Transfer of mitochondrial DNA from the northern red-backed vole (Clethrionomys rutilus) to the bank vole (C. glareolus)

Summary Using a silver staining method to detect DNA fragments produced by restriction enzymes, it was possible to compare mitochondrial DNAs (mtDNAs) from 85 individuals of the bank vole (Clethrionomys glareolus) trapped at 25 localities in Fennoscandia. There are two distinctly different mtDNA lineages, one occurring in southern and central Fennoscandia and the other in the northern parts. A fragment comparison method shows about 12.7% nucleotide sequence divergence between these two lineages. This major difference between animals of the same species could theoretically be explained by intraspecific lineage survivorship independent of species hybridization, or by introduction of an atypical mtDNA via hybridization with a closely related species. Analysis of mtDNAs from the two otherClethrionomys species present in Fennoscandia (C. rutilus andC. rufocanus) shows that the mtDNA of northernC. glareolus is very similar to that ofC. rutilus and that the mtDNA lineages of these two species cluster together in a phenogram, with small genetic distances among them. By contrast, electrophoresis of proteins encoded by 17 nuclear loci reveals fixed allelic differences between these two species at 8 loci. Hence the presence of two distinctly different mtDNA lineages withinC. glareolus may be a consequence of a limited episode of hybridization betweenC. glareolus andC. rutilus, probably during the postglacial recolonization of Fennoscandia 8000–13,000 years ago.     

Gene conversion in the mitochondrial genome on interspecific hybridization in voles of the <Emphasis Type="Italic">Clethrionomys</Emphasis> genus

The phenomenon of interspecific hybridization accompanied by transfer of the mitochondrial genome from the northern red-backed vole (Clethrionomys rutilus) to the bank vole (Cl. glareolus) in northeastern Europe is well known already for 25 years. However, the possibility of recombination between homologous segments of maternal and paternal mtDNAs of the voles during fertilization was not previously studied. Analysis of data on variability of nucleotide sequences of the mitochondrial gene for cytochrome b in populations of red-backed and bank voles in the area of their sympatry has shown that as a result of interspecific hybridization, the mitochondrial gene pool of bank voles contains not only mtDNA haplotypes of red-backed vole females, but also mtDNA haplotypes of bank voles bearing short nucleotide tracts of red-backed vole mtDNA. This finding supports the hypothesis that an incomplete elimination of red-backed vole paternal mtDNA during the interspecific hybridization between bank vole females and red-backed vole males leads to the gene conversion of bank vole maternal mtDNA tracts by homologous ones of mtDNA of red-backed vole males.     

Densities of microtime rodents along a pollution gradient from a copper-nickel smelter

Population densities of microtine rodents were studied along an air pollution gradient in the Kola Peninsula, Russia, by long-term and short-term trapping. The study area is affected by high sulphur dioxide and heavy metal emissions from the Severonikel copper-nickel smelter in Monchegorsk. The density of Clethrionomys rufocanus, the most abundant vole species in the area, was lowest close to the smelter and increased with distance up to the farthest, less polluted trapping sites. Clethrionomys glareolus, C. rutilus and Lemmus lemmus were absent from the most severely damaged area and were also scarce at the moderately polluted area 28 km south of the smelter. Although the population of C. glareolus has previously been cyclic at the moderately polluted area, we were unable to demonstrate any regular cycle. The most likely explanation for the low number of microtine rodents in the damaged and moderately polluted areas in a decrease in the quantity of important food plants: epiphytic lichens for C. glareolus and possibly C. rutilus, mosses for L. lemmus and seed plants, especially Vaccinium myrtillus, for C. rufocanus. Close to the smelter, direct toxic effects of heavy metals may also reduce population densities. The results show that pollutants may change the relative proportions of microtine species.     

