Introduced Rana ridibunda currently replace the native waterfrogs R. lessonae and R. esculenta in several areas of central Europe. The unusual reproductive system in waterfrogs of the Rana esculenta complex suggests that this replacement may be driven by a genetic mechanism: Rana esculenta, a hybrid between R. ridibunda and R. lessonae, eliminates the lessonae genome from the germline and clonally transmits the ridibunda genome (hybridogenesis). Hybrids form mixed populations with R. lessonae (L-E-system) in which they persist by backcrossing with the parental species. Matings between hybrids are unsuccessful, because their ridibunda genomes contain fixed recessive deleterious mutations. When introduced into a L-E-system, R. ridibunda can mate with both native taxa, producing R. ridibunda offspring with R. esculenta,and R. esculenta offspring with R. lessonae (primary hybridizations). If primary hybrids are hybridogenetic, they produce viable R. ridibunda offspring in matings with other hybrids, because their clonal genomes are unlikely to share the deleterious alleles present in the ancient clones. Thus, R. ridibunda will increase in the population at the expense of both native taxa, eventually leaving a pure R. ridibunda population. We provide three lines of evidence for this process from a currently invaded population in Switzerland: (1) Primary hybridizations take place, as roughly 10% of hybrids in the population possess ridibunda genomes derived from the introduced frogs. (2) Hybridogenesis occurs in primary hybrids, although at a low frequency. (3) Many hybrid × hybrid matings in the population indeed produce viable offspring. Hence, the proposed genetic mechanism appears to contribute to the species replacement, although its importance may be limited.
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273The pool frog, Rana lessonae, has historically been considered an introduction into Britain, with Italy the most likely source. Recently, the possibility of native status for a Norfolk pool frog population was raised. We used random amplified DNA (RAPD) analyses to clarify the status issue. Nine arbitrarily designed primers detected 160 polymorphisms in 174 pool frogs from 11 European locations. Polymorphism levels varied widely, being lowest in the northern populations and highest in the south. As with many isolated populations, the peninsular and insular populations of Sweden, Norway and Britain showed relatively little polymorphism. Principal component and cluster analysis showed clear geographical groupings. The Norfolk, Norwegian and Swedish individuals formed a closely related group a northern clade, substantiating native status for the Norfolk population, which, interestingly, had idiosyncratic features suggestive of a distinctive evolutionary history. Phylograms constructed from RAPD data were consistent with an unexpected postglacial colonization route, in which the northern clade derived from migration out of Italy, via Hungary and Poland, though cryptic glacial refugia in Eastern Europe are an alternative explanation. Our RAPD analyses concur with a parallel microsatellite investigation. Both genetic studies and bioacoustic and archaeozoological findings support native status for Norfolk pool frogs and have prompted a programme for re-establishing the northern clade in England.
We tested for environmental factors that may lead to balancing selection and to the maintenance of a genetic polymorphism at the enzyme locus lactate dehydrogenase B (LDH-B) in the pool frog, Rana lessonae. We raised tadpoles individually in a factorial experiment in which we manipulated temperature, food level, and food quality. The only statistically significant difference among LDH-B genotypes was in growth rate, with the heterozygote performing best. Although the difference was not significant, heterozygotes also tended to perform best for size at metamorphosis. However, heterozygotes did not perform best in terms of other traits (age at metamorphosis and rates of survival and metamorphosis), where differences among LDH-B genotypes were also not significant. The size of the effect of LDH-B genotype depended on the environment, which suggests that the locus may be selectively neutral in some environments. There were no genotype–environment interactions in the sense that reaction norms along environmental gradients did not cross. When we raised tadpoles in groups, e/e homozygotes had a significantly higher body mass and developed at the significantly highest rate. In addition, there may be a trade-off between larval and adult performance: adult frogs show a different ranking in performance of LDH-B genotypes than tadpoles do. These results suggest that this genetic polymorphism is maintained through heterozygote advantage, possibly in conjunction with antagonistic pleiotropy.
Based on material collected in 2001 – 2002, The diet of R. ridibunda in Mordovia included 200 different species. Arthropoda (Insecta, Arachnida) were common. Among vertebrates, the consumption of tailless amphibians and fish indicate well-developed cannibalism in this species. Mammals were also a part of the diet.
По материалам, собранным в 2001 – 2002 гг., в пище озерной лягушки встречаются около 200 различных видов животных, среди которых преобладают бесхвостые земноводные, что указывает на довольно сильный развитый каннибализм. В диету также входят различные виды рыб и млекопитающих.
Water frogs are involved in several hybridogenetic complexes. However, investigations in Southern and Western France produced evidence of new hybridization events between R. lessonae and R. perezi (some new hybrids tadpoles were discovered), and the existence of new assemblages [R-G, L-P, R-P, P-R-G: (R, ridibunda; G, grafi; L, lessonae; P, perezi)]. In respect to hybridogenesis, these assemblages offer opportunities for both primary and secondary hybridizations. Such peculiarities may be explained either by introductions or by relic populations and may influence water frog evolution.
