Genetic differentiation and habitat partition of Robertsonian house mouse populations (Mus musculus domesticus) of Tunisia

The karyological analysis of Tunisian populations of house mice revealed the existence of a Robertsonian (Rb) chromosomal race carrying nine pairs of metacentric chromosomes in central Tunisia. Divergence estimates showed that they are genetically differentiated from local all-acrocentric populations and have a reduced level of genic variability. The Rb populations are restricted to urban habitats, whereas all-acrocentric populations occur in rural areas. Contact zones between these two types of habitat yield chromosomally polymorphic populations. Analysis of gene flow indicates that it is reduced and limited to populations bordering the contact areas. The reduced genic variability and patchy distribution exhibited by the Tunisian Rb mice do not agree with results from previous studies of the European Rb populations. Two hypotheses are presented to account for this discrepancy based on local differentiation versus introduction of the Tunisian Rb race.     

New Robertsonian populations of the house mouse (Mus musculus domesticus) from north of the Alps]

New Robertsonian (Rb) populations of the house mouse (Mus musculus domesticus) carrying different combinations of centric fusions are reported in France, Switzerland, and Germany. In Alsace (France), the diploid number varied from 2n = 38 to 34; four fusions were present, with Rb(4.12) homozygous in all populations whereas Rb(5.10), Rb(5.7), or Rb(10.14) were found to be segregating. In Switzerland, only all-acrocentric mice (2n = 40) were present in Bern while Rb(5.7) and Rb(9.16) occurred in Basel. In the Konstanz locality from southern Germany, all the mice were homozygous for nine Rb fusions: Rb(1.18), Rb(2.5), Rb(3.6), Rb(4.12), Rb(7.15), Rb(8.17), Rb(9.14), Rb(10.11), and Rb(13.16). The phylogenetic relationship of these new Rb populations with those already known is discussed.     

Geometric morphometrics on Greek house mouse populations (Mus musculus domesticus) with Robertsonian and all-acrocentric chromosomal arrangements

This work aims to give the first comprehensive morphometric analysis of intraspecific variation for the different populations of the western house mouse (Mus musculus domesticus), in the Robertsonian (Rb) system of the N-NW Peloponnisos. Furthermore, we study all-acrocentric karyotype populations (2n = 40) of the species coming from several localities of Greece. We apply 2D shape analysis, i.e. landmark analysis and Elliptic Fourier Analysis, on the dorsal and ventral side of skull and the occlusal view of the first upper molar (M¹), respectively. Although significant genetic divergence between typical and Rb populations and even ongoing speciation processes have been reported for this species, this was not the case for the Greek populations studied. However, our analyses herein reveal morphologically differentiated chromosome groups in N-NW Peloponnisos Rb system and a clear geographical discrimination of the all-acrocentric (2n = 40) populations for all characters studied. We suggest that in all-acrocentric (2n = 40) karyotype mice the geographical distance drives their differentiation while within the Rb system of N-NW Peloponnisos, karyotype is the key factor that acts on their phenotypic variation.     

Dispersal mechanisms in a captive wild house mouse population (Mus domesticus Rutty)

The ecological implications of dispersal have been discussed in many studies of wild animals in the field but little is known about the social mechanisms leading to the emigration of certain members of a group. To study the social background of dispersal in wild house mice ( Mus domesticus Rutty) a population cage system was evaluated that allowed permanent observation of individually marked animals. It consisted of ten cages connected to a central cage by transparent plastic tubes. Two of these cages were defined as 'dispersal cages' and could be reached only by swimming through a water basin. Dispersal was defined as a permanent stay in one of these cages for at least 4 days. At the beginning of the experiment one pair of house mice with their litter was placed into the cage system. Each of six experiments lasted for 6 months during which data on spacing, social interactions, body condition, reproduction, mortality and dispersal were collected by daily observations. Results regarding this study could be summarized as follows: (1) dispersal in house mice is male-biased; (2) there are interfamiliar differences in dispersal age, dispersal rate, and in the development of the population structure; (3) after reaching sexual maturity subdominant males are evicted by the dominant one; (4) reproductive rate among females drops with increasing birth order, thus only the oldest females within a group reproduce; (5) females born under high population density conditions can only reproduce after dispersal.     

