Genic differentiation in M. m. domesticus populations from Europe, the Middle East and North Africa: geographic patterns and colonization events

Comparison of the exploratory behaviour of wild and laboratory mouse species

In this study, we compared the exploratory behaviour of mound-building mice (Mus spicilegus) and house mice (Mus musculus) with domesticated laboratory mouse strains (BALB/c and C57BL/6). The animals spent 15 minutes in the furnished test box before the exit to the outside world became free. During the 5-minute test, it was noted whether the animal left the familiar environment; if it did, it was recorded in how many seconds. Based on our results, the wild mouse species were more likely to leave the familiar mouse box and explore the outside environment earlier than the laboratory mice. We also found a difference within the wild mouse species, the mound-building mouse being the one that explored the external environment to a greater extent and faster. The effect of domestication manifests in the fact that laboratory mouse strains are less likely to leave their familiar environment and are significantly less active than their wild ancestors.

LOW FREQUENCY OF t HAPLOTYPES IN NATURAL POPULATIONS OF HOUSE MICE (MUS MUSCULUS DOMESTICUS)

t haplotypes are a naturally occurring, autosomal, meiotic-drive system found on chromosome 17 of the house mouse. They show non-Mendelian transmission from heterozygous +/t males, such that 90% or more of the male's offspring inherit the t-bearing chromosome. Although they are expected to become rapidly fixed, surveys of natural populations typically report low overall frequencies of only ~15-25% +/t heterozygotes. Generally, such studies of t haplotypes in wild populations have sampled only small numbers of individuals due to the need to genotype mice by breeding, thus we have conducted a large survey of wild mice, Mus musculus domesticus, using DNA markers to examine the frequency and distribution of t haplotypes in natural populations. The overall frequency of +/t heterozygotes from our entire sample was 0.062, which is much lower than all previous estimates of t haplotype frequency. t haplotypes were patchily distributed and rare, and were present in only 46% of the populations we sampled. There were no significant sex-specific differences in the frequency of t haplotypes. Our data suggest that the frequency of +/t heterozygotes in independent populations varies with respect to population size and stability: t haplotypes were at low frequency in all large, relatively persistent populations, whereas they were at more variable, and often higher, frequencies in small, temporally unstable populations. The extinction and recolonization of many of the smaller populations may contribute to the greater variation in t haplotype frequency observed, and small populations may be important reservoirs of t haplotypes in the wild. The highest frequencies of t haplotypes were obtained from populations with semilethal, or complementing lethal, t haplotypes, where t/t homozygous mice were present.

THE ROLE OF DEME SIZE, REPRODUCTIVE PATTERNS, AND DISPERSAL IN THE DYNAMICS OF t-LETHAL HAPLOTYPES

The t-lethal haplotypes (t) found in house mouse (Mus musculus) populations are recessive lethals favored by gametic selection whereby male heterozygotes exhibit a non-Mendelian transmission ratio of about 95% t. The expected equilibrium frequency is 0.385; however, empirical values are lower, averaging close to 0.13. We examined the hypothesis that interdemic selection is the cause of the low empirical values by using a deme-structured simulation model that included overlapping generations, a realistic breeding system, differential deme productivity, and a large total population. We found that under some conditions interdemic selection could lower t frequency below 0.13 in the face of immigration rates up to 5%. Low frequencies were correlated with effective deme size (n_e ), regardless of whether n_e was changed through changing deme size (n) or through changing the proportion of breeding adults. Earlier workers showed how the first two phases of interdemic selection (random genetic differentiation and mass selection) interacted to reduce the haplotype frequency, but here we show the importance of the third phase (differential productivity of demes) once demes are linked by dispersal. The effect of this phase is not due to the (negative) covariation between deme productivity and haplotype frequency, but occurs when differential deme productivity generates a difference in t frequency between the population of juveniles recruited into their natal deme and the population of juvenile dispersers. This difference was maximized when the average productivity of demes was low, either because few adult females bred at any one time and/or because fecundity was low. Contrary to an earlier prediction, male-biased dispersal also reduced haplotype frequency, and this probably stems from the relative excess of wild-type genotypes among dispersers compared to the deme residents. Another unexpected finding was that the randomly generated excess of heterozygotes (F_IS < 0) found in small demes favored t haplotypes; however, the effect was only seen when the more powerful influence of the third phase of interdemic selection was removed. Simulations of neutral polymorphisms showed that a deme structure giving F_ST ≤ 0.6 is inconsistent with a haplotype frequency below 0.13. Based on current empirical estimates of F_ST (about 0.2), we concluded that immigration rates in the field are too high for interdemic selection alone to cause the observed deficit of lethal haplotypes. One factor that could combine with population structure effects is the observation that the transmission ratio is lowered to around 0.6 in litters produced from postpartum estrus (PPE). Incorporating this factor, we showed that interdemic selection could be effective in lowering the frequency of t below 0.13 when F_ST was above 0.43 even when migration rates were up to 10%. These results suggest that if empirical haplotype and F_ST estimates are accurate, then additional factors such as a lowered fitness of heterozygotes may be involved.