Genetic variation and phylogeography of the bank vole (<Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>, Arvicolinae,Rodentia) in Russia with special reference to the introgression of the mtDNA of a closely related species,red-backed vole (<Emphasis Type="Italic">Cl. rutilus</Emphasis>)

Totally, 294 bank voles (Clethrionomys glareolus) and 18 red-backed voles (Cl. rutilus) from 62 sites of European Russia were studied. Incomplete sequences (967 bp) of the mitochondrial cytochrome b gene were determined for 93 Cl. glareolus individuals from 56 sites and 18 Cl. rutilus individuals from the same habitats. Analysis of the cytochrome b gene variation has demonstrated that practically the entire European part of Russia, Ural, and a considerable part of Western Europe are inhabited by bank voles of the same phylogroup, displaying an extremely low genetic differentiation. Our data suggest that Cl. glareolus very rapidly colonized over the presently occupied territory in the post-Pleistocene period from no more than two (central European and western European) refugia from ancestral populations with a small effective size. PCR typing of the mitochondrial cytochrome b gene allowed us to assess the scale of mtDNA introgression from a closely related species, Cl. rutilus, and to outline the geographical zone of this introgression. Comparison with the red-backed vole haplotypes in the habitats shared by both species favors the hypothesis of an ancient hybridization event (mid-Holocene) and a subsequent introgression. These results suggest that the hybridization took place in the southern and middle Pre-Ural region.     

The effect of diet quality on gut anatomy in British voles (Microtinae)

Three species of British voles, the bank vole Clethrionomys glareolus, the field vole Microtus agrestis and the water vole Arvicola terrestris were maintained on diets of seed and plant leaf material to investigate changes in gut anatomy. C. glareolus and M. agrestis showed significant changes in most regions of the gut; they developed longer and heavier tracts when on a highfibre diet. This response may be important in enabling these animals to withstand seasonal changes in diet quality.     

Morphological and molecular characterisation of Paranoplocephala buryatiensis n. sp. and P. longivaginata Chechulin & Gulyaev, 1998 (Cestoda: Anoplocephalidae) in voles of the genus Clethrionomys

A new species, Paranoplocephala buryatiensis n. sp. (Cestoda:Anoplocephalidae), is described from the grey-sided vole Clethrionomys rufocanus (Sundevall) in the Republic of Buryatia (Russian Federation) and compared with P. longivaginata Chechulin & Gulyaev, 1998, a parasite of the red vole C. rutilus (Pallas) in the same region. P. buryatiensis n. sp. and P. longivaginata both have an exceptionally long vagina and cirrus, unique features among known species of Paranoplocephala Lühe, 1910. The new species differs from P. longivaginata primarily by its wider and more robust body, lower length/width ratio of mature proglottides, tendency of testes to occur in two separate groups, seminal receptacle of a different shape and the position of the cirrus-sac with respect to the ventral longitudinal osmoregulatory canal. The cytochrome oxidase subunit I (COI) sequence data support the independent status of these species, and show that they form a monophyletic assemblage within Paranoplocephala (sensu lato). Assuming cospeciation, an indirect calibration using host speciation dates estimated a rate of mtDNA substitution of 1.0–1.7% pairwise (0.5–0.85% per lineage) sequence divergence per million years. A faunistic review of Paranoplocephala species in C. rufocanus and C. rutilus in the Holarctic region is presented.     

Experiments on habitat selection in voles: Implications for the inverse distribution of two common European species

Summary Several pairs of species of microtine rodents show inverse distributions or abundance. Here I determine if the inverse abundance relationship of the bank vole Clethrionomys glareolus (a forest species) and the field vole Microtus agrestis (an open field species) could be explained by habitat selection. Habitat selection was examined by choice experiments with combinations of soil and vegetation types, and with separate habitat factors. The effects of early experience and social behaviour were also studied, e.g. in a large outdoor enclosure. M. agrestis showed a stronger dependence on shelter (e.g. easily excavated soil or thick grassy vegetation) than C. glareolus, while the presence of preferred food did not affect habitat selection for either species. In both species, laboratory-bred specimens showed considerably fewer preferences. Habitat selection in subadult C. glareolus was influenced by social clumping, while territorial behaviour made more even distributions in M. agrestis.The less rigid habitat selection and weaker territorial behaviour observed in the forest species (Clethrionomys) compared with a species from temporary habitats (Microtus) is not consistent with present concepts of r- and K-selection.     

