Hybridogenetic water frogs

The genome of R. ridibunda is the most widespread water frog genome in Europe. It appears not only in the "good" species R. ridibunda, but also in hybrids, which originated by matings of R. ridibunda with other "good" species. In contrast to other interspecies hybrids, such bastards are fertile, but reproductively behave more like R. ridibunda than the other parental species, as it almost exclusively produces gametes containing the R. ridibunda genome. This reproductive peculiarity was first described for the fish genus Poeciliopsis and later on also proved for the genus Rana and was named hybridogenesis. For hybridogenetic gametogenesis it was assumed that the non-R. ridibunda genomes is excluded prior to meiosis, with a subsequent endoreduplication of the remaining R. ridibunda genome conducted to the gametes.

For evolutionary reasons it was discussed earlier that the hybridogenetic hybrids could be vectors for the R. ridibunda genome. Such hypothesis becomes even more interesting, if not only R. ridibunda itself induces hybridogenesis, but also its hybridogenetic associates R. grafi (parental species: R. perezi), R. esculenta (p. s.: R. lessonae) or R. hispanica (p. s.: R. bergeri) and therefore play a important role for the distribution of the R. ridibunda genome. The multiple combinations of the R. ridibunda genome with other water frog taxa yield a wide environmental plasticity. Hence, the R. ridibunda genome might distribute far beyond the borders of the different hybridogenetic systems. As introgression between the synoptic non-R. ridibunda taxon and the R. ridibunda genome has been proved by different authors, it also contains the possibility to gain genes from different "good" species.

 

Scandinavian populations of the common frog Rana temporaria

Understanding biodiversity requires understanding of the processes creating and maintaining this diversity. Studies of marginal populations, and their role in speciation and diversification processes, have had a central role in the development of the modern evolutionary synthesis (Mayr 1963). Despite of this relationship between emergence of modern evolutionary biology and study of marginal populations, we are still largely ignorant of possible major intra-specific evolutionary diversifications within species occupying large geographical ranges (e.g. Avise 1994; Conover & Schultz 1995; Foster 1999). One reason for this is that autecological studies of marginal populations are often logistically difficult, not least because they often inhabit areas not easily accessible for researchers. Furthermore, since even the most elementary background information is typically lacking, it is difficult to devise research strategies and plans to carry out autecological investigations in marginal populations.

The common frog (Rana temporaria) is geographically the most widely distributed amphibian species of Europe (Gasc et al. 1997; Fig. 1). Latitudinally, its distribution range spans from the Pyrenees and northern Greece to North Cape and Barent's Sea in the north. Longitudinally, it occurs from Spanish west coast beyond the Ural mountains in east. It also inhabits a wide range of altitudes occurring from sea level up to 2660 m in the Pyrenees (Gasc et al. 1997) and up to 1000 m in Scandinavian mountains (Fog et al. 1997). Because the species is abundant throughout of its distribution range, it has been subject to a number of detailed studies conducted in temperate regions (e.g. Haapanen 1970; Elmberg 1990, 1991; Ryser 1996; Miaud et al. 1999). However, as to the most northern part of its distribution range north of the Arctic Circle, hardly anything is known about basic biology of adult common frogs (Merilä 2000). Our studies of larval stages show that the northern common frogs have diverged genetically from their southern conspecifics in many key life history traits (e.g. Merilä et al. 2000; Laurila et al. 2001; Timenes Laugen et al. 2001). Preliminary studies indicate that also many aspects of adult life histories (e.g. maturation patterns, extrinsic mortality rates, longevity, body size), behaviours (e.g. diurnal activity patterns) and sex ratios among adults are dramatically different for subarctic frogs as compared to their southern conspecifics (Merilä 2000; see below).

 

Reproductive success

The challenge for poikilothermic animals posed by cold climate and short summers is reflected in the fact that only one out of the 209 species of European amphibians has managed to establish viable populations into the subarctic zone. Our preliminary investigations of biology of the common frog populations in subarctic Finland have revealed a number of interesting features, which differentiate these northern populations from the southern conspecifics.

In particular, detailed life-table analyses based on capture-recapture studies will be used to test whether there are fundamental differences in maturation, growth and survival patterns between the northern and southern populations, as well as to identify the most critical life stages for ensuring stable population sizes in the south and north, respectively.

The infrastructure established for capture-recapture studies will also allow us to estimate the distribution of reproductive success among individual males and females in these populations, as well as to quantify natural selection on different adult traits (e.g. size, age, breeding time) using 'genetic tagging' - something which has never been assessed in explosive breeding amphibians before.

 

Biogeography of the common frog

In general, amphibians are known to exhibit a higher degree of population subdivision than any other major animal taxa, but large scale population genetic surveys of widely distributed species are still scarce, especially in the Eurasian continent. Using microsatellite markers and mtDNA sequences, we investigated large-scale population genetic structure of the common frog ( Rana temporaria ) – the most wide-spread amphibian of the Paleartic region.