Mitochondrial DNA variation in house mice, Mus domesticus (Rutty)

This article summarizes knowledge of house mouse diversity based on restriction analysis of mtDNA from 202 individuals representing 83 localities in Europe, North Africa, the Middle East, and the Americas. It begins by describing the variation among 34 newly collected European mice and relating the 15 new types of mtDNA found in them to the 37 types known from previous work. None of the new types represent deep new branches in the tree. Moreover, the order of branching in the tree reinforces the view that mice north of the Alps have southern origins. Two possible time scales are under consideration for the process of colonization of north-western Europe. According to one hypothesis, which assumes that the mtDNA clock in mice ticks at the standard mammalian rate, commensal mice colonized Europe north of the Alps roughly 30 000–70 000 years ago, perhaps in concert with the spread of anatomically modern Homo sapiens . Another hypothesis, which requires accelerated mtDNA evolution in mice, would have the colonization take place 5000–6000 years ago, or even more recently, following the spread of agriculture. The study also shows that mtDNAs closely resembling those M. domesticus mtDNAs that introgressed into the Danish and Swedish populations of M. musculus are widespread in M. domesticus populations of the region near the hybrid zone between these two species in Germany.     

Patterns of infection by Salmonella and Yersinia spp. in commensal house mouse (Mus musculus domesticus) populations

AIMS: This study sought to examine the risk posed by house mice transmitting pathogens to livestock on typical mixed-agriculture farms in the UK. METHODS AND RESULTS: In a 10-month longitudinal study at one farm, 222 faecal samples were taken from mice and 57 swabs from the farm environment; 3.2% and 15.8%, respectively, were positive for Yersinia. Seventy-five intestinal samples were taken from house mice from three other farms and 9.3% were positive for Yersinia. The commonest species was Y. enterocolitica (of a wide range of serotypes); all isolates were non-pathogenic, except one of Y. pseudotuberculosis. Salmonella was not isolated from any sample. CONCLUSION: This study provides additional evidence that house mice are generally not significant vectors of either pathogenic Yersinia strains or Salmonella species. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first longitudinal study of Yersinia in any small mammal population, and shows infection to be a dynamic series of generally non-pathogenic, transient infections.     

The dynamic of the t-haplotype in wild populations of the house mouse Mus musculus domesticus in Israel

The t-haplotype, a variant of the proximal part of the mouse chromosome 17, is composed of at least four inversions and is inherited as a single genetic unit. The haplotype causes embryonic mortality or male sterility when homozygous. Genes within the complex are responsible for distortion of Mendelian transmission ratio in males. Thus, the t-haplotype in heterozygous males is transferred to over 95% of the progeny. We examined the dynamic and behavior of the t-haplotype in wild populations of the house mouse in Israel. The Israeli populations show high frequency (15%–20%) of both partial and complete t-carrying mice, supporting the suggestion that the t-complex evolved in the M. domesticus line in the Israeli region. In one population that had the highest frequency of t-carrying individuals, we compared the level of gene diversity between t-carrying and normal mice in the marker’s loci: H-2 locus of the major histocompatibility complex (MHC) on the t-haplotype of chromosome 17, three microsatellites on other chromosomes, and the mitochondrial D-loop. Genetic variability was high in all tested loci in both t and (+) mice. All t mice carried the same chromosome and showed the same H-2 haplotype. While t-carrying mice showed significant H-2 heterozygotes access, (+) mice expressed significant H-2 heterozygote deficiency. There were no differences in the level of gene diversity between t and (+) mice in the other loci. Heterozygosity level at the MHC may be an additional factor in the selective forces balancing the t-haplotype polymorphism.     