Effects of competition and season on survival and maturation of young bank vole females

In territorial microtines intra-specific density dependent processes can limit the maturation of individuals during the summer of their birth. This may have demographic consequences by affecting the number and the age distribution of breeding individuals in the population. Little is known about this process on a community level, though populations of many northern microtine species fluctuate in synchrony and are known to interfere socially with each other. We experimentally studied the influence of the field vole Microtus agrestis on maturation, breeding, space use and survival of weanling bank voles, Clethrionomys glareolus. Two additive competition experiments on bank vole populations were conducted in large outdoor enclosures, half of them additionally housing a field vole population. In a mid-summer experiment low population density and absence of older breeding females minimised intra-specific competition. Survival was not affected by the presence of field voles. Season had a significant effect on both the probability of maturation and breeding of the weanlings. Competition with field voles significantly delayed breeding, and coupled with seasonal effects decreased the probability of breeding. In a late-summer experiment breeding and survival of bank vole weanlings were studied for three weeks as part of a high density breeding bank vole population. Weanlings did not mature at all nor were their space use and survival affected by the presence of field voles. Our results show that competition with other species can also have an impact on breeding of immatures. In an extreme seasonal environment, even a short delay of breeding may decrease survival chances of offspring. Seasonal and competition effects together may thus limit the contribution of year born females to reproductive output of the population. Other studies have shown that adult breeding bank voles suffer lower survival in the presence of field voles, but this study showed no survival effects on the weanlings. Thus it might be beneficial for weanlings to stay immature especially in the end of the breeding season and postpone reproduction to the next breeding season if densities of competing species are high.     

Chromosomal evolution in the genus Clethrionomys

The karyotypes of Clethrionomys rufocanus and C. glareolus are presented using G- and C-banding techniques. There is intraspecific polymorphism in Clethrionomys rufocanus for a pericentric inversion. The main difference between the karyotypes of the two species is considered to result from a reciprocal translocation.     

Spatial dynamics in fluctuating vole populations

Summary Patterns and consistency of distribution, spatial and temporal components, and the extent of spatial density-dependence were compared between semi-cyclic and cyclic populations of the vole species Clethrionomys glareolus and Microtus agrestis in south-central and north Sweden. Cyclic populations were less clumped and only C. glareolus showed a consistency in distribution between years. Spatial variation contributed little to the distributions in cyclic populations while the spatial and temporal variations were of the same magnitude in the semi-cyclic populations. The latter populations could be subdivided into areas with different spatial and temporal components. The spatial density-dependence increased from increase to decline years in C. glareolus but not in M. agrestis, which differed conspicuously between reregions in population development. The data imply that spatial dynamics should be considered as much as temporal ones for non-cyclic populations, that the same regulating or limiting factors may be at work in both spatial and temporal components and that, in addition, social behaviour may be important in explaining spatial dynamics. However, the latter effects may be fairly species-specific.     

Functional features of microbial communities in the digestive tract of field voles (Microtus rossiaemeridionalis and Clethrionomys glareolus)

The nitrogen-fixating and cellobiohydrolase activity, the nitrogen (N) and carbon (C) contents, and the number of microorganisms in the prestomach, cecum, and colon of two vole species were studied: the southern vole (Microtus rossiaemeridionalis) and the bank vole (Clethrionomys glareolus), which is characterized by a mixed type of diet. The nitrogen-fixating activity in the cecum was found to be the highest in the voles compared with the mammals studied earlier. The seasonal dynamics of both nitrogenase and cellobiohydrolase activities was registered in the southern vole. The structure of the microbial complex in the southern vole is more varied and includes microorganisms associated with plant substrates.     

Sheep grazing and rodent populations: evidence of negative interactions from a landscape scale experiment

Inter-specific competition, facilitation and predation influence herbivore assemblages, but no study has experimentally explored the interactions between large ungulates and small rodents. In a fully replicated, landscape scale experiment, we manipulated densities of domestic sheep in mountain pastures in Norway. We then determined population growth and densities of rodents by live trapping in each of the areas with different sheep densities. We found that the (summer) population growth rate and autumn density of the field vole (Microtus agrestis) was lower at high sheep density. This provides the first experimental evidence of negative interactions between an ungulate and small rodent species. There was no effect on the bank vole (Clethrionomys glareolus), whose diet differs from sheep. Sheep density, therefore, potentially alters the pattern of inter-specific population synchrony amongst voles. Our study shows that negative interactions between large ungulates and small rodents may be species-specific and negative population consequences for the rodent population appear above threshold ungulate densities.Electronic supplementary material is available for this article at     

Adaptive phylogeography: functional divergence between haemoglobins derived from different glacial refugia in the bank vole

