B chromosome polymorphism in populations of Apodemus flavicollis in Yugoslavia

Four populations of yellow necked mice Apodemus flavicollis were karyologically studied for the presence of B chromosomes. One or two B chromosomes were found in all populations except one, in frequencies which differ significantly. The Bs cannot be accurately distinguished from chromosomes of normal complement even after application of differential staining.     

A comparative study of the heterochromatin of Apodemus sylvaticus and Apodemus flavicollis

The two closely related species Apodemus sylvaticus and Apodemus flavicollis (Muridae) differ in the distribution of their heterochromatin. Two major repetitive sequences known to occur in both species were isolated from A. flavicollis after digestion of total nuclear DNA with the restriction enzymes HindIII and EcoRI respectively and characterized in both species by filter hybridisation and in situ hybridisation to metaphase chromosomes. The EcoRI clone detects a dispersed repetitive sequence family in the genome of both species. Southern blot hybridisation with the HindIII satellite DNA probe reveals major similarities and minor differences in the two species. In situ hybridisation with the HindIII probe labels all chromosomes of A. flavicollis exclusively in the centromeric heterochromatin, whereas in A. sylvaticus several autosomes are also labelled distally. The labelling patterns correspond to the distribution of heterochromatin in the two species. It is concluded that the additional distal heterochromatin of A. sylvaticus contains similar sequences to those of the centromeric heterochromatin of both species. The distal heterochromatin in A. sylvaticus most likely evolved by transposition and amplification of centromeric satellite DNA elements, after the separation of the two species.     

Epigenetic polymorphism in populations of Apodemus sylvaticus and Apodemus flavicollis (Rodentia, Muridae)

The main object of this study was to use discontinuous variation as a means of measuring divergence between populations of A. sylvaticus (L.) and A. flavicollis (Melchior) from Britain and Eurasia. Samples from 17 populations (671 skulls) were each classified for 20 discontinuous morphological variants. The investigation has shown that certain epigenetic variants have a characteristic incidence in populations of the two species and that the relative incidence of the variants could be used as an aid to specific separation. The calculation of degree of divergence between the populations has made it possible to suggest the order in which populations are related to each other.     

Effects of gonadotrophin on the ovary of Apodemus flavicollis in winter anoestrus

Wild-caught female Apodemus flavicollis were given daily subcutaneous injections of PMSG + HCG on two consecutive days (2xPMSG + HCG), HCG on two days (2 X HCG), PMSG + HCG until their vagina became perforate (Perforate), and PMSG + HCG until they became perforate and were left undisturbed for another five days (Perforate + 5). Untreated females served as controls. The ovaries and uteri increased in weight as a result of the treatment. The number of healthy follicles increased in "perforate" females and decreased in "2 X HCG". Luteinization of the follicles and possibly of the interstitial tissue was seen in "2 X HCG". In this group, the mean diameter of the corpora lutea increased, and it decreased in "perforate" and "perforate + 5". The mean number of corpora lutea was unaffected by the treatment. The activity of 3 beta hydroxysteroid dehydrogenase (3 beta HSD) was strong in the corpora lutea of the untreated females and remained so throughout treatment. The interstitial tissue of the untreated females showed only weak 3 beta HSD activity, but the activity was strong in all the experimental groups. One female ovulated, but it is not clear if it was in response to treatment.     

Genetic differentiation in populations of the yellow-necked mouse,Apodemus flavicollis,harbouring B chromosomes in different frequencies

