Cytogenetics of the Hybridogenetic Frog Pelophylax grafi and Its Parental Species Pelophylax perezi

Hybrid taxa from the genus Pelophylax can propagate themselves in a modified way of sexual reproduction called hybridogenesis ensuring the formation of clonal gametes containing the genome of only one parental (host) species. Pelophylax grafi from South-Western Europe is a hybrid composed of P. ridibundus and P. perezi genomes and it lives with a host species P. perezi (P-G system). Yet it is unknown, whether non-Mendelian inheritance is fully maintained in such populations. In this study, we characterize P. perezi and P. grafi somatic karyotypes by using comparative genomic hybridization, genomic in situ hybridization, fluorescent in situ hybridization, and actinomycin D-DAPI. Here, we show the homeology of P. perezi and P. grafi somatic karyotypes to other Pelophylax taxa with 2n = 26 and equal contribution of ridibundus and perezi chromosomes in P. grafi which supports F1 hybrid genome constitution as well as a hemiclonal genome inheritance. We show that ridibundus chromosomes have larger regions of interstitial (TTAGGG)n repeats flanking the nucleolus organizing region on chromosome no. 10 and a high quantity of AT pairs in the centromeric regions. In P. perezi, we found species-specific sequences in metaphase chromosomes and marker structures in lampbrush chromosomes. Pericentromeric RrS1 repeat sequence was present in perezi and ridibundus chromosomes, but the blocks were stronger in ridibundus. Various cytogenetic techniques applied to the P-G system provide genome discrimination between ridibundus and perezi chromosomal sets. They could be used in studies of germ-line cells to explain patterns of clonal gametogenesis in P. grafi and broaden the knowledge about reproductive strategies in hybrid animals.

Genome elimination from the germline cells in diploid and triploid male water frogs Pelophylax esculentus

Hybridogenesis is a hemiclonal reproductive strategy in diploid and triploid hybrids. Our study model is a frog P. esculentus (diploid RL and triploids RLL and RRL), a natural hybrid between P. lessonae (LL) and P. ridibundus (RR). Hybridogenesis relies on elimination of one genome (L or R) from gonocytes (G) in tadpole gonads during prespermatogenesis, but not from spermatogonial stem cells (SSCs) in adults. Here we provide the first comprehensive study of testis morphology combined with chromosome composition in the full spectrum of spermatogenic cells. Using genomic in situ hybridization (GISH) and FISH we determined genomes in metaphase plates and interphase nuclei in Gs and SSCs. We traced genomic composition of SSCs, spermatocytes and spermatozoa in individual adult males that were crossed with females of the parental species and gave progeny. Degenerating gonocytes (24%–39%) and SSCs (18%–20%) led to partial sterility of juvenile and adult gonads. We conclude that elimination and endoreplication not properly completed during prespermatogenesis may be halted when gonocytes become dormant in juveniles. After resumption of mitotic divisions by SSCs in adults, these 20% of cells with successful genome elimination and endoreplication continue spermatogenesis, while in about 80% spermatogenesis is deficient. Majority of abnormal cells are eliminated by cell death, however some of them give rise to aneuploid spermatocytes and spermatozoa which shows that hybridogenesis is a wasteful process.

Maintenance of pure hybridogenetic water frog populations: Genotypic variability in progeny of diploid and triploid parents

An intriguing outcome of hybridisation is the emergence of clonally and hemiclonally reproducing hybrids, that can sustain, reproduce, and lead to the emergence of polyploid forms. However, the maintenance of diploid and polyploid hybrid complexes in natural populations remains unresolved. We selected water frogs from the Pelophylax esculentus complex to study how diploid and triploid hybrids, which reproduce hemiclonally via hybridogenesis, are maintained in natural populations. During gametogenesis in diploid hybrids, one of the parental genomes is eliminated, and the remaining genome is endoreplicated. In triploid hybrids, the single-copy genome is typically eliminated, while genome endoreplication does not occur. To investigate how diploid and triploid hybrid frogs reproduce in populations without parental species, we crossed these hybrid animals from two separate pure hybrid populations located in Poland. Using cytogenetic analysis of tadpoles that emerged from the crosses, we established which gametes were produced by parental hybrids. The majority of hybrid females and hybrid males produced one type of gamete with the P. ridibundus genome. However, in both studied populations, approximately half of the diploid and triploid hybrids simultaneously produced gametes with different genome compositions and ploidy levels, specifically, the P. ridibundus and P. lessonae genomes, as well as diploid gametes with genomes of both parental species. Triploid hybrid males and females mostly produced haploid gametes with the P. lessonae genome; however, gametes with the P. ridibundus genome have also been observed. These results suggest that not all hybrids follow the classical hybridogenetic reproduction program and reveal a significant level of alterations in the gametogenesis pathways. In addition, we found a variable survival rate of particular progeny genotypes when we crossed hybrid females with different males suggesting the important role of postzygotic barriers on the maintenance of pure hybrid systems. We suggest that the observed variability in produced gametes and the different survival rate of the progeny with certain genotypes is crucial for the existence of pure hybrid systems.

Morphometry of two cryptic tree frog species at their hybrid zone reveals neither intermediate nor transgressive morphotypes

Under incomplete reproductive isolation, secondary contact of diverged allopatric lineages may lead to the formation of hybrid zones that allow to study recombinants over several generations as excellent systems of genomic interactions resulting from the evolutionary forces acting on certain genes and phenotypes. Hybrid phenotypes may either exhibit intermediacy or, alternatively, transgressive traits, which exceed the extremes of their parents due to epistasis and segregation of complementary alleles. While transgressive morphotypes have been examined in fish, reptiles, birds, and mammals, studies in amphibians are rare. Here, we associate microsatellite-based genotypes with morphometrics-based morphotypes of two tree frog species of the Hyla arborea group, sampled across a hybrid zone in Poland, to understand whether the genetically differentiated parental species also differ in morphology between each other and their hybrids and whether secondary contact leads to the evolution of intermediate or transgressive morphotypes. Using univariate approaches, explorative multivariate methods (principal component analyses) as well as techniques with prior grouping (discriminant function analyses), we find that morphotypes of both parental species and hybrids differ from each other. Importantly, hybrid morphotypes are neither intermediate nor transgressive but found to be more similar to H. orientalis than to H. arborea.

