Per Sjocren // Biological  Journal  of  the Linnean Sociely  (1991),  42:  135-147.

Local  extinction  along  the  intrinsic  isolation  gradient  within  metapopulations  is  reviewed  with particular  reference  to  a  study  of  the  pool  frog  (Rana  lessonae)  on  the  northern  periphery  of  its geographical  range.  As  in  the  pool  frog,  many  other  different  taxa  show  significantly  increased extinrtion probabilities with increased interpopulation distance. Present data imply that the relative impact  of  demographic  and  genetic  factors  in  such  storhastic extinctions  depends  on  the genetic history  of  thc melapopulation; data also  imply  that populations fluctuate more greatly  in size than predirted  from  demographic models  which  have  been  commonly  referred  to.  By  mitigating  such fluctuations and  inbreeding,  and compensating  for emigration, immigration  undoubtedly  'rescues' local populations from extinction. In this way, and not  just  in  terms of recolonization, connectivity  is ronrluded  to  be  a  key  to  metapopulation  persistenre.  Implications  for  conservation  are  also presented.

For the last 24 year (1963 - 1986) the author has been working on European water frogs. He has crossed more than 740 pairs of water frogs, and has counted and measured the eggs from over 600 females of all six Europeanspecies and their five hybrids whith live in wild, and other hybrids receved in crossing experiments in laboratory. He has reared over 20 000 metamorphosed froglets of which more than 4000 individuals were devoted for further rearing to study their various features during their ontogenetic development. During this long period he has elaborated numerous rearing and investigative methods whith he has gathered in this paper.

Metamorphosed F1 specimens produced by crosses of three forms of green frogs, viz. Rana lessonae Camerano, R. esculenta Linnaeus and R. ridibunda Pallas, fall into three morphological groups corresponding with them. The indices analysed suggest that only the progeny of lessonae and ridibunda inberit characters of their parents, whereas the offspring of esculenta exhibit, almost exclusivery, characters typical of ridibunda. The group which reveals esculenta characters consist of hybrids obtained by crossing the form lessonae with esculenta or ridibunda. The author concludes that out of the three forms lessonae with esculenta or ridibunda. The author concludes that out of three forms of green frogs, those of lessonae and ridibunda are species and the form esculenta is a hybrid produced, above all, by crossing the two others

In eastern Ukraine, the Rana esculenta complex consists of three species: R. lessonae, R. ridibunda, and hybrid R. esculenta. The first one was rare, whereas two latter frog taxa were very common. Based on DNA flow cytometry, mass occurrence of the triploidy in Rana esculenta has been revealed in 14 localities of Kharkov, Donetsk, and Lugansk Provinces. One hybrid specimen from Kharkov Province was tetraploid. All polyploids were recorded along the middle part of Seversky Donets River (above 450 km). Triploids comprised two groups with different genome composition (LLR and LRR), and were found in three types of population systems (E, R–E, and L–E–R). Geographic distribution of polyploidy in European green frogs is briefly outlined. Different methods of ploidy level identification are discussed. The chromosome count and nuclear DNA cytometry provide the most reliable data.

The purpose of this investigation was to determine the feeding habits of the marsh frog, Rana ridibunda, populations inhabiting Turkish Thrace. Analysis of the stomach contents of 53 (19 ♂♂, 34 ♀♀) adult individuals was performed. The frog diet consisted of a wide variety of arthropods; Diptera (42.62%) and Coleoptera (21.84%) were especially prominent. Aquatic forms did not contribute much to the frog diet. The prey items identified indicate that individuals of this species, like other ranids, are generalist opportunistic predators whose diet is most strongly influenced by prey availability.

The objective of our study was to bring data upon the feeding of Rana lessonae, Rana arvalis from Reci region, Covasna County. We watched at the trophic spectrum of this two Rana species the variation depending on species, sex, habitat and diurnal activity. The feeding of moor frogs is more intense in twilight period. The largest diversity of preys was presented in the stomach contents of Rana lessonae samples captured from the pool. The females both of the two Rana species eat a greatest variety of preys vis a vis the males. Only Rana lessonae captured from the permanent pool present in stomach contents a relatively high number of aquatic preys.

Christoph Vorburger // Ecology Letters, (2001) 4: 628-636

The hemiclonal waterfrog Rana esculenta, a hybrid between R. ridibunda and R. lessonae, eliminates the lessonae genome from the germline and clonally transmits the ridibunda genome (hybridogenesis). Such genomes are prone to accumulate deleterious mutations, which may explain why offspring from matings between hybrids are typically inviable. Here I present ®eld data from a population for which experimental crossings showed that some R. esculenta pairs produce viable R. ridibunda offspring. I demonstrate: (1) that R. ridibunda metamorphs are also produced and survive under natural conditions; (2) that their genotypes are consistent with combinations of clonal ridibunda genomes found in hybrids; and (3) that all R. ridibunda are female. These females possibly recombine the clonal genomes they inherited and, upon mating with syntopic R. lessonae, produce new hemiclones with novel combinations of alleles. Hence, occasional recombination between otherwise clonal ridibunda genomes seems plausible and may provide an escape from the evolutionary dead end they were proposed to be trapped in.

The hemiclonal waterfrog Rana esculenta (RL genotype), a bisexual hybrid between R. ridibunda (RR) and R. lessonae (LL), eliminates the L genome from its germline and clonally transmits the R genome (hybridogenesis). Matings between hybrids produce R. ridibunda offspring, but they generally die at an early larval stage. Mortality may be due to fixed recessive deleterious mutations in the clonally inherited R genomes that were either acquired through the advance of Muller’s ratchet or else frozen in these genomes at hemiclone formation. From this hypothesis results a straightforward prediction: Matings between different hemiclones, that is, between R. esculenta possessing different R genomes of independent origin, should produce viable R. ridibunda offspring because it is unlikely that different clonal lineages have become fixed for the same mutations. I tested this prediction by comparing survival and larval performance of tadpoles from within- and between-population crossings using R. esculenta from Seseglio (Se) in southern, Alpnach (Al) in central, and Elliker Auen (El) in northern Switzerland, respectively. Se is isolated from the other populations by the Alps. Enzyme electrophoresis revealed that parents from Se belonged to a single hemiclone that was different from all hemiclones found north of the Alps. Parents from Al also belonged to one hemiclone, but parents from El belonged to three hemiclones, one of which was indistinguishable from the one in Al. Rana esculenta from Se produced inviable tadpoles when crossed with other hybrids of their own population, but when crossed with R. esculenta from Al and El, tadpoles successfully completed metamorphosis, supporting the hypothesis I tested. Within-population crosses from Al were also inviable, but some within-population crosses from El, where three hemiclones were present, produced viable offspring. Only part of the crosses between Al and El were viable, but there was no consistent relationship between hemiclone combination and tadpole survival.When backcrossed with the parental species R. ridibunda, hybrids from all source populations produced viable offspring. Performance of these tadpoles with a sexual and a clonal genome was comparable to that of normal, sexually produced R. ridibunda tadpoles. Thus, in the heterozygous state, the deleterious mutations on the clonal R genomes did not appear to reduce tadpole fitness.