Variability in size of the nuclear genome in pygmy wood mouse Sylvaemus uralensis (Rodentia, Muridae)

Earlier, in an integral genetic study, the Asian and European races were distinguished within the species Sylvaemus uralensis (pygmy wood mouse) and the European race was divided into the East European and South European forms. Each of these groups differed from the others, in particular, in the quantity of the centromeric heterochromatin in karyotypes of the animals. To establish the pattern of its changes in S. uralensis, in the present study the DNA content in splenocyte nuclei in all races and forms of pygmy wood mice was assessed using DNA flow cytometry. The heterochromatin amount in karyotypes and genome size were shown to be correlated. The East European chromosomal race of S. uralensis (Central Chernozem and Non-Chernozem regions of Russia, Crimea Peninsula, Middle Volga region, and Southern Ural) and the Asian race of this species (East Kazakhstan, Uzbekistan, and East Turkmenistan), which have respectively the highest and the lowest amounts of centromeric heterochromatin in the karyotype, exhibit the greatest difference in the DNA content in the genome. On average, the difference is approximately 8% in males and 6.7% in females; in both cases, the ranges of variability were distinctly different. Against the general background of the trait variation, the Asian race, whose members have the smallest DNA amount in their cells, looks homogeneous. The genome of the South European chromosomal form of S. uralensis (Caucasus, Transcaucasia, Carpathians, and Balkan Peninsula), which exhibits an intermediate content of the centromeric heterochromatin in the karyotype, is smaller that the genome of the East European race (by 3.2% in the group of males and by 1.9%, in the group of females), but larger than that of the Asian race (by 5% in either sex). Thus, the variability of size of centromeric C-blocks in pygmy wood mouse is likely to be associated with elimination (or, conversely, an increase in the amount) of the genetically inert chromatin. It is suggested that a significant contribution to the variability of genome size in S. uralensis is made by heterochromosomes, or, more precisely, their variable regions, which seem to be largely heterochromatic.     

Allozyme variation in pygmy wood mouse Apodemus uralensis (Rodenita, Muridae) in the Ural region

Variation of 17 allozyme loci was examined in 530 Apodemus uralensis individuals caught in the Ural region in 2005 through 2007. In the populations examined, the mean value of the population genetic differentiation index F(ST) constituted 0.169. It was demonstrated that F(ST) values for the samples obtained from the 1-km2 plot in different years, as well as for the samples trapped at a distance from 0.3 to 5 km during one year, could be remarkably higher than the mean value, pointing to their high, statistically significant differentiation. It seems likely that this differentiation was caused by spatial population subdivision, associated with the mice migrations, temporal change of the population structure, and the gene drift. In A. uralensis, inhabiting the zone of East Ural radioactive trace, no differences in the allozyme sets and their frequencies were observed.     

Allozyme Variation of the Pygmy Wood Mouse Sylvaemus uralensis (Rodentia,Muridae) and Estimation of the Divergence of Its Chromosome Forms

The genetic divergence between the eastern European, southern European, and Asian chromosome forms of the pygmy wood mouse Sylvaemus uralensis, whose karyotypes differ from one another in the amount of centromeric heterochromatin, has been reevaluated using allozyme analysis. In general, Asian chromosome forms in S. uralensis living in eastern Kazakhstan, eastern Turkmenistan (the Kugitang Ridge), and Uzbekistan are more monomorphic than European populations of this species. However, the allozyme differences between all chromosome forms of the pygmy wood mouse are comparable with the interpopulation differences within each form and are an order of magnitude smaller than those between good species of the genus Sylvaemus. Thus, the chromosome forms of S. uralensis cannot be considered to be separate species. The concept of races as large population groups that have not diverged enough to regard them as species but differ from one another in some genetic characters is used to describe the differentiation of S. uralensis forms more adequately. The currently available evidence suggests the existence of two S. uralensis races, the Asian and the European ones, and two chromosome forms (eastern and southern) of the European race. The possible historical factors that have determined the formation of the races of the pygmy wood mouse are considered. According to the most plausible hypothesis, the shift and fragmentation of the broad-leaved forest zone during the most recent glacial period (late Pleistocene) were the crucial factors of the formation of these races, because they resulted in a prolonged isolation of the European and Asian population groups ofS. uralensis from each other.     