Analyses of Cytochrome b sequences revealed evidence for two distinct lineages inhabiting western and eastern parts of Europe . The separation of these lineages c. 750 000 years ago may have been induced by the onset of the Middle Pleistocene continental glaciations. Analyses of the variability of microsatellite loci within each of the clades revealed evidence for evolution of a high degree of population subdivision ( F ST ~ 0.23) even in northern Fennoscandia, colonized less than 10 000 years ago. The high level of substructuring is puzzling in the face of an apparently high dispersal capacity, as evidenced by the rather rapid recolonization of northern Europe . This suggests that processes other than restricted dispersal capacity need to be explored as explanations for the high degree of population subdivision in amphibians. The colonization of northern Europe has been accompanied by loss of genetic variability as evidenced by decreasing levels of intrapopulational genetic variability in microsatellite loci from south to north across Europe .

 

The invasiveness of R. ridibunda

The introduction of waterfrog species within the " Rana ridibunda " group ( Rana ridibunda , R. bedriagae , R. shquiperica , R. epeirotica ) from oriental Europe has began in France approximately in the 1950s on gastronomic demand. The origin of the introduced taxa, the history of introductions and the impact on autochthonous species were hardly investigated until recently. The range expansion of R. ridibunda in France , however, was alarming, illustrating the need to investigate the genetic and ecological consequences of these introductions. The system is appealing as hybridization processes occur between " R. ridibunda " and autochthonous taxa ( R. lessonae , R. perezi ), producing viable hybrids ( R. esculenta and R. grafi , respectively) reproducing by hybridogenesis. The viability of these hybrids is a result of a meiosis distortion induced by the introduced genome (i.e. ridibunda ). It leads to a hemiclonal transmission of the maternal ridibunda genome to the gametes and an exclusion of the paternal genome from the germ line prior to meiosis. Hence, the gamete pool of R. ridibunda is supported by hybrids, likely strengthening the competitive abilities of R. ridibunda in mixed populations. In this context, contact between introduced and autochthonous species within a hybridogenetic complex can generate risks in a genetic and an ecological point of view. Ecological consequences and risks of R. ridibunda introductions include the overlapping of ecological niches, competition and replacement of autochthonous species. The invasibility of the introduced species has been highlighted in analyzing the ecological preferences of adults, the larval performance during different development stages, and the competitive abilities relative to the hybrid taxa and the autochthonous parental species in various environments. Although the invasive species principally occupy breeding habitats different from those inhabited by autochtonous waterfrog taxa, recently some ridibunda individuals are suspected to be present in those habitats. Moreover, tadpoles of the invasive species well develop in those habitats.

From a genetic point of view, hybridogenesis increases the probability that a recombined R. ridibunda hemigenome will re-enter the R. ridibunda gene pool. This is of special interest, as waterfrogs have been extensively introduced to France and may enrich the gene pool of R. ridibunda.

Evolutionary Ecology of the bullhead Cottus gobio

In focus of this research project is a fluvial river system in France, which is seperated in different section by damms. Further, the water chemistry is different in two major arms of the rivers system, with a highly variable and a environmental stable side arm (temperature, pH, salinity).

Do the genotypes differ in these different environments? Do they perform differently? Are there phenotypes selected for?

 

Bet-hedging in frogs

Earlier models of energy expenditure by parents propose a single optimal level of investment in all young. However, these models do not account for the effect of environmental variability on offspring fitness. Such a variation may lead to multiple fitness functions; when this occurs there is no single optimal egg size. Divergence in propagule size within and among clutches is an expression of phenotypic variation allowing progeny to cope with environmental instabilities, leading to increased parental fitness. Highly variable and unpredictable offspring conditions should select for intra-clutch variation in offspring size. In unpredictable environments, large offspring should survive even under harsh environmental conditions, while all offspring can survive during "good" years. One possible evolutionary response of this bet-hedging strategy should be within-clutch variation in egg yolk volume. There is a substantial body of theory regarding how propagule size should evolve under various scenarios, but a backup of these theories with empirical data hardly exists. My study can contribute to these theories, as frogs occur in habitats with great variability in water duration, oxygen saturation of water, and food availability. My study aims to document if propagule size varies, and to derive a clear image of the reproductive strategies of frogs under different environmental conditions.

Biodiversity Monitoring and Conservation Policy

I am currently coordinating the EU-Project EuMon, which has several aims:

Chytridiomycosis in amphibians

Chytridiomycosis is an infectious disease that affects amphibians worldwide. It is caused by the chytrid fungus (Batrachochytrium dendrobatidis), a fungus capable of causing sporadic deaths in some amphibian populations and 100% mortality in others. It is emerging character is linked to global climate change. The fungus is so small that it is not visible with the bare eye. In the Pyrenees Atlantiques mass die-offs in amphibian populations at high altitude were already documented for 2007, and the disease is spreading fast. How the disease can spread so fast is still enigmatic and subject of an international project at the Station d’Ecologie Experimentale. Project RACE.

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The following short film was produced by BiodivERsA and talks about the RACE project, featuring the work of RACE members in Moulis in the French Pyrenées.