Gamete types, sex determination and stable equilibria of all-hybrid populations of diploid and triploid edible frogs (Pelophylax esculentus)

BACKGROUND: Triploid individuals often play a key role in speciation by hybridization. An understanding of the gamete types (ploidy and genomic content) and stability of hybrid populations with triploid individuals is therefore of importance for exploring the role of hybridization in evolution. The all-hybrid populations of the edible frog, Pelophylax esculentus, are unique in their composition and genetic dynamics: Diploid (genotype LR) and triploid (LLR and LRR) hybrids depend on each other's different gamete contributions for successful reproduction and maintenance of the populations, as the parental genotypes P. lessonae (LL) and P. ridibundus (RR) are absent among adults. This study provides data and interpretations on gamete types and sex determination that are essential for understanding the function, evolutionary potential and threats of this intriguing system. RESULTS: Dissection of metamorphs from a crossing experiment confirmed that sex determination is an XX-XY system with the Y confined to the L genome. From microsatellite analysis of parents and offspring from the crossings, gamete frequencies could be deduced: Triploids of both sexes mostly made haploid gametes with the genome they had in double dose, however LLR females also made approximately 10% LL gametes by automixis. LR frogs showed much variation in their gamete production. In LRR-rich populations, their LR sperm production was sufficiently high (22%) to explain the observed proportion of LRR males, the formation of which has not previously been understood. A model was constructed to calculate equilibrium genotype proportions for different population types on the basis of the gamete proportions found. These equilibria agreed well with empirical literature data. CONCLUSION: If population differentiation with respect to genotype proportions is really driven by gamete patterns, as strongly suggested by the present study, all-hybrid populations constitute not one, but several intrinsically different breeding systems. Tetraploidization could occur if the survival or fertility of both males and females increased. Whether introduction of hybrid or parental species individuals would threaten the all-hybrid populations cannot be predicted without further knowledge on the mechanisms behind non-hybrid inviability, but at least R genomes with Y factor are predicted to be invasive, if introduced, and could bring the populations to collapse

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

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Coexistence of diploid and triploid hybrid water frogs: population differences persist in the apparent absence of differential survival

Background: The role of differential selection in determining the geographic distribution of genotypes in hybrid systems has long been discussed, but not settled. The present study aims to asses the importance of selection in structuring all-hybrid Pelophylax esculentus populations. These populations, in which the parental species (P. lessonae with genotype LL and P. ridibundus with genotype RR) are absent, have pond-specific proportions of diploid (LR) and triploid (LLR and LRR) genotypes. Results: With data from 12 Swedish ponds, we first show that in spite of significant changes in genotype proportions over time, the most extreme ponds retained their differences over a six year study period. The uneven distribution of genotypes among ponds could be a consequence of differential selection varying among ponds (selection hypothesis), or, alternatively, of different gamete production patterns among ponds (gamete pattern hypothesis). The selection hypothesis was tested in adults by a six year mark-recapture study in all 12 ponds. As the relative survival and proportion of LLR, LR and LRR did not correlate within ponds, this study provided no evidence for the selection hypothesis in adults. Then, both hypotheses were tested simultaneously in juvenile stages (eggs, tadpoles, metamorphs and one year old froglets) in three of the ponds. A gradual approach to adult genotype proportions through successive stages would support the selection hypotheses, whereas the presence of adult genotype proportions already at the egg stage would support the gamete pattern hypothesis. The result was a weak preference for the gamete pattern hypothesis. Conclusions: These results thus suggest that selection is of little importance for shaping genotype distributions of all-hybrid populations of P. esculentus, but further studies are needed for confirmation. Moreover, the study provided valuable data on genotype-specific body lengths, adult survival and sex ratios

Sex-related differences in aging rate are associated with sex chromosome system in amphibians