Comparative FISH analysis of C-positive regions of chromosomes of wood mice (Rodentia, Muridae, Sylvaemus)

The homology of DNA of C-positive centromeric regions of chromosomes in wood mice of the genus Sylvaemus (S. uralensis, S. fulvipectus, S. sylvaticus, S. flavicollis, and S. ponticus) was estimated for the first time. DNA probes were generated by microdissection from the centromeric regions of individual autosomes of each species, and their fluorescence in situ hybridization (FISH) with metaphase chromosomes of representatives of all studied wood mouse species was carried out. Unlike in the chromosomal forms and races of S. uralensis, changes in the DNA composition of the chromosomal centromeric regions in the wood mouse species of the genus Sylvaemus (including closely related S. flavicollis and S. ponticus) are both quantitative and qualitative. The patterns of FISH signals after in situ hybridization of the microdissection DNA probes with chromosomes of the species involved in the study demonstrate significant differences between C-positive regions of wood mouse chromosomes in the copy number and the level of homology of repetitive sequences as well as in the localization of homologous repetitive sequences. It was shown that C-positive regions of wood mouse chromosomes can contain both homologous and distinct sets of repetitive sequences. Regions enriched with homologous repeats were detected either directly in C-positive regions of individual chromosomes or only on the short arms of acrocentrics, or at the boundary of C-positive and C-negative regions.     

Analysis of genetic variation and differentiation in the pygmy wood mouse <Emphasis Type="Italic">Sylvaemus uralensis</Emphasis> (Rodentia,Muridae) aided by the RAPD-PCR method

The present work ascertains distinct differentiation of the pygmy wood mouse into two groups of populations, which correspond to the European and Asian races, but cannot, however, be considered valid species due to the low values of genetic distances between them. Neither of the used statistical methods explicitly verifies genetic isolation of the East European and South European chromosomal forms, which are close to one another and together constitute the European race. However, these chromosomal forms/races of the pygmy wood mouse differentiates each from other by a character of polymorphism. It can be explained both by the history of their formation (the degree of proximity to the ancestral population complex, isolation, etc.) and by unequal current environmental conditions.     

On the taxonomic rank of ciscaucasicus and its relationships with the pygmy wood mouse Sylvaemus uralensis inferred from the mtDNA cytochrome b gene sequence

To specify the taxonomic rank of form ciscaucasoides (independent species Sylvaemus ciscaucasoides, or intraspecific form of pygmy wood mouse, S. uralensis), a 402-bp the mtDNA cytochrome b gene fragment (402 bp) was examined in S. ciscaucasoides individuals from six geographic localities of the Caucasus and Ciscaucasus, (Krasnodar krai and Adygeya Republic) and 17 S. uralensis individuals from seven localities of the Russian Plai (Saratov oblast, Smolensk oblast, Voronezh oblast, Tula oblast, Moscow oblast, and Tver' oblast). For comparison, the cytochrome b gene was partly sequenced in the samples of yellow necked, S. flavicollis (n = 2, Samara oblast), and Caucasian, S. ponticus (n = 6, Krasnodar krai), wood mice. One Mus musculus specimen from Western Europe, whose nucleotide sequences were deposed in the GenBank, was used as intergeneric outgroup. Phylogenetic trees for the forms examined were constructed based on the mtDNA sequence variation and using the neighbor joining and maximum parsimony methods. The network of the cytochrome b haplotypes was also constructed. The level of genetic divergence was evaluated using Kimura's two-parameter algorithm. Based on the data on the sequence variation in a 402-bp mtDNA cytochrome b gene fragment, the hypothesis on the species status of the ciscaucasicus form was. The mean intergroup distances (d) between the geographic groups of S. uralensis varied from 0.0036 to 0.0152. At the same time, the distances between the pygmy wood mice and the group of S.flavicollis-S. ponticus varies in the range from 0.0860 to 0.0935, and the level of intergeneric genetic differentiation (Sylvaemus-Mus) is higher than the latter index (d = 0.142). Ciscaucasoides should be considered as geographic substitution form of S. uralensis. Furthermore, the Caucasian populations of S. uralensis (= ciscaucasoides) were characterized by a threefold lower value of intergroup genetic divergence (d = 0.0062) than the East European populations (d= 0.0179). This finding pointed to some isolation of Caucasian populations of pygmy wood mouse and depletion of their gene pool. However other molecular genetic data (similarity of nucleotide composition and consistence of the levels of intra- and intergroup distances) suggest the absence of geographic subdivision between Caucasian and East European populations ofS. uralensis relative to the molecular marker examined.     