Body size and life history traits of the fire salamander Salamandra salamandra from Poland

The fire salamander Salamandra salamandra is a widespread taxon in Europe, exhibiting great intraspecific diversity in phenotype and life history traits across its geographical distribution. Here, we studied body size, sexual dimorphism, age, growth rate and condition of fire salamanders from the north-eastern margin of its range. In total, 2,102 individuals from 23 populations representing the Polish parts of the Sudetes and the Carpathian Mountains were sampled between 2004 and 2016. Body traits and age showed significant differences between the western (the Sudetes) and eastern (the Carpathians) groups of populations. Salamanders from the Carpathians tended to be longer, heavier and older. Female-biased sexual size dimorphism was found only in the Carpathians. Body condition at the beginning of the season was poor, then increased to reach a peak in early June, and deteriorated toward the end of the season. Age estimated by skeletochronology on phalangeal bones ranged from 2 to 16 years in both females and males, with the highest share of 7- to 9-year-old individuals. Age of juveniles ranged from 1 to 5 years in the Sudetes and from 1 to 4 years in the Carpathians. Growth curves (fitted using von Bertalanffy’s model) were asymptotic throughout the individual lifespans, but exhibited differences between sexes and mountain ranges. Altitude did not explain differences in characteristics of populations living in the two mountain ranges, but these differences most probably resulted from habitat quality (better in the Carpathians) and adverse human impact (higher in the Sudetes).

Impacts of the synthetic androgen Trenbolone on gonad differentiation and development - comparisons between three deeply diverged anuran families

Using a recently developed approach for testing endocrine disruptive chemicals (EDCs) in amphibians, comprising synchronized tadpole exposure plus genetic and histological sexing of metamorphs in a flow-through-system, we tested the effects of 17β-Trenbolone (Tb), a widely used growth promoter in cattle farming, in three deeply diverged anuran families: the amphibian model species Xenopus laevis (Pipidae) and the non-models Bufo(tes) viridis (Bufonidae) and Hyla arborea (Hylidae). Trenbolone was applied in three environmentally and/or physiologically relevant concentrations (0.027 µg/L (10-10 M), 0.27 µg/L (10-9 M), 2.7 µg/L (10-8 M)). In none of the species, Tb caused sex reversals or masculinization of gonads but had negative species-specific impacts on gonad morphology and differentiation after the completion of metamorphosis, independently of genetic sex. In H. arborea and B. viridis, mounting Tb-concentration correlated positively with anatomical abnormalities at 27 µg/L (10-9 M) and 2.7 µg/L (10-8 M), occurring in X. laevis only at the highest Tb concentration. Despite anatomical aberrations, histologically all gonadal tissues differentiated seemingly normally when examined at the histological level but at various rates. Tb-concentration caused various species-specific mortalities (low in Xenopus, uncertain in Bufo). Our data suggest that deep phylogenetic divergence modifies EDC-vulnerability, as previously demonstrated for Bisphenol A (BPA) and Ethinylestradiol (EE2).

Shedding Light on a Secretive Tertiary urodelean Relict: Hynobiid salamanders (Paradactylodon persicus s.l.) from Iran, Illuminated by Phylogeographic, Developmental and Transcriptomic Data

The Hyrcanian Forests present a unique Tertiary relict ecosystem, covering the northern Elburz and Talysh Ranges (Iran, Azerbaijan), a poorly investigated, unique biodiversity hotspot with many cryptic species. Since the 1970s, two nominal species of Urodela, Hynobiidae, Batrachuperus (later: Paradactylodon) have been described: Paradactylodon persicus from northwestern and P. gorganensis from northeastern Iran. Although P. gorganensis has been involved in studies on phylogeny and development, there is little data on the phylogeography, systematics, and development of the genus throughout the Hyrcanian Forests; genome-wide resources have been entirely missing. Given the huge genome size of hynobiids, making whole genome sequencing hardly affordable, we aimed to publish the first transcriptomic resources for Paradactylodon from an embryo and a larva (9.17 Gb RNA sequences; assembled to 78,918 unigenes). We also listed 32 genes involved in vertebrate sexual development and sex determination. Photographic documentation of the development from egg sacs across several embryonal and larval stages until metamorphosis enabled, for the first time, comparison of the ontogeny with that of other hynobiids and new histological and transcriptomic insights into early gonads and timing of their differentiation. Transcriptomes from central Elburz, next-generation sequencing (NGS) libraries of archival DNA of topotypic P. persicus, and GenBank-sequences of eastern P. gorganensis allowed phylogenetic analysis with three mitochondrial genomes, supplemented by PCR-amplified mtDNA-fragments from 17 museum specimens, documenting <2% uncorrected intraspecific genetic distance. Our data suggest that these rare salamanders belong to a single species P. persicus s.l. Humankind has a great responsibility to protect this species and the unique biodiversity of the Hyrcanian Forest ecosystems.

Smaller hippocampal volume following sexual assault in women is associated with post-traumatic stress disorder

OBJECTIVE

Exposure to sexual assault is a significant risk factor to develop post-traumatic stress disorder (PTSD) in females. The early neurobiological changes leading to the development of PTSD remain understudied and unclear in this population.