Sex-related differences in mortality are widespread in the animal kingdom. Although studies have shown that sex determination systems might drive lifespan evolution, sex chromosome influence on aging rates have not been investigated so far, likely due to an apparent lack of demographic data from clades including both XY (with heterogametic males) and ZW (heterogametic females) systems. Taking advantage of a unique collection of capture-recapture datasets in amphibians, a vertebrate group where XY and ZW systems have repeatedly evolved over the past 200 million years, we examined whether sex heterogamy can predict sex differences in aging rates and lifespans. We showed that the strength and direction of sex differences in aging rates (and not lifespan) differ between XY and ZW systems. Sex-specific variation in aging rates was moderate within each system, but aging rates tended to be consistently higher in the heterogametic sex. This led to small but detectable effects of sex chromosome system on sex differences in aging rates in our models. Although preliminary, our results suggest that exposed recessive deleterious mutations on the X/Z chromosome (the "unguarded X/Z effect") or repeat-rich Y/W chromosome (the "toxic Y/W effect") could accelerate aging in the heterogametic sex in some vertebrate clades.Peer reviewe

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

The western Palaearctic water frogs in the genus Pelophylax comprise several distinct species and three hybridogenetic hybrid forms. In this study, we focus on the Pelophylax esculentus complex, which consists of two sexual species, Pelophylax ridibundus and Pelophylax lessonae, and their hybridogenetic hybrid, Pelophylax esculentus. Specifically, we investigated taxonomic composition and ploidy level of water frogs sampled in three different types of wetland habitats in the Hortobagy National Park (HNP), eastern Hungary. Using variation in serum albumin intron 1 (SAI-1) and 15 microsatellite loci, we detected the presence of all members of the P. esculentus complex in the studied localities. In one locality, all three taxa occurred syntopically, while in others water frog populations consisted of P. ridibundus and P. esculentus exclusively. The genomic composition of the 63 examined hybrid specimens analysed with microsatellites showed the occurrence of diploid genotypes only. We used a population genetic approach (allelic richness, gene diversity, multilocus genotypes and multilocus disequilibrium) to infer the breeding system of water frogs at HNP. Our data indicate that at least in two populations, hybrids form gametes with clonally transmitted P. ridibundus genome and produce a new hybrid generation by mating with P. lessonae

Fajösszetétel és ploiditás vizsgálata a kelet-magyarországi zöld békákon (Pelophylax)

Pelophylax génuszba tartozó és Nyugat-palearktiszi elterjedéssel bíró zöldbékák közé jelenleg 12 fajtsorolunk, amelyek egy részénél megfigyelhető hibridogenetikus fajkomplexek jelenléte. Vizsgálatunkbana Magyarországon is előforduló kecskebéka fajkomplexre (Pelophylax esculentus komplex) irányítottukfigyelmünket. Ennek a komplexnek a tagjai a tavi béka (Pelophylax ridibundus, genotípus RR) a kis tavibéka (Pelophylax lessonae, genotípus LL) illetve ezen két szülői faj hibridizációjából létrejövő kecskebéka(Pelophylax esculentus, genotípus LR). A hibrid egyedek a szülői fajoktól morfológiai jellegeik alapjánnehezen elkülöníthetőek, ezért molekuláris taxonómiai megközelítést (polimorfia detektálása a szérumalbuminintron 1 génben) használtunk arra, hogy az egyes élőhelyeken a fajösszetételt meghatározzuk,továbbá a hibrid egyedek genotípusát mikroszatellita vizsgálattal feltárjuk, majd populáció genetikai indexekszámításával következtessünk a szaporodási rendszerükre. A három éves mintagyűjtés során 164 zöldbékától vettünk DNS mintát a Hortobágyi Nemzeti Park (HNP) három különböző élőhelyi paraméterekkelrendelkező vizes élőhelyén. A komplex mind a három tagjának jelenlétét észleltük a mintavételi területen.Egyik vizes élőhelyen mindhárom taxon együtt fordult elő, míg a másik két élőhelyen a tavi béka és akecskebéka együttes előfordulását detektáltuk. A ploiditás vizsgálat során 63 hibrid kecskebéka genotípusátanalizáltuk és azt találtuk, hogy az egyedek diploid (LR) genotípussal rendelkeztek. Triploid genotípusú(LLR és LRR) békákat nem találtunk. Az eredményeink azt indikálják, hogy a három élőhelyből legalábbkettőben LE (lessonae-esculenta) szaporodási rendszert figyelhetünk meg, ahol a hibridek a ridibundus(R) genomjaikat klonális módon adják tovább a következő hibrid generációnak és csak a kis tavi békávalvisszakereszteződve képes sikeresen szaporodni

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