Allozyme variability and the population genetic structure of the mice Apodemus agrarius,Mus musculus,and Sylvaemus uralensis (Rodenita,Muridae) in Western Siberia

Of the three mouse species inhabiting Western Siberia, the striped field mouse is characterized by the highest level of genetic variation, a uniform distribution of polymorphism indicators, lower values of genetic differentiation, and higher values of gene flow, as compared to the house mouse and pygmy wood mouse. The house mouse populations have abrupt changes in the parameters of protein polymorphism in different localities. This is due to the considerable spatial dissociation of the cities of Siberia and the differing histories of their settlement. Inhabitation of an urban area leads to the partitioning of striped wood mouse populations into groups that considerably differ in allele frequencies and genetic variation indices. These changes are not related to the degree of urbanization and are determined by genetic drift. Some characteristics of these groups, like reduced variability, heterozygote deficit, and deviation from the Hardy-Weinberg equilibrium, limit their adaptive potential and make them dependent on the populations of inter-settlement territories.     

Allozyme variation of the pygmy wood mouse <Emphasis Type="Italic">Apodemus uralensis</Emphasis> (Rodenita,Muridae) in the ural region

Variation of 17 allozyme loci was examined in 530 Apodemus uralensis individuals caught in the Ural region from 2005 to 2007. In the populations examined, the mean value of the genetic differentiation index F _ST constituted 0.169. It was demonstrated that F _ST values for the samples obtained from the 1-km2 plot in different years, as well as for the samples trapped at a distance from 0.3 to 5 km during one year, could be remarkably higher than the mean value, pointing to their high, statistically significant differentiation. It seems likely that this differentiation was caused by spatial population subdivision, associated with the mice migrations, temporal change of the population structure, and the gene drift. Population of A. uralensis from radioactively contaminated zone displayed no specificities in the allozyme set and frequencies, which could basically distinguish these animals from the other Ural populations.     

Systematics and geographic variation of Ethiopian Arvicanthis (Rodentia, Muridae)

Arvicanthis , the unstriped grass-rat, is a widespread genus occurring in many regions of Africa and is major pest in agricultural farmland. Despite its economic importance for developing countries, the taxonomy of the genus is stillin a chaotic state. We used univariate and multivariate morphometrics to investigate two species, A. abyssinicus and A. dembeensis , occurring in Ethiopia. Results show that these taxa are well separated, this contradicting authors who lump all Arvicanthis as A. niloticus . There is a longitudinal cline in the morphology of A. dembeensis from eastern regions along the Rift Valley. Differences between the species in morphlogy seem to reflect adaptation to different ecological niches at high ( A. abyssinicus ) and low ( A. dembeensis ) altitudes.     

Comparative FISH analysis of C-positive blocks of centromeric chromosomal regions of pygmy wood mice <Emphasis Type="Italic">Sylvaemus uralensis</Emphasis> (Rodentia,Muridae)

The composition and homology of centromeric heterochromatin DNA has been compared in representatives of the Asian race and two chromosomal forms (Eastern European and Southern European) of the European race of the pygmy wood mouse Sylvaemus uralensis by means of in situ hybridization with metaphase chromosomes of microdissection DNA probes obtained from centromeric C-blocks of mice of the Southern European chromosomal form and the Asian race. Joint hybridization of both DNA probes yielded all possible variants of centromeric regions in terms of the presence of repetitive sequences homologous to those of some or another dissection region, which indicates a diversity of centromeric regions differing in DNA composition. However, most variations of the fluorescent in situ hybridization (FISH) patterns are apparently related to quantitative differences of repetitive elements of the genome. Experiments with the DNA probe obtained from the genome of the Southern European form of the pygmy wood mouse have shown that the number of intense FISH signals roughly corresponds to the number of large C-segments in representatives of the European race, which is characterized by a large amount of the centromeric C-heterochromatin in the karyotype. However, intense signals have been also detected in experiments on hybridization of this probe with chromosomes of representatives of the Asian race, which has no large C-blocks in the karyotype; thus, DNA sequences homologous to heterochromatic ones are also present in nonheterochromatic regions adjacent to C-segments. Despite the variations of the numbers of both intense and weak FISH signals, all chromosomal forms/races of S. uralensis significantly differ of the samples from one another in these characters. The number of intense FISH signals in DNA in pygmy wood mice of the samples from eastern Turkmenistan (the Kugitang ridge) and southern Omsk oblast (the vicinity of the Talapker railway station) was intermediate between those in the European and Asian races, which is apparently related to a hybrid origin of these populations (the hybridization having occurred long ago in the former case and recently in the latter case).     