The effect of multiple simple Robertsonian heterozygosity on chromosome pairing and fertility of wild-stock house mice (Mus musculus domesticus)

The influence of Robertsonian (Rb) heterozygosity on fertility has been the subject of much study in the house mouse. However, these studies have been largely directed at single simple heterozygotes (heterozygous for a single Rb metacentric) or complex heterozygotes (heterozygous for several to many metacentrics which share common chromosome arms). In this paper we describe studies on male multiple simple heterozygotes, specifically the F(1) products of crosses between wild-stock mice homozygous for four or seven metacentrics and wild-stock mice with a standard all-acrocentric karyotype; these F(1) products were characterized by four and seven trivalents at meiosis I, respectively. Mice with the same karyotype, but two different genetic backgrounds were examined. Although a range of meiotic and fertility studies were conducted, particular emphasis was paid to analysis of chromosome pairing, previously not well-described in multiple simple heterozygous mice. The progression of spermatocytes through prophase I was followed by electron microscopy of surface spread material. As previously shown for single simple Rb heterozygotes, the trivalents that characterize multiple simple heterozygotes initially showed delayed pairing of the centromeric region and later showed side arm formation, resulting from non-homologous pairing by the centromeric ends of the acrocentric chromosomes. In the four trivalent groups of mice, 15 and 32% of trivalents showed unpairing in the centromeric region at mid pachytene; equivalent values were 29 and 39% for the seven trivalent groups. Pairing abnormalities (largely attachments and interlocks between trivalents and between a trivalent and the XY configuration) were observed in 18 and 23% of mid pachytene cells in the four trivalent groups and 36 and 49% of cells in the seven trivalent groups. The greater level of pachytene irregularity (unpairing and pairing abnormalities) in seven versus four trivalent heterozygotes was mirrored in terms of higher anaphase I nondisjunction frequency and lower germ cell counts. However, while pachytene irregularities appear to contribute to germ cell death, examples of male sterility in our material undoubtedly also involve genic incompatibilities.     

Cryptosporidium tyzzeri and Cryptosporidium muris originated from wild West-European house mice (Mus musculus domesticus) and East-European house mice (Mus musculus musculus) are non-infectious for pigs

Three and 8 week old pigs were inoculated with Cryptosporidium muris HZ206 (Mus musculus musculus isolate), Cryptosporidium tyzerri CR2090 (M. m. musculus isolate) or C. tyzzeri CR4293 (isolate from a hybrid between Mus musculus domesticus and M. m. musculus) at a dose of 1 × 10(7) oocysts per animal. Inoculated pigs showed no detectable infection and no clinical symptoms of cryptosporidiosis during 30 days post infection (DPI), and no macroscopic changes were detected in the digestive tract following necropsy. Developmental stages were not detected in gastrointestinal tract tissue by histology or PCR throughout the duration of the experiment. The infectivity of isolates was verified on SCID mice, in which oocysts shedding started from 4 to 8 DPI. Based on our findings, it can be concluded that pigs are not susceptible to C. muris or C. tyzzeri infection.     

A hybrid zone comprising staggered chromosomal clines in the house mouse (Mus musculus domesticus).

In the vicinity of John o'Groats (Caithness, Scotland) there is a small karyotypic race of the house mouse (Mus musculus domesticus) characterized by a diploid number of 32 chromosomes, including the metacentrics 4.10, 9.12, 6.13 and 11.14. This race forms a hybrid zone with the standard British race (fully acrocentric chromosomes, 2n = 40). Although hybrid zones normally consist of several (or many) narrow character clines at the same position, this zone is unusual in that the chromosomal clines do not coincide. The cline for arm combination 11.14 is staggered relative to the 6.13 cline and both are separate from the clines for 4.10 and 9.12 (which may or may not coincide). A variety of explanations for the structure of the hybrid zone are discussed. It is possible that this may be a case of 'zonal raciation'.     

Of mice and 'convicts': origin of the Australian house mouse, Mus musculus

The house mouse, Mus musculus, is one of the most ubiquitous invasive species worldwide and in Australia is particularly common and widespread, but where it originally came from is still unknown. Here we investigated this origin through a phylogeographic analysis of mitochondrial DNA sequences (D-loop) comparing mouse populations from Australia with those from the likely regional source area in Western Europe. Our results agree with human historical associations, showing a strong link between Australia and the British Isles. This outcome is of intrinsic and applied interest and helps to validate the colonization history of mice as a proxy for human settlement history.     