METHODS

Participants were 27 adult females recruited within a month following exposure to sexual assault (T1) and 20 age-matched non-exposed controls. Among the victims, 10 participants met (PTSD+) and 15 did not meet (PTSD-) DSM-IV criteria for PTSD 6 months post-trauma (T2). At both visits, hippocampal and amygdala volumes were extracted from magnetic resonance imaging scans, and indices of total diurnal cortisol changes were derived from individual areas under the curve relative to the ground (AUCg). Measures at T1 were compared between groups at T1, measures at T2 between groups at T2, and measures at T1 between groups at T2.

RESULTS

At T1, victims had significantly smaller bilateral hippocampal volumes, but not AUCg, than controls. At T2, neither hippocampal volume nor AUCg significantly differed among the groups. However, the PTSD+ group had significantly smaller hippocampal volumes at T1 than the control group, but not compared to the PTSD- group.

CONCLUSIONS

This study indicates that having smaller hippocampal volumes is a risk factor to develop PTSD in females exposed to sexual assault.

Batrachochytrium dendrobatidis is present in Poland and associated with reduced fitness in wild populations of Pelophylax lessonae

The fungus Batrachochytrium dendrobatidis (Bd) is a pathogen associated with global declines of amphibians. We used qPCR to detect Bd in 255 samples from 10 Polish populations of 8 species. We found Bd infection in 3 species (Bombina variegata, Pelophylax lessonae, P. esculentus). The infection intensity in P. lessonae reached a maximum of 58400 genomic equivalents of zoospores (GE), and the 2 most heavily infected individuals died. Previous observations of the populations that included infected individuals showed reduced body size, failure to reproduce, and mortalities of adults. These data highlight the importance of emerging diseases, and the need to recognize them as an important factor in conservation of the genus Pelophylax in Poland and Central and Eastern Europe.

Variability of NOR patterns in European water frogs of different genome composition and ploidy level

We studied water frogs from a complex composed of two species: Pelophylax lessonae (Camerano, 1882) (genome LL, 2n = 26) and P. ridibundus (Pallas, 1771) (RR, 2 = 26), and their natural hybrid P. esculentus (Fitzinger, 1843) of various ploidy and genome composition (RL, 2n = 26, and RRL or RLL, 3n = 39). Tetraploids RRLL were found (4n = 52) in juveniles. We applied cytogenetic techniques: AgNO₃, chromomycin A_3, PI and fluorescent in situ hybridization with a 28S rDNA probe. Results obtained by silver staining corresponded well with those stained with CMA₃, PI and FISH. As a rule, NORs are situated on chromosomes 10. The number of Ag-NORs visible on metaphase plates was the same as the number of Ag-nucleoli present in interphase nuclei of the same individual. In all analyzed metaphases, NORs exhibited variations in size after AgNO₃ and CMA₃ stainings. Sixty-six individuals (out of 407 analyzed) were polymorphic for the localization and number of NORs. Fifty-one diploids had NORs only on one chromosome of pair 10. Three triploids (LLR and RRL) displayed two NORs, and two other triploid RRL individuals displayed one, instead of expected three NORs. In ten individuals extra NORs were detected on chromosomes other than 10 (chromosomes 2 and 9).

Prespermatogenesis and early spermatogenesis in frogs

Spermatogenesis in frogs was for the first time divided into two phases: prespermatogenesis, when gonocytes proliferate in developing tadpole testes, and active spermatogenesis when spermatogonial stem cells (i.e. descendants of gonocytes), either self-renew or enter into meiotic cycles within cysts formed by Sertoli cells. We argue that amphibian larval gonocytes are homologues to mammalian gonocytes, whereas spermatogonial stem cells (SSCs) in adult frogs are homologous to mammalian single spermatogonia (A_s). Gonocytes constitute sex cords, i.e. the precursors of seminiferous tubules; they are bigger than SSCs and differ in morphology and ultrastructure. The nuclear envelope in gonocytes formed deep finger-like invaginations absent in SSCs. All stages of male germ cells contained lipid droplets, which were surrounded by glycogen in SSCs, but not in gonocytes. Mitochondria in gonocytes had enlarged edges of cristae, and in SSCs also lamellar mitochondria appeared. Minimal duration of prespermatogenesis was 46days after gonadal sex differentiation, but usually it lasted longer. SSCs give rise to secondary spermatogonia (equal to mammalian A, In, and B). Their lowest number inside a cyst was eight and this indicated the minimal number of cell cycles (three) of secondary spermatogonia necessary to enter meiosis. We sorted them according to the number of cell cycles (from 8 to 256 cells). This number is similar to that recorded for mammals as the result of a single A_s proliferation. The number of secondary spermatogonia correlates with the volume of a cyst. The general conclusion is that spermatogenesis in amphibians and mammals follows basically the same scheme.

Preliminary genetic data suggest the occurrence of the Balkan water frog, Pelophylax kurtmuelleri, in southwestern Poland

Recent molecular studies have detected the occurrence of exotic water frog species (Pelophylaxsp.) in central and western European populations. Here, we report genetic evidence for the occurrence of the Balkan water frog,Pelophylax kurtmuelleri, in southwestern Poland. We found a high frequency of an allele of serum albumin intron-1 and a mitochondrial cytochromebhaplotype specific for this southern taxon in frogs from the Barycz river drainage system. We interpret this finding as evidence of admixture betweenP. kurtmuelleriand the localridibundus-esculentuswater frog population. The origin of the exoticP. kurtmuellerimitochondrial and nuclear alleles in southwestern Poland could be due to (i) hybridization after a human-mediated introduction ofP. kurtmuelleri, (ii) the persistence of ancestral polymorphism in central EuropeanP. ridibundus, or (iii) hybridization betweenP. kurtmuelleriandP. ridibundusin the Balkans followed by the northward expansion of admixedP. ridibundus. Identical mtDNA haplotypes found in southwestern Poland and localities on the borders between Greece, Albania and Macedonia suggest that this region harboured the source population ofP. kurtmuelleriat the studied site.