Two alternative models are used to explain maintenance of polymorphism of B chromosomes (Bs) in populations of a great number of species. The parasitic model suggests deleterious effects of Bs on fitness of carriers, while the heterotic model assumes that, in the absence of drive, equilibrium is produced by beneficial effects of Bs at low numbers. In order to determine the potential contribution of Bs to genetic differentiation and diversity, four populations of Apodemus flavicollis, differing in frequency of Bs (from 0.23 to 0.38) and settled in ecologically different habitats, were analyzed by 471 AFLP markers. Although numerous loci were demonstrated to be population specific, none of them was associated with individuals with Bs. AMOVA showed that the presence of Bs does not affect population differentiation, pointing to greater genetic similarity of Bs to A chromosomes. The greatest genetic diversity (0.241) was found in the population settled in optimal conditions for this species featured by the lowest frequency of animals with Bs (0.23). We found that the majority of loci marked as loci under directional selection, are characteristic of either a population with lower or one with a higher frequency of Bs. Several loci detected as outliers were associated with environmental variables that could directly and/or indirectly influence population dynamics of A. flavicollis. Thus, we suggest that the different frequency of Bs carriers in populations is related to adaptive differentiation to diverse habitats, which is in accordance with the heterotic model of Bs maintenance.     

Impact of radio-collars on Yellow-necked Mice, Apodemus flavicollis (Mammalia, Rodentia)

The impact of wearing a radio-collar was investigated for 124 Yellow-necked Mice Apodemus flavicollis, in mixed deciduous woodlands. Collared mice neither lost more weight nor lost weight more frequently than control individuals. The cost of wearing a collar was not greater for small individuals than for larger ones. The impact of wearing a radio-collar was mostly due to the shape and size of the radio-collar and the external antenna rather than the package weight. This aspect was particularly acute in the presence of ticks. One solution to this problem may be to build and use radio-transmitter packages as compact (but nevertheless resistant to gnawing) and symmetrical as possible and to use collars impregnated with tick repellent.     

B chromosomes and developmental homeostasis in the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia): effects on nonmetric traits

B chromosomes are found in almost all populations of the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia). Their effects on developmental homeostasis in this species were analyzed using morphological nonmetric traits (number of foramina) in a sample of 218 animals from locality Mt Jastrebac in the former Yugoslavia. Variations of the parameters of developmental homeostasis (the degree of fluctuating asymmetry--FA, the number of asymmetrical characters per individual--NA, and the total phenotypic variability--PV) were examined in three groups: in animals without Bs, with one B chromosome, and with more than one B chromosome. Significant differences in the level of FA between groups of animals were found for two characters. Carriers of one B chromosome displayed the highest level of phenotypic variability. Parameters of developmental stability (DS) were monitored in the population in which significant variations in the frequency of animals with Bs (fB) were established during the season earlier. The FA levels for four foramina out of a total of 12 examined followed the changes of frequencies of animals with Bs. Furthermore, a significant seasonal correlation between NA and fB was found. The presence of B does not cause a disturbance of homeostasis in a way that allows changes in homeostasis to be directly related to B chromosome's presence. However, carriers of B react differently to environmental changes than do noncarriers.     

Intra individual variation in the number of B chromosomes in the yellow-necked mouse, Apodemus flavicollis (Mammalia, Rodentia)

The yellow-necked mouse, Apodemus flavicollis, is characterized by a frequent occurrence of B chromosomes. The frequency of intra individual mosaicism of Bs was studied in 995 animals collected at six localities in Serbia. It was found that 329 (33.06%) possessed B chromosomes. Among these, 87 animals (26.44%) were mosaics. A total of 32 mosaic animals with more than one B chromosome were analyzed for distribution of Bs which was found to be quite different between groups of animals with different numbers of Bs and increases with their number. The frequency of mosaics differs between localities and ranges from 0.22 to 0.55.     

Differences in the ecology of Bartonella infections of Apodemus flavicollis and Myodes glareolus in a boreal forest

The epidemiology of Bartonella species infecting Apodemus flavicollis and Myodes glareolus in a forest in Eastern Poland was followed for 2 years using mark-recapture. Infections could be acquired in any month, but prevalence, and probability of infection, peaked in the summer. There were significant differences in the pattern of infections between the two species. Both hosts were primarily infected as juveniles, but the probability of infection was highest for A. flavicollis, which, evidence suggests, experienced longer-lasting infections with a wider range of Bartonella genotypes. There was no evidence of increased host mortality associated with Bartonella, although the infection did affect the probability of recapture. Animals could become re-infected, generally by different Bartonella genotypes. Several longer lasting, poorly resolved infections of A. flavicollis involved more than 1 genotype, and may have resulted from sequential infections. Of 22 Bartonella gltA genotypes collected, only 2 (both B. grahamii) were shared between mice and voles; all others were specific either to A. flavicollis or to M. glareolus, and had their nearest relatives infecting Microtus species in neighbouring fields. This heterogeneity in the patterns of Bartonella infections in wild rodents emphasizes the need to consider variation between both, host species and Bartonella genotypes in ecological and epidemiological studies.     