Ejaculate allocation under varying sperm competition risk in the house mouse, Mus musculus domesticus

A common mechanism through which males can enhance their successin postcopulatory contests over paternity is to inseminate moresperm than their rivals. However, ejaculate production is costlyand the evolution of prudent sperm allocation strategies sensitiveto variation in local levels of sperm competition has now beendemonstrated in diverse taxa, including mammals. Theory predictsan increased sperm allocation in response to an elevated riskof sperm competition, but here we show that male house mice(Mus musculus domesticus) instead ejaculate fewer sperm perejaculate when mating in the presence of a rival male. Thissurprising sperm allocation pattern may be a necessary consequenceof adaptive changes in copulatory behavior, enabling males toachieve more rapid sperm transfer and/or to ejaculate repeatedlyunder risk of sexual competition. The size of a second ejaculatecomponent, the copulatory plug, is unaffected by sperm competitionrisk. Our results highlight how the often complex interplaybetween different reproductive traits can affect the evolutionof sperm competition phenotypes.     

Higher differentiation among subspecies of the house mouse (Mus musculus) in genomic regions with low recombination

In the early stages of reproductive isolation, genomic regions of reduced recombination are expected to show greater levels of differentiation, either because gene flow between species is reduced in these regions or because the effects of selection at linked sites within species are enhanced in these regions. Here, we study the patterns of DNA sequence variation at 27 autosomal loci among populations of Mus musculus musculus, M. m. domesticus, and M. m. castaneus, three subspecies of house mice with collinear genomes. We found that some loci exhibit considerable shared variation among subspecies, while others exhibit fixed differences. We used an isolation-with-gene-flow model to estimate divergence times and effective population sizes (N(e) ) and to disentangle ancestral variation from gene flow. Estimates of divergence time indicate that all three subspecies diverged from one another within a very short period of time approximately 350,000 years ago. Overall, N(e) for each subspecies was associated with the degree of genetic differentiation: M. m. musculus had the smallest N(e) and the greatest proportion of monophyletic gene genealogies, while M. m. castaneus had the largest N(e) and the smallest proportion of monophyletic gene genealogies. M. m. domesticus and M. m. musculus were more differentiated from each other than either were from M. m. castaneus, consistent with greater reproductive isolation between M. m. domesticus and M. m. musculus. F(ST) was significantly greater at loci experiencing low recombination rates compared to loci experiencing high recombination rates in comparisons between M. m. castaneus and M. m. musculus or M. m. domesticus. These results provide evidence that genomic regions with less recombination show greater differentiation, even in the absence of chromosomal rearrangements.     

Norwegian house mice (Mus musculus musculus/domesticus): distributions, routes of colonization and patterns of hybridization

We investigated the distributions and routes of colonization of two commensal subspecies of house mouse in Norway: Mus musculus domesticus and M. m. musculus. Five nuclear markers (Abpa, D11 cenB2, Btk, SMCY and Zfy2) and a morphological feature (tail length) were used to differentiate the two subspecies and assess their distributions, and mitochondrial (mt) D‐loop sequences helped to elucidate their colonization history. M. m. domesticus is the more widespread of the two subspecies, occupying the western and southern coast of Norway, while M. m. musculus is found along Norway’s southeastern coast and east from there to Sweden. Two sections of the hybrid zone between the two subspecies were localized in Norway. However, hybrid forms also occur well away from that hybrid zone, the most prevalent of which are mice with a M. m. musculus‐type Y chromosome and an otherwise M. m. domesticus genome. MtDNA D‐loop sequences of the mice revealed a complex phylogeography within M. m. domesticus, reflecting passive human transport to Norway, probably during the Viking period. M. m. musculus may have colonized earlier. If so, that leaves open the possibility that M. m. domesticus replaced M. m. musculus from much of Norway, with the widely distributed hybrids a relict of this process. Overall, the effects of hybridization are evident in house mice throughout Norway.     