Ultrasonography: a method used for pregnancy imaging of the fire salamander (Salamandra salamandra)

Ultrasound imaging has more frequently been used in veterinary medicine of amphibians and reptiles. In this study, we have verified the usefulness of ultrasound imaging in pregnancy determination of the fire salamander Salamandra salamandra. We have also undertaken to estimate the number of larvae and their developmental stage directly in the oviducts. Three gravid females from Lower Silesia (southern Poland) were examined. Due to the small size of the scanned animals, and the particular arrangement of embryos in the oviducts and ultrasound beams dispersal, the method proved to be inaccurate. Therefore, the minimum number of well-visualized larvae was determined. The maximum number of larvae was established on the basis of the visible fragments of embryos. After birth, we found that the number of larvae born was included in the "min-max" range in only one case. In the remaining two salamanders the number of larvae was higher than estimated in 3 to 7 individuals. The results showed that ultrasound imaging allows the minimum number of larvae in salamander; oviducts to be specified. However, total length measurements were possible only for single and clearly visible embryos.

The plasticizer bisphenol A affects somatic and sexual development, but differently in pipid, hylid and bufonid anurans

Due to their terrestrial habitats and aquatic reproduction, many amphibians are both very vulnerable and highly suitable bioindicators. The plasticizer bisphenol A (BPA) is one of the most produced chemical substances worldwide, and knowledge on its impacts on humans and animals is mounting. BPA is used for the industrial production of polycarbonate plastics and epoxy resins and found in a multitude of consumer products. Studies on BPA have involved mammals, fish and the fully aquatic anuran model Xenopus laevis. However, our knowledge about the sexual development of non-model, often semi-terrestrial anuran amphibians remains poor. Using a recently developed experimental design, we simultaneously applied BPA to two non-model species (Hyla arborea, Hylidae; Bufo viridis, Bufonidae) and the model X. laevis (Pipidae), compared their genetic and phenotypic sex for detection of sex reversals, and studied sexual development, focusing on anatomical and histological features of gonads. We compared three concentrations of BPA (0.023, 2.28 and 228 μg/L) to control groups in a high-standard flow-through-system, and tested whether conclusions, drawn from the model species, can be extrapolated to non-model anurans. In contrast to previous studies on fish and Xenopus, often involving dosages much higher than most environmental pollution data, we show that BPA causes neither the development of mixed sex nor of sex-reversed individuals (few, seemingly BPA-independent sex reversals) in all focal species. However, environmentally relevant concentrations, as low as 0.023 μg/L, were sufficient to provoke species-specific anatomically and histologically detectable impairments of gonads, and affected morphological traits of metamorphs. As the intensity of these effects differed between the three species, our data imply that BPA diversely affects amphibians with different evolutionary history, sex determination systems and larval ecologies. These results highlight the role of amphibians as a sensitive group that is responsive to environmental pollution.

Impaired gonadal and somatic development corroborate vulnerability differences to the synthetic estrogen ethinylestradiol among deeply diverged anuran lineages

Amphibians are undergoing a global decline. One poorly investigated reason could be the pollution of aquatic habitats by endocrine disrupting compounds (EDCs). We tested the susceptibility to the synthetically stabilized estrogen 17α-ethinylestradiol (EE2) in three deeply diverged anuran species, differing in sex determination systems, types of gonadogenesis and larval ecologies. To understand whether data from the amphibian model Xenopus laevis (Pipidae) are analogous and applicable to only distantly related non-model amphibians, tadpoles of X. laevis, Hyla arborea (Hylidae) and Bufo viridis (Bufonidae) were simultaneously exposed to 50, 500 and 5000ng/L EE2 from hatching until completion of metamorphosis, using a flow-through-system under identical experimental conditions. Comparing molecularly established genetic with histologically assessed phenotypic sex in all species, we have recently shown that EE2 provoked numerous genetic-male-to-phenotypic-female sex reversals and mixed sex individuals, confirming overall its expected feminizing effect. In the present study, we focus on the influence of EE2 on gonadal and somatic development. Anatomy and histology revealed several species-specific effects. In both non-model species, H. arborea and B. viridis, high numbers of anatomically impaired gonads were observed. In H. arborea, exposed to 5000ng/L EE2, numerous underdeveloped gonads were detected. Whereas EE2 did not alter snout-to-vent length and body weight of X. laevis metamorphs, H. arborea showed a treatment-dependent decrease, while B. viridis exhibited an increase in body weight and snout-to-vent length. Apart from a concentration-dependent occurrence of yellowish skin color in several H. arborea, no organ-specific effects were detected. Since EE2 ubiquitously occurs in many aquatic ecosystems and affects sexual and somatic development, among EDCs, it may indeed contribute to amphibian decline. The inter-species variation in developmental EE2-effects corroborates species-specific vulnerability differences towards EDCs between deeply diverged amphibian groups.

Sex reversal assessments reveal different vulnerability to endocrine disruption between deeply diverged anuran lineages

Multiple anthropogenic stressors cause worldwide amphibian declines. Among several poorly investigated causes is global pollution of aquatic ecosystems with endocrine disrupting compounds (EDCs). These substances interfere with the endocrine system and can affect the sexual development of vertebrates including amphibians. We test the susceptibility to an environmentally relevant contraceptive, the artificial estrogen 17α-ethinylestradiol (EE2), simultaneously in three deeply divergent systematic anuran families, a model-species, Xenopus laevis (Pipidae), and two non-models, Hyla arborea (Hylidae) and Bufo viridis (Bufonidae). Our new approach combines synchronized tadpole exposure to three EE2-concentrations (50, 500, 5,000 ng/L) in a flow-through-system and pioneers genetic and histological sexing of metamorphs in non-model anurans for EDC-studies. This novel methodology reveals striking quantitative differences in genetic-male-to-phenotypic-female sex reversal in non-model vs. model species. Our findings qualify molecular sexing in EDC-analyses as requirement to identify sex reversals and state-of-the-art approaches as mandatory to detect species-specific vulnerabilities to EDCs in amphibians.