Over the years, researchers have used presumptively neutral molecular variation to infer the origins of current species'' distributions in northern latitudes (especially Europe). However, several reported examples of genic and chromosomal replacements suggest that end-glacial colonizations of particular northern areas may have involved genetic input from different source populations at different times, coupled with competition and selection. We investigate the functional consequences of differences between two bank vole (Clethrionomys glareolus) haemoglobins deriving from different glacial refugia, one of which partially replaced the other in Britain during end-glacial climate warming. This allows us to examine their adaptive divergence and hence a possible role of selection in the replacement. We determine the amino acid substitution Ser52Cys in the major expressed β-globin gene as the allelic difference. We use structural modelling to reveal that the protein environment renders the 52Cys thiol a highly reactive functional group and we show its reactivity in vitro. We demonstrate that possessing the reactive thiol in haemoglobin increases the resistance of bank vole erythrocytes to oxidative stress. Our study thus provides striking evidence for physiological differences between products of genic variants that spread at the expense of one another during colonization of an area from different glacial refugia.     

High genomic diversity in the bank vole at the northern apex of a range expansion: The role of multiple colonizations and end‐glacial refugia

The history of repeated northern glacial cycling and southern climatic stability has long dominated explanations for how genetic diversity is distributed within temperate species in Eurasia and North America. However, growing evidence indicates the importance of cryptic refugia for northern colonization dynamics. An important geographic region to assess this is Fennoscandia, where recolonization at the end of the last glaciation was restricted to specific routes and temporal windows. We used genomic data to analyse genetic diversity and colonization history of the bank vole (Myodes glareolus) throughout Europe (>800 samples) with Fennoscandia as the northern apex. We inferred that bank voles colonized Fennoscandia multiple times by two different routes; with three separate colonizations via a southern land‐bridge route deriving from a “Carpathian” glacial refugium and one via a north‐eastern route from an “Eastern” glacial refugium near the Ural Mountains. Clustering of genome‐wide SNPs revealed high diversity in Fennoscandia, with eight genomic clusters: three of Carpathian origin and five Eastern. Time estimates revealed that the first of the Carpathian colonizations occurred before the Younger Dryas (YD), meaning that the first colonists survived the YD in Fennoscandia. Results also indicated that introgression between bank and northern red‐backed voles (Myodes rutilus) took place in Fennoscandia just after end‐glacial colonization. Therefore, multiple colonizations from the same and different cryptic refugia, temporal and spatial separations and interspecific introgression have shaped bank vole genetic variability in Fennoscandia. Together, these processes drive high genetic diversity at the apex of the northern expansion in this emerging model species.     

Environmental fluctuations facilitate species co-existence and increase decomposition in communities of wood decay fungi

Correlations between mast fruiting of bilberry Vaccinium myrtillus and peak levels of Clethrionomys-voles have been reported from both Norway and Finland, but there has been a discussion whether this is a bottom-up or a top-down relationship. In a multiple regression model, 65% of the variation in a bilberry production index calculated from game reports from southern Norway 1932–1977 could be explained by the berry index of the two preceding years and climate factors acting during key stages of the flowering cycle. High vole populations in previous years did not contribute to explain the fluctuation in berry production. I used the selected model and climate data to predict bilberry production for the period 1978–2004. Predicted berry indices of the current and previous year explained 38% and the total amount of precipitation in May–June explained 16% of the variation in a log-transformed snap-trapping index of bank vole Clethrionomys glareolus 1980–2004. The vole index was not related to any of the climate variables used to predict berry production. This pattern supports the hypothesis that vole cycles are generated by changes in plant chemistry due to climate-synchronized mast fruiting.     

Sexual maturation and age-related dynamics of corticosterone in Clethrionomys rutilus and Cl. rufocanus voles (Rodentia, Cricetidae) under experimental conditions

Differences in the mechanisms regulating reproduction between the northern red-backed vole (Clethrionomys rutilus) and large-toothed red-backed vole (Cl. rufocanus) have been revealed under vivarium conditions. In Cl. rufocanus, the date of birth is the most important ??signal?? factor for sexual maturation of male young of the year, while the effect of population density is significant only for males of the spring generation. Males of the summer generation fail to mature in the same year and are completely excluded from reproduction, which may be accounted for by a rise in the level of corticosterone (measured in fecal samples). Such a mechanism for regulating the numbers of reproductive individuals is absent in Cl. rutilus voles. Throughout the breeding season, males of this species respond to high population density by a decrease in the rate of sexual maturation. No definite relationship between sexual maturation and the level of adrenocortical activity has been revealed in Cl. rutilus.     