Chromosomal variability of the field mouse Apodemus agarius (Rodentia, Muridae)

Chromosome sets of 114 Apodemus agrarius mice from 29 localities in Moldova, Ukraine, Siberia, and Far East were studied by means of G-, C-, and NOR-banding. In all populations studied, the Y chromosome was shown to be a medium-size acrocentric chromosome consisting of heterochromatin. Chromosome polymorphism observed in populations from Primorskii krai concerned (1) the morphology of the first two autosome pairs (variants A/A, A/ST, and ST/ST), (2) the number of metacentric chromosomes (from 6 to 8), and (3) heterochromatin localization in the pericentromeric regions of two metacentric chromosome pairs. A karyotype with an additional heterochromatic microchromosome found in all the metaphases studied was described in one mouse from a locality of western Primorye that has not been studied previously. In the karyotype of 15 mice from four populations of Primorye, the pool of nucleolus organizer regions is distributed over three autosome pairs rather than over four, as is the case A. agrarius from Europe. Based on the analysis of literature sources and our own data, the problem of chromosome polymorphism in the field mouse is discussed.     

An input of heterochronies to the formation of interspecific differences in wood mice, Sylvaemus (Rodentia)

Difference in adult size and body shape among the closely related species is the result of diversification in morphogenetic processes. We examined age variation in various external and craniometric characters of four Sylvaemus species from Ukrane fauna, S. uralensis, S. arianus, S. sylvaticus, and S. tauricus, using cross-sectional techniques. The intensive growth period occurred to be different in various species. Rapid growth of S. tauricus continues during all life span, while in other species growth rate becomes slower at different stages. This peculiarity of S. tauricus may be the result of prolonged and more intensive production of somatotropin by adenohypophysis. Side effects of adenohypophysis activation are resulted from changes in S. tauricus skull shape and more intensive pigmentation in comparison with three other species. Prolongation of growth could be developmental mechanism for size increasing in evolution of S. tauricus. The independence of body size and pigmentation is principally the same at interspecific and intraspecific level.     

Genetic variation and evolution in the genus Apodemus (Muridae: Rodentia)

Genetic variation was studied using protein electrophoresis of 28–38 gene loci in 1347 specimens of Apodemus agrar-ius, A. peninsulae, A. flavicollis, A. sylvaticus, A. alpicola, A. uralensis, A . cf. hyrcanicus, A. hermonensis, A. m. mystacinus and A. m. epimelas , representing 121 populations from Europe, the Middle East, and North Africa. Mean values of heterozygosity per locus for each species ranged from 0.02 to 0.04. Mean values of Nel's genetic distance ( D ) between the taxa ranged from 0.06 (between A. flavicollis and A. alpicola) to 1.34 (between A. uralensis and A. agrarius ). The highest values of D were found between A. agrarius and other Apodemus species (0.62-1.34). These values correspond to those generally observed between genera in small mammals. Our data show that A. agrarius and A. peninsulae are sister species, well-differentiated from other taxa. High genetic distance between A. m. mystacinus and A. m. epimelas leads us to consider them distinct species and sister taxa to other Western Palaearctic species of the subgenus Sylvaemus . The data also suggest a recent separation of members of the latter group from a common ancestor, and subsequent rapid radiation, making it difficult to infer phylogenetic relationships. Some taxonomic implications of the results are discussed further.© 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 395–419.     

Population dynamics of the wood mouse Apodemus sylvaticus (Rodentia: Muridae) in a Sitka spruce successional mosaic

A population of wood mice, Apodemus sylvaticus (L.), was studied by capture-mark-recapture, in five 0.81 ha grids within a habitat mosaic produced by the felling of a plantation of mature Sitka spruce ( Picea sitchensis Carriere). From May 1990 to June 1992, we estimated population sizes, survival (total and by weight classes), recruitment, and home-range sizes, usually at monthly intervals, for the study area taken as a whole. Inter-grid movements were frequent for both sexes, supporting the assumption that the population could be analysed in this way. Wood mice were more abundant in 1991 than in 1990, associated with increased seed supply. In both years, numbers were low in spring and increased gradually towards an autumn peak, which was of similar size in both years. Male home-range areas varied seasonally, and were negatively correlated with population sizes, while female home ranges showed little seasonal variation. Survival and recruitment both declined sharply in late autumn, apparently in a densitydependent way. Except for this latter result, the population dynamics in a heterogeneous coniferous woodland were consistent with those described previously for this species in more homogeneous deciduous woodlands, suggesting that the same regulatory processes are operating in both habitats.     

Bibliography of the genus Apodemus (Rodentia, Muridae)

A complete bibliography of the genus Apodemus for the period 1970–1980 is presented, with a selection of the more important papers from earlier years, and a few references for 1981. A coded index to species and subjects is provided. All titles have been translated into English.     