Genetic structure of the fire salamander Salamandra salamandra in the Polish Sudetes

We analysed genetic variation within and differentiation between nineteen populations of the fire salamander Salamandra salamandra inhabiting the north-eastern margin of the species range in the Sudetes Mountains (south-western Poland). The results were compared with those obtained recently for the Polish part of the Carpathians. Variation of 10 nuclear microsatellite loci was analysed in 744 individuals to estimate genetic structure, gene flow, isolation and to test for a geographic gradient of genetic variation. Mitochondrial DNA control region (D-loop) of 252 specimens from all localities was used to identify the origin of populations currently inhabiting its north-eastern range. We found little genetic differentiation among populations in the Sudetes indicating substantial recent or ongoing gene flow. The exceptions were one isolated peripheral population located outside the continuous distribution range which displayed extremely reduced genetic variation probably due to a combination of long term isolation and low population size, and one population located at the eastern margin of the Polish Sudetes. Populations inhabiting the Sudetes and the Carpathians formed two separate clusters based on microsatellite loci. In accordance with available phylogeographic information, single mitochondrial haplotype (type IIb) fixed in all populations indicates their origin from a single refugium and may suggest colonization from the Balkan Peninsula. The analysis of geographic gradient in variation showed its decline in the westerly direction suggesting colonization of Poland from the east, however, alternative scenarios of postglacial colonization could not be rejected with the available data.

Genetic structure and differentiation of the fire salamander Salamandra salamandra at the northern margin of its range in the Carpathians

Amphibian populations occurring at the margin of the species range exhibit lower genetic variation due to strong genetic drift and long-term isolation. Limited mobility and site fidelity together with habitat changes may accelerate genetic processes leading to local extinction. Here, we analyze genetic variation of the fire salamander subspecies Salamandra s. salamandra inhabiting the Outer Carpathian region in Poland, at the northern border of its distribution. Nuclear DNA polymorphism based on 10 microsatellite loci of 380 individuals sampled in 11 populations were analysed to measure gene flow between subpopulations and possible long-term isolation. Mitochondrial DNA control region analysis among 17 individuals representing 13 localities was used to detect the origin of populations which colonized Northern Europe after the last glaciation. Overall, pairwise FST’s and AMOVA test of ‘among group’ variation showed little differences in the allele frequencies and relatively high local gene flow. However, Bayesian clustering results revealed subtle structuring between eastern and western part of the studied region. Two extreme marginal populations from the Carpathian Piedmont revealed reduced genetic variation which may be attributed to strong influence of genetic drift. Only one mitochondrial DNA haplotype (type IIb) was found in all individuals and suggest that after the Last Glacial Maximum Salamandra salamandra migrated to the North-Western Europe from the single glacial refugium placed in the Balkan Peninsula.

Sex-chromosome differentiation parallels postglacial range expansion in European tree frogs (Hyla arborea)

Occasional XY recombination is a proposed explanation for the sex-chromosome homomorphy in European tree frogs. Numerous laboratory crosses, however, failed to detect any event of male recombination, and a detailed survey of NW-European Hyla arborea populations identified male-specific alleles at sex-linked loci, pointing to the absence of XY recombination in their recent history. Here, we address this paradox in a phylogeographic framework by genotyping sex-linked microsatellite markers in populations and sibships from the entire species range. Contrasting with postglacial populations of NW Europe, which display complete absence of XY recombination and strong sex-chromosome differentiation, refugial populations of the southern Balkans and Adriatic coast show limited XY recombination and large overlaps in allele frequencies. Geographically and historically intermediate populations of the Pannonian Basin show intermediate patterns of XY differentiation. Even in populations where X and Y occasionally recombine, the genetic diversity of Y haplotypes is reduced below the levels expected from the fourfold drop in copy numbers. This study is the first in which X and Y haplotypes could be phased over the distribution range in a species with homomorphic sex chromosomes; it shows that XY-recombination patterns may differ strikingly between conspecific populations, and that recombination arrest may evolve rapidly (<5000 generations).

Gonadal sex differentiation in frogs: how testes become shorter than ovaries

Testis differentiation in anuran amphibians is the result of two opposing processes: degeneration of the distal part, and development of the proximal part, which becomes a functional male gonad. Undifferentiated gonad differentiates directly into a testis without a transition phase. We described the morphology of developing testes in Rana temporaria and Hyla arborea, and made careful histology and ultrastructure in Pelophylax lessonae. The developing testis was divided into 10 stages (I-III, undifferentiated gonad, IV-X, testis). The earliest morphological symptoms of testis differentiation were observed in 4- to 5-week-old tadpoles at Gosner stage 27-28. At that time an undifferentiated gonad, composed of 6-9 metameres, differentiates into a testis. The proximal metameres (2-3 in the right gonad and 3-4 in the left one) differentiate into a functional testis, while the distal ones degenerate. The difference between left and right gonad size is maintained until adulthood (stage X). Degeneration of the distal part progresses along the posterior-anterior gradient and starts at stage IV. It affects first the germ cells with accompanying precursor Sertoli cells, and then the mesenchymal cells and fibroblasts. Finally the external epithelium forms a "sleeve" around the almost empty distal part. The total length of the testes stays the same until stage VIII at Gosner stage 41 (age 74-148 days). Active spermatogenesis starts at stage IX (juveniles after their first hibernation), during which the distal part eventually disappears and the proximal part starts growing considerably due to progressing spermatogenesis.