Variability of mitochondrial DNA cytochrome <Emphasis Type="Italic">b</Emphasis> gene in the red vole <Emphasis Type="Italic">Clethrionomys rutilus</Emphasis> Pallas, 1779, population in the flood-plain middle stream of the Kolyma River

The intrapopulation variability of a cytochrome b gene fragment and the corresponding amino acid sequence was studied in the red vole Clethrionomys rutilus Pallas, 1779 from the flood-plain of the Kolyma River. Wide polymorphic variability of these properties was observed. Differences in the cytochrome b gene sequence were determined between the red voles of the studied population and the species collected in neighborhoods of Novosibirsk and Omsk. The revealed results point to the urgency of the cytochrome b gene nucleotide sequence and the variants of the respective amino acid sequence as genetic markers of originality of different red vole populations.     

Molecular and ecological signs of mitochondrial adaptation: consequences for introgression?

The evolution of the mitochondrial genome and its potential adaptive impact still generates vital debates. Even if mitochondria have a crucial functional role, as they are the main cellular energy suppliers, mitochondrial DNA (mtDNA) introgression is common in nature, introducing variation in populations upon which selection may act. Here we evaluated whether the evolution of mtDNA in a rodent species affected by mtDNA introgression is explained by neutral expectations alone. Variation in one mitochondrial and six nuclear markers in Myodes glareolus voles was examined, including populations that show mtDNA introgression from its close relative, Myodes rutilus. In addition, we modelled protein structures of the mtDNA marker (cytochrome b) and estimated the environmental envelopes of mitotypes. We found that massive mtDNA introgression occurred without any trace of introgression in the analysed nuclear genes. The results show that the native glareolus mtDNA evolved under past positive selection, suggesting that mtDNA in this system has selective relevance. The environmental models indicate that the rutilus mitotype inhabits colder and drier habitats than the glareolus one that can result from local adaptation or from the geographic context of introgression. Finally, homology models of the cytochrome b protein revealed a substitution in rutilus mtDNA in the vicinity of the catalytic fraction, suggesting that differences between mitotypes may result in functional changes. These results suggest that the evolution of mtDNA in Myodes may have functional, ecological and adaptive significance. This work opens perspective onto future experimental tests of the role of natural selection in mtDNA introgression in this system.     

The prey of the Barn owl (Tyto alba alba) in East Norfolk

Some 5000 Barn owl pellets, collected from sites in East Norfolk during the past decade, have been examined. The most important prey species, by weight, were the Field vole (Microtus agrestis) 52%, the Brown rat (Rattus norvegicus) 17%, and the Common shrew (Sorex araneus) 12%. The prey varies over different habitats; Wood mice (Apodemus sylvaticus) and Bank vole (Clethrionomys glareolus) forming a higher proportion in localities with hedges, scrub and woodland than in open grasslands. These results are comparable with those of other recent work. However, when compared with studies conducted over 30 years ago, it would appear that the Field vole now constitutes a higher proportion, and the Brown rat a lower proportion, of the prey taken.     

Breeding suppression in the bank vole as antipredatory adaptation in a predictable environment

Summary In northern Fennoscandia, microtine rodent populations fluctuate cyclically. The environment of an individual vole can be considered to be predictable when the risks of predation and intra- and interspecific competition change with the cycle, such that both are high during the population highs of voles. The risk of predation is also high during the vole crash. After the crash, the vole population is characterized by low intra- and interspecific competition and low predation pressure. The main predators affecting voles during the crash are the small mustelids, least weasel and stoat. The density of these specialist predators declines drastically during the winter after the vole crash. We studied experimentally the impact of the perceived presence of stoats on the breeding and mating behaviour of voles. In a series of breeding experiments with bank voles,Clethrionomys glareolus, both old and young females suppressed breeding when exposed to the odour of stoats,Mustela erminea. The weights of females decreased in both experimental and control groups, but more among the voles under odour exposition. It seems that females actively avoided copulations under high predation risk and that breeding suppression is mediated by a change in female mating behaviour. There was no change in male behaviour or physical condition between the experimental and control treatments. An alternative mechanism for the observed breeding suppression could be the one caused by decreased feeding in females mediated with low energy intake which does not allow breeding. Regardless of its mechanism, delay of breeding should increase the probability of non-breeding females to survive to the next breeding season. The females surviving the crash should gain a strong selective advantage in a predator-free environment of the subsequent breeding season, which could explain the adaptive function of this antipredatory strategy.