Allozyme variation and divergence in the phyllotine rodent Calomys hummelincki (Husson, 1960)

The aim of the present study was to assess the degree of genetic variation and divergence among six populations of Calomys hummelincki, a phyllotine rodent distributed in northern South America. With this information we will try to evaluate the two hypotheses of possible colonization and differentiation of this group of rodents postulated by Baskin and Reig. We studied 34 loci by electrophoretic analysis: 21 were monomorphic for all populations and 13 were polymorphic in at least one population, being P _1% = 21.6% the mean value for all populations. The mean value of heterozygosity per locus was H = 0.075. Low values of genetic distance were observed among populations of the Llanos region (0.001 < D < 0.006). There was a larger genetic distance (D = 0.024) between the population from Isiro, in the northwestern semiarid region, and those from the Llanos region. The insular population of Aruba displayed the lowest value of genetic distance with the population from Isiro (D = 0.014). The specimens from Sipao, on the right side of the Orinoco river, displayed the highest values of genetic distances in comparison with other populations of C. hummelincki (0.070 < D < 0.095). The relatively high differentiation was due to the fixation of new alleles, not found in other populations of C. hummelincki, at loci Idh-1 and Est-2. F-statistics and Nm values indicated reduced gene flow among the populations sampled. Despite the limited data, the results seem to support Reig's hypothesis about south to north colonization of genus Calomys in South America. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on the Robertsonian chromosomal variation of Mus musculus domesticus (Rodentia, Muridae) in Greece

The karyotype of the house mouse, Mus musculus domesticus , was examined in 282 specimens from 44 localities, in an effort to gain better understanding of the Robertsonian (Rb) variation known to exist in Greece. We consider that an Rb system exists in Peloponnisos, southern Greece, distributed in an area that is substantially larger than previously known. It consists of at least three Rb races with 2 n  = 30, 2 n  = 24 and 2 n  = 28, respectively, the last being reported for the first time in this paper and carrying Rb(3.6), Rb(8.12), Rb(10.14), Rb(13.15), Rb(9.16) and Rb(11.17) in a homozygous state. Additional instances of variation in this Rb system include individuals with 2 n  = 31 and 32 of variable Rb constitution and hybrids between the Rb races with 2 n  = 30 and 2 n  = 24. In southern Peloponnisos, Rb(10.14) was found in either a homozygous or a heterozygous state (2 n  = 38 or 39). The relationships among the Rb populations of Peloponnisos are discussed and hypotheses for their evolution are proposed. Rb variation was also recorded in two new locations of eastern Sterea Ellas (2 n  = 28 and 29) and one in Ipiros, north-west Greece (2 n  = 38). These findings corroborate the existence of two separate Rb systems in those two areas. Moreover, among a number of islands surveyed, Rb variation was only found in Kythira island, with Rb(10.14) in a heterozygous state (2 n  = 39). Finally, the typical all-acrocentric karyotype (2 n  = 40) was found in 51 of the animals studied from 13 localities. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 503–513.     

Seasonal variation in growth of Mastomys natalensis (Rodentia: Muridae) in Morogoro, Tanzania

The paper describes seasonal growth variation in a free living population of Mastomys natalensis (Smith, 1834) from Morogoro, Tanzania. Growth patterns are associated with rainfall patterns. Young animals enter a growth stop at the end of the dry season (October-November) and growth is only resumed after the first heavy rains. Reproduction is more related to size than to age. If the heavy rains come early in the rainy season, animals start reproducing in the beginning of the year. The resulting young reach their maximal size without growth stop and start reproducing immediately. Possible causes for the growth stop are discussed. Growth phenomena are thought to explain at least partially the relation between rainfall and reproduction in Mastomys .

Résumé

Cet article décrit la variation de croissance saisonnière d'une population de Mastomys natalensis (Smith, 1834) vivant en libertéà Morogoro, en Tanzanie. Les graphiques de croissance sont associés à ceux des chutes de pluie. Les jeunes animaux connaissent un arrêt de la croissance à la fin de la saison sèche (octobre-novembre) et la croissance ne reprend qu'après les premières fortes pluies. La reproduction est plus liée à la taille qu'à l'âge. Si les fortes pluies surviennent tôt en saison des pluies, les animaux commencent à se reproduire au début de l'année. Les jeunes atteignent alors leur taille maximale sans avoir connu d'arrêt de croissance et commencent à se reproduire immédiatement. On pense que les phénomènes de croissance expliquent au moins partiellement la relation entre les chutes de pluie et la reproduction chez Mastomys .