Maintenance of ancestral sex chromosomes in Palearctic tree frogs: direct evidence from Hyla orientalis

Contrasting with the situation found in birds and mammals, sex chromosomes are generally homomorphic in poikilothermic vertebrates. This homomorphy was recently shown to result from occasional X-Y recombinations (not from turnovers) in several European species of tree frogs (Hyla arborea, H. intermedia and H. molleri). Because of recombination, however, alleles at sex-linked loci were rarely diagnostic at the population level; support for sex linkage had to rely on multilocus associations, combined with occasional sex differences in allelic frequencies. Here, we use direct evidence, obtained from anatomical and histological analyses of offspring with known pedigrees, to show that the Eastern tree frog (H. orientalis) shares the same pair of sex chromosomes, with identical patterns of male heterogamety and complete absence of X-Y recombination in males. Conservation of an ancestral pair of sex chromosomes, regularly rejuvenated via occasional X-Y recombination, seems thus a widespread pattern among Hyla species. Sibship analyses also identified discrepancies between genotypic and phenotypic sex among offspring, associated with abnormal gonadal development, suggesting a role for sexually antagonistic genes on the sex chromosomes.

A stockpile of ova in the grass frog Rana temporaria is established once for the life span. Do ovaries in amphibians and in mammals follow the same evolutionary strategy?

Most anuran amphibians produce high numbers of eggs during several consecutive breeding seasons. The question is still open whether oocytes are formed anew as a result of oogonial proliferation after each spawning or the definitive pool of oocytes is established during the juvenile period and is sufficient for the whole reproductive life span of a female. Our quantitative studies show that primary oogonia in adult female frogs can proliferate, but they fail to differentiate further and do not enter meiosis, and thereby there is no supplementation of new generations of oocytes after each spawning. Ovaries of one-year-old grass frogs contain (median) 53,447 diplotene oocytes, in two-years-old frogs this number decreased to 33,583 and eventually reached 25,679 in virgin mature females. More than 50% decrease in the total oocyte number was accompanied by massive degeneration (atresia) of oocytes. The final number of oocytes in a female forms a stock for 11-12 breeding seasons and exceeds the number of eggs produced during the potential reproductive life span of this species. The phylogenetic context of oocyte recruitment modes in the major clades of vertebrates is discussed in respect to their ability to replenish the stock (a renewable stock in ovaries named "open" vs. a non-renewable stock in ovaries named "closed").

Genome dosage effect and colouration features in hybridogenetic water frogs of the Pelophylax esculentus complex

There are three taxons of central European water frogs of the Pelophylax esculentus complex: two morphologically distinct species, Pelophylax lessonae (LL) and Pelophylax ridibundus (RR), and hybrids Pelophylax esculentus, which can be either diploid (RL) or triploid (LLR or RRL). The morphology of hybrids is supposed to follow genome dosage effect. We describe colouration of water frogs with genome composition verified by chromosome analysis. Typical colouration features in LL were: spots on the ventral side, brown limbs, “weak” femur spotting pattern, brown dorsal folds and yellow-green colour in “waist”. Typical RR had dark-green or olive-green limbs, “full” femur spotting pattern, no hip spot and no yellow colour in “waist”. For all hybrids the most typical features were strong spots on the ventral side and a mosaic of green and brown colour on limbs. Typical LLR had brown-greenish dorsal folds underlined by a partial black line, “weak” femur spotting pattern and yellow-greenish colour in “waist” and on femur. Typical RL had greenish hind limbs, green dorsal folds, no yellow colour in “waist”, and no hip spot. Typical RRL was similar to RL, but had a continuous black line under dorsal folds. There were, however, numerous exceptions to these trends, with the most prominent being much higher than expected variability of colouration of Pelophylax lessonae. Therefore caution must be advised when trying to estimate genome composition of water frogs solely on the basis of colouration.

A simplified molecular method for distinguishing among species and ploidy levels in European water frogs (Pelophylax)

Western Palearctic water frogs in the genus Pelophylax are a set of morphologically similar anuran species that form hybridogenetic complexes. Fully reliable identification of species and especially of hybrid ploidy depends on karyological and molecular methods. In central Europe, native water frog populations consist of the Pelophylax esculentus complex, that is, P. lessonae (LL), P. ridibundus (RR) and the hybrid form P. esculentus that can have different karyotypes (RL, LLR and RRL). We developed existing molecular methods further and propose a simple PCR method based on size-differences in the length of the serum albumin intron-1 and the RanaCR1, a non-LTR retrotransposon of the chicken repeat (CR) family. This PCR yields taxon-specific banding patterns that can easily be screened by standard agarose gel electrophoresis and correctly identify species in all of the 160 samples that had been identified to karyotype with other methods. To distinguish ploidy levels in LR, LLR and RRL specimens, we used the ratio of the peak heights of the larger (ridibundus specific) to the smaller (lessonae specific) bands of fluorescently labelled PCR products resolved on a capillary DNA sequencer and obtained a correct assignment of the karyotype in 93% of cases. Our new method will cut down time and expenses drastically for a reliable identification of water frogs of the P. esculentus complex and potentially for identification of other hybridogenetic complexes and/or taxa, and it even serves as a good indicator of the ploidy status of hybrid individuals.