Chromosomal polymorphism in the house mouse (Mus domesticus) of Greece and Yugoslavia

A total of 88 wild mice from the Dalmatian coast of Yugoslavia (35 animals), and Peloponnesus (30 animals) and Thebes (23 animals) on mainland Greece were karyotyped. In all but five animals Robertsonian translocations were found. Mice from the Dalmatian region were homozygous for translocations Rb(5.15), Rb(6.12), Rb(8.17), Rb(9.13), and Rb(10.14); they were homo-or heterozygous for the translocation Rb(1.11). Some of them lacked the Rb(1.11) translocation altogether so that the diploid numbers in the Yugoslavian mice were 2n=28, 29, 30, or 40. The mice from the vicinity of Olympia in northwestern Peloponnesus were homozygous for eight Robertsonian translocations: Rb(1.3), Rb(2.5), Rb(4.6), Rb(8.12), Rb(9.16), Rb(10.14), Rb(11.17), and Rb(13.15). Their diploid chromosome number was therefore 2n=24. Mice from the vicinity of Patras in northwest Peloponnesus carried all except the first three of these eight translocations; their chromosome number was 2n=30. Finally, the mice from Thebes were homozygous for translocations Rb(2.15), Rb(4.14), Rb(5.12), and Rb(10.13). They were homo- or heterozygous for Rb(6.9), Rb(8.17), and Rb(1.11); some mice lacked the Tb(1.11) translocation altogether. The translocations Rb(6.9)40Tu and Rb(10.13)42Tu represent new arm combinations not found previously in any wild mouse population. the remaining translocations have previously been found in different Mediterranean countries, in Scotland and in southern Germany. The findings suggest that each translocation arose only once and that different translocations have come together in different populations to generate a unique karyotype characterizing this population.     

Two types of mouse (Mus musculus domesticus) Y chromosomes in Quebec.

A Y chromosomal repetitive sequence identified two types of Y chromosomes in mice (Mus musculus domesticus) caught near Ste. Anne de Bellevue, Quebec. One type is apparently identical to the Y chromosome found in Maryland, Delaware, and California, whereas the other type is similar, but not identical, to the Y chromosome present in M.m. poschiavinus, an Alpine race of M.m. domesticus. These findings suggest that the domesticus Y chromosome is highly polymorphic and thus useful for elucidating the relationships among American and European house mouse populations.     

Meiotic recombination and spermatogenic impairment in Mus musculus domesticus carrying multiple simple Robertsonian translocations

We quantitatively analyzed the spermatogenic process, including evaluation of seminiferous tubules with defective cycles, rates of germ cell death and sperm morphology, in adult male mice with standard telocentric chromosomes (2n = 40, CD1 strain), homozygous (2n = 24, Mil II population) and heterozygous (2n = 24 x 40) for Robertsonian (Rb) rearrangements. The animals were analyzed at three different ages: three, five and seven months after birth. The number and position of crossover events were also determined by chiasmata counting and immunostaining with an antibody against mouse MLH1 protein. Our analysis of spermatogenesis confirms the impairment of the spermatogenic process in multiple simple heterozygotes due to both germ cell and abnormal sperm morphology. The detrimental effects exerted by Rb heterozygosities were found to be at least partially buffered with time: the frequency of defective tubules was lower and germ cell survival and sperm morphology better in 7-month-old animals than in the 3- and 5-month-old mice. While there are previously published data on germ cell death in multiple simple heterozygotes, this is the first report of a partial rescue of spermatogenesis with time. The mean frequency of MLH1 foci was lower in Rb homozygous and heterozygous mice than in mice carrying all telocentric chromosomes. The lower number of foci in Rb mice can be ascribed to a decrease in the number of multiple chiasmata and the maintenance of single chiasmata preferentially located in the terminal region of both the telocentric and metacentric chromosomes.     

Genetical variation in three Pacific House mouse (Mus musculus) populations

Studies of allozymic variation at 36 loci in samples of House mouse ( Mus musculus ) populations collected from the island of Hawaii and two islets in the Enewetak Atoll, Marshall Islands are reported. All three populations carried a great deal of inherited variation: the Hawaiian sample had a mean heterozygosity of 16.6% per locus, which is the highest value so far found for a mouse population. The mean heterozygosities for the two atoll populations were 11.4% and 10.9%.
There were no changes in either heterozygosity or allozyme frequencies with age. In this respect, these populations from the tropical Pacific differed from populations living in cold temperate regions where natural selection has been shown to affect allele frequencies in different ways at different stages of the life cycle.