabilizing the mixed 2n-3n hybrid populations. By demonstrating these contrasting roles in male reproduction, we draw attention to a new significant evolutionary potential for animals with nonsexual reproduction, namely reproductive plasticity.