Cryptic diversity among Western Palearctic tree frogs: postglacial range expansion, range limits, and secondary contacts of three European tree frog lineages (Hyla arborea group)

We characterize divergence times, intraspecific diversity and distributions for recently recognized lineages within the Hyla arborea species group, based on mitochondrial and nuclear sequences from 160 localities spanning its whole distribution. Lineages of H. arborea, H. orientalis, H. molleri have at least Pliocene age, supporting species level divergence. The genetically uniform Iberian H. molleri, although largely isolated by the Pyrenees, is parapatric to H. arborea, with evidence for successful hybridization in a small Aquitanian corridor (southwestern France), where the distribution also overlaps with H. meridionalis. The genetically uniform H. arborea, spread from Crete to Brittany, exhibits molecular signatures of a postglacial range expansion. It meets different mtDNA clades of H. orientalis in NE-Greece, along the Carpathians, and in Poland along the Vistula River (there including hybridization). The East-European H. orientalis is strongly structured genetically. Five geographic mitochondrial clades are recognized, with a molecular signature of postglacial range expansions for the clade that reached the most northern latitudes. Hybridization with H. savignyi is suggested in southwestern Turkey. Thus, cryptic diversity in these Pliocene Hyla lineages covers three extremes: a genetically poor, quasi-Iberian endemic (H. molleri), a more uniform species distributed from the Balkans to Western Europe (H. arborea), and a well-structured Asia Minor-Eastern European species (H. orientalis).

Evidence for integrity of parental genomes in the diploid hybridogenetic water frog Pelophylax esculentus by genomic in situ hybridization

The Western Palearctic water frogs Pelophylax ridibundus and P. lessonae were identified as parental (sexual) species and P. esculentus as their interspecific, hybridogenetically reproducing hybrid with hemiclonal heredity. We used genomic in situ hybridization (GISH) to identify parental chromosomes of P.lessonae and P.ridibundus in diploid P. esculentus karyotypes (2n = 26). GISH probes were made by fluorochrome labeling of total genomic DNA extracted from the sexual progenitors. The labeled probe from one species was hybridized to chromosomes of P. esculentus in the presence of excess of unlabeled genomic DNA from the other species. Thus, the P. lessonae probe was blocked by P. ridibundus unlabeled DNA, and vice versa. We successfully discriminated each of the 13 respective parental chromosomes in metaphase complements of the hybrids according to species-specific hybridization signals. GISH enabled us to confirm additional differences between parental chromosomes in size (smaller chromosomes belong to P. lessonae) and in the presence of DAPI-positive centromeric heterochromatin (detected in chromosomes of P. ridibundus, but not in P. lessonae). The fact that no visible intergenomic exchanges were found in metaphase chromosomes of diploid P. esculentus provides important information on the genomic integrity of hemiclonal transmission and supports hybridogenesis as a reproductive mode at the chromosome level for the specimens examined.

Age structure, size and growth rate of water frogs from central European natural Pelophylax ridibundus-Pelophylax esculentus mixed populations estimated by skeletochronology

Abstract

Central European water frog Pelophylax esculentus (formerly known as Rana esculenta) is a natural hybrid between P. lessonae and P. ridibundus. The hybrids reproduce by hybridogenesis and usually share populations with one of the parental species. Natural ridibundus-esculentus (R-E) mixed populations are rare. The population described herein is composed of 80% P. ridibundus and 20% P. esculentus represented by both sexes. We analyzed 159 adults and 228 juveniles. Age of adults collected from breeding sites ranged from 2 to 6 years in males and from 3 to 7 years in females of both taxa. The percentage of individuals older than 5 years was low. Average age of P. ridibundus was higher than that of P. esculentus. In P. ridibundus the average age of females was higher than that of males. In P. esculentus the difference between ages of females and males was not significant. Measurements of yearly radial growth of long bones revealed that the frogs grew intensively before reaching sexual maturity (3 years for females and 2 years for males). In the group of juveniles before I hibernation, P. esculentus were significantly bigger than P. ridibundus, however, there was no difference in body size between both taxa after I hibernation i.e., before the start of a new growth season. Mean LAG-1 diameters were significantly greater in adults P. ridibundus than in juveniles after I hibernation, but not in P. esculentus.

Differential expression of two vasa/PL10-related genes during gametogenesis in the special model system Rana

Germline cell fate decisions are primarily controlled at the post-transcriptional level with DEAD-box RNA helicases playing a crucial role in germline development. In this study, we report the identification of two DEAD-box vasa/PL10 orthologues (RlVlg and RlPL10) in a species complex of the genus Rana, characterized by hybridogenetic reproduction, an enigmatic process that involves the exclusion of an individual genome, and endoreduplication events. Both genes were expressed during the early stages of gametogenesis of R. ridibunda, R. lessonae, and their natural hybrid R. esculenta. RlVlg expression was germline specific. On the other hand, RlPL10 was also expressed in somatic tissues, although only at low levels. The two genes were expressed in different phases of mitotic and meiotic spermatogenetic divisions as demonstrated by immunostaining with an anti-H3 phosphohistone antibody. The data indicate that RlVlg and RlPL10 may represent useful markers for dissecting the molecular aspects of genome exclusion and endoreduplication of the hybridogenetic gametogenesis.

Development and growth of long bones in European water frogs (Amphibia: Anura: Ranidae), with remarks on age determination

Differentiation and development of long bones were studied in European water frogs: Rana lessonae, R. ridibunda, and R. esculenta. The study included premetamorphic larvae (Gosner Stage 40) to frogs that were 5 years old. Femora, metatarsal bones, and proximal phalanges of the hindlimb exhibit the same pattern of periosteal bone differentiation and the same pattern of growth. Longitudinal and radial growth of these bones was studied by examination of the diaphyses and epiphyses, particularly where the edge of periosteal bone is inserted into the epiphysis. The periosteum seems to be responsible for both longitudinal and radial growth. Investigation of the formation, length, and arrangement of lines of arrested growth reveals that the first line is present only in the middle 25-35% of the length of the diaphysis of an adult bone; therefore, only the central portion of the diaphysis should be used for age estimation in skeletochronological studies. Comparison of the shapes and histological structures of epiphyses in the femur, metatarsal bones, and phalanges revealed that epiphyseal cartilages are composed of an inner and outer part. The inner metaphyseal cartilage has distinct zones and plugs the end of the periosteal bone cylinder; its role in longitudinal growth is questioned. The outer epiphyseal cartilage is composed of articular cartilages proper, in addition to lateral articular cartilages. Differences in the symmetry of the lateral articular cartilages of distal epiphyses of the femur and toes may reflect adaptations to different kinds of movements at the knee and in the foot.