The role of host sex in parasite dynamics: field experiments on the yellow-necked mouse Apodemus flavicollis

We investigated the role of host sex in parasite transmission and questioned: ‘Is host sex important in influencing the dynamics of infection in free living animal populations?’ We experimentally reduced the helminth community of either males or females in a yellow‐necked mice (Apodemus flavicollis) population using an anthelmintic, in replicated trapping areas, and subsequently monitored the prevalence and intensity of macroparasites in the untreated sex. We focussed on the dominant parasite Heligmosomoides polygyrus and found that reducing parasites in males caused a consistent reduction of parasitic intensity in females, estimated through faecal egg counts, but the removal of parasites in females had no significant influence on the parasites in males. This finding suggests that males are responsible for driving the parasite infection in the host population and females may play a relatively trivial role. The possible mechanisms promoting such patterns are discussed.     

Genetic interaction between distinct Dobrava hantavirus subtypes in Apodemus agrarius and A. flavicollis in nature

Dobrava virus (DOBV) occurs in two different rodent species, Apodemus flavicollis (DOBV-Af) and A. agrarius (DOBV-Aa). We sequenced the S and M genomic segments from sympatric DOBV-Af and DOBV-Aa strains which fell into two distinct genetic lineages. Molecular phylogenetic analyses gave evidence for genetic reassortment between S and M segments of DOBV-Af and DOBV-Aa and indicated homologous recombination events in DOBV evolution. DOBV-Af and DOBV-Aa are distinct but also subject to genetic exchanges that affect their evolutionary trajectories.     

MHC diversity and the association to nematode parasitism in the yellow-necked mouse (Apodemus flavicollis)

In vertebrates, the genes of the major histocompatibility complex (MHC) are among the most debated candidates accounting for co-evolutionary processes of host-parasite interaction at the molecular level. The exceptionally high allelic polymorphism found in MHC loci is believed to be maintained by pathogen-driven selection, mediated either through heterozygous advantage or rare allele advantage (= frequency dependent selection). While investigations under natural conditions are still very rare, studies on humans or mice under laboratory conditions revealed support for both hypotheses. We investigated nematode burden and allelic diversity of a functional important MHC class II gene (DRB exon2) in free-ranging yellow-necked mice (Apodemus flavicollis). Twenty-seven distinct Apfl-DRB alleles were detected in 146 individuals with high levels of amino acid sequence divergence, especially at the antigen binding sites (ABS), indicating selection processes acting on this locus. Heterozygosity had no influence on the infection status (being infected or not), the number of different nematode infections (NNI) or the intensity of infection, measured as the individual faecal egg count (FEC). However, significant associations of specific Apfl-DRB alleles to both nematode susceptibility and resistance were found, for all nematodes as well as in separate analyses of the two most common nematodes. Apodemus flavicollis individuals carrying the alleles Apfl-DRB*5 or Apfl-DRB*15 revealed significantly higher FEC than individuals with other alleles. In contrast, the allele Apfl-DRB*23 showed a significant association to low FEC of the most common nematode. Thus, our results provide evidence for pathogen-driven selection acting through rare allele advantage under natural conditions.     

Studies on the distributions of Apodemus sylvaticus (L.) and A. flavicollis (Melchior) in Britain

The known distributions of A. sylvaticus and A. flavicollis were examined with respect to four woodland classifications and two soil types. The distribution of A. flavicollis in Britain is associated with drier areas and with the distribution of mature, deciduous woodland. A. sylvaticus is found in both wet and dry areas and is distributed independently of deciduous woodland.