DNA content and genome composition of diploid and triploid water frogs belonging to the Rana esculenta complex (Amphibia, Anura)

The Central European water frog Rana esculenta L., 1758 is a natural hybrid between Rana lessonae Camerano, 1882 (LL) and Rana ridibunda Pallas, 1771 (RR). Hybrids are usually diploid (RL) or triploid (LLR or RRL). Distinguishing LL from RL, RR from RL, and LLR from RRL according to external morphology is ambiguous. In this study we checked whether the DNA content in erythrocyte nuclei measured by image cytometry is useful in determination of the taxonomic status of diploids (LL, RR, and RL) and the genome composition of triploids (LLR and RRL). For exact and direct identification of parental species, as well as for determination of genome composition in hybrids, we applied actinomycin D – 4',6-diamidino-2-phenylindole chromosome staining to metaphase plates. We analyzed 43 LL, 12 RR, and 32 RL diploids, and 37 LLR and 19 RRL triploids. All diploid hybrids had 2n = 26 chromosomes, and all triploid hybrids had 3n = 39 chromosomes. Neither aneuploid nor mosaic hybrids were detected. The expected numbers of 13 R. lessonae (L) and 13 R. ridibunda (R) chromosomes in RL hybrids were recorded in about 31% of individuals. In the rest of the sample the composition was variable, ranging from 9 to 14 R chromosomes and the corresponding number of L chromosomes. The expected composition of 26 L and 13 R chromosomes was detected in about 32% of LLR triploids, whereas in the rest of the sample the composition of chromosomes ranged from 8 to 15 R chromosomes and the corresponding number of L chromosomes. The expected numbers of 26 R and 13 L chromosomes were detected in about 26% of RRL triploids, whereas in the rest of the sample the composition of chromosomes ranged from 19 to 28 R chromosomes and the corresponding number of L chromosomes. The DNA content densitometry showed that RR and RL diploids had 9.5% and 3.8% more DNA, respectively, than LL diploids. These differences, although statistically significant, were not sufficient to unequivocally discriminate LL from RL and RR from RL. Triploids had about 50% more DNA than LL diploids (49% in LLR and 51% in RRL), but these differences were too small for unequivocal determination of their genome composition.

Pattern and rate of ovary differentiation with reference to somatic development in anuran amphibians

The rate of somatic development of anuran amphibians is only roughly correlated with the rate of gonad differentiation and varies among species. The somatic stage of a tadpole often does not reflect its age, which seems to be crucial for gonad differentiation rate. We compared the morphology and differentiation of developing ovaries at the light and electron microscopy level, with reference to somatic growth and age of a female. Our observations were performed on 12 species of six families (Rana lessonae, R. ridibunda, R. temporaria, R. arvalis, R. pipiens, R. catesbeiana, Bombina bombina, Hyla arborea, Bufo bufo. B. viridis, Xenopus laevis, Pelobates fuscus) and compared with the results obtained by other authors. This allowed us to describe the unified pattern of anuran female gonad differentiation. Ovary differentiation was divided into 10 stages: I-III, undifferentiated gonad; IV, sexual differentiation; V, first nests of meiocytes; VI, first diplotene oocytes; VII-IX, increasing number of diplotene oocytes and decreasing number of oogonia and nests; X, fully developed ovary composed of diplotene oocytes with rudimental patches of oogonia. We distinguished three types of ovary differentiation rate: basic (most species), retarded (genus Bufo), and accelerated (green frogs of the subgenus Pelophylax genus Rana).

Development of testes and differentiation of germ cells in water frogs of the Rana esculenta - complex (Amphibia, Anura)

Abstract

The European water frog, Rana esculenta, is a hybrid whose genome is composed of haploid chromosome sets of its parental species R. lessonae and R. ridibunda. Prior to meiosis one of the parental sets is discarded and the other is duplicated (hybridogenesis). In the parental species sex differentiation begins at tadpole stages 28-30 (Gosner, 1960), at stages 30-36 the testes are composed of proliferating pale spermatogonia 1°. At stages 36-39 a new class of spermatogonia I° (dark) appears. Before first hibernation, seminiferous lobules are filled with cysts containing germ cells at various stages of spermatogenesis up to elongating spermatids. In R. esculenta gonad development is affected from the earliest stages: the gonads are smaller and composed of reduced number of spermatogonia I°. The phase of pale spermatogonia I° proliferation is prolonged up to the second year of life. The structure of the gonads, as well as that of germ cells themselves, are often abnormal.

Spontaneous haploid-triploid mosaicism in the progeny of a Rana kI. esculenta female and Rana lessonae males

Abstract

A female Rana kl. esculenta was crossed with three different R. lessonae males. Three froglets with unexpected phenotypes appeared. Two of them were chimerae with one side of the body of lessonae, and the other of escutenta phenotype, and one displayed lessonae, instead of the expected esculenta, phenotype. Blood of three individuals was composed of small haploid and large triploid erythrocytes. These results indicate that the froglets were 1n/3n mosaics.

The fate of intramitochondrial paracrystalline inclusion bodies in germ line cells of water frogs (Amphibia, Anura)

Numerous intramitochondrial paracrystalline inclusion bodies (ICIB) were observed in the germinal plasm of a mid-blastula, and in primordial germ cells (PGCs) after their migration to the germinal ridges, in Rana ridibunda, R. lessonae and R. esculenta. In oogonia the number of ICIB decreases rapidly. Single ICIB are observed in the germ cells up to the leptotene stage; they have never been observed in pachytene oocytes. In diplotene oocytes that have reached a diameter of about 100 microns ICIB are visible again, and their number increases concomitantly with oocyte growth.