Reduced recombination patterns in Robertsonian hybrids between chromosomal races of the house mouse: chiasma analyses

The recombination suppression models of chromosomal speciation posit that chromosomal rearrangements act as partial barriers to gene flow allowing these regions to accumulate genetic incompatibilities, thus contributing to the divergence of populations. Empirical and theoretical studies exploring the requirements of these models have mostly focused on the role of inversions. Here, the recombination landscape of heterozygosity for Robertsonian (Rb) fusions is investigated in the house mouse. Laboratory-bred F1 males and females between highly differentiated races from Tunisia (Rb: 2n=22, Standard, St: 2n=40) were produced in which all Rb fusions are present as trivalents in meiosis. Recombination patterns were determined by the analysis of chiasmata and compared with previous data on the Tunisian parental mice. A comparative analysis was performed on wild-caught male mice spanning the hybrid zone between two Italian races (2n=40, 2n=22). The results showed that the chiasma characteristics of both male and female Tunisian F1 and Italian hybrids clearly differed from those of Rb and St mice. Not only was the mean chiasma number (CN) intermediate between those of the parental mice in both geographic samples, but the distribution of chiasmata along the chromosomal arms of the F1 showed a distinct mosaic pattern. In short, the proximal region in the F1 exhibited a reduced CN similar to that observed in homozygous Rb, whereas distal regions more closely matched those in St mice. These results suggest that Rb rearrangements (homozygous or heterozygous) reduce recombination in the proximal regions of the chromosomes supporting their potential role in recombination-mediated speciation models.

Demographic history of a recent invasion of house mice on the isolated Island of Gough

Island populations provide natural laboratories for studying key contributors to evolutionary change, including natural selection, population size and the colonization of new environments. The demographic histories of island populations can be reconstructed from patterns of genetic diversity. House mice (Mus musculus) inhabit islands throughout the globe, making them an attractive system for studying island colonization from a genetic perspective. Gough Island, in the central South Atlantic Ocean, is one of the remotest islands in the world. House mice were introduced to Gough Island by sealers during the 19th century and display unusual phenotypes, including exceptionally large body size and carnivorous feeding behaviour. We describe genetic variation in Gough Island mice using mitochondrial sequences, nuclear sequences and microsatellites. Phylogenetic analysis of mitochondrial sequences suggested that Gough Island mice belong to Mus musculus domesticus, with the maternal lineage possibly originating in England or France. Cluster analyses of microsatellites revealed genetic membership for Gough Island mice in multiple coastal populations in Western Europe, suggesting admixed ancestry. Gough Island mice showed substantial reductions in mitochondrial and nuclear sequence variation and weak reductions in microsatellite diversity compared with Western European populations, consistent with a population bottleneck. Approximate Bayesian computation (ABC) estimated that mice recently colonized Gough Island (~100 years ago) and experienced a 98% reduction in population size followed by a rapid expansion. Our results indicate that the unusual phenotypes of Gough Island mice evolved rapidly, positioning these mice as useful models for understanding rapid phenotypic evolution.

Genetic differentiation of the house mouse around the Mediterranean basin: matrilineal footprints of early and late colonization

The molecular signatures of the recent expansion of the western house mouse, Mus musculus domesticus, around the Mediterranean basin are investigated through the study of mitochondrial D-loop polymorphism on a 1313 individual dataset. When reducing the complexity of the matrilineal network to a series of haplogroups (HGs), our main results indicate that: (i) several HGs are recognized which seem to have almost simultaneously diverged from each other, confirming a recent expansion for the whole subspecies; (ii) some HGs are geographically delimited while others are widespread, indicative of multiple introductions or secondary exchanges; (iii) mice from the western and the eastern coasts of Africa harbour largely different sets of HGs; and (iv) HGs from the two shores of the Mediterranean are more similar in the west than in the east. This pattern is in keeping with the two-step westward expansion proposed by zooarchaeological data, an early one coincident with the Neolithic progression and limited to the eastern Mediterranean and a later one, particularly evident in the western Mediterranean, related to the generalization of maritime trade during the first millennium BC and onwards. The dispersal of mice along with humans, which continues until today, has for instance left complex footprints on the long ago colonized Cyprus or more simple ones on the much more recently populated Canary Islands.

Of mice and (Viking?) men: phylogeography of British and Irish house mice

The west European subspecies of house mouse (Mus musculus domesticus) has gained much of its current widespread distribution through commensalism with humans. This means that the phylogeography of M. m. domesticus should reflect patterns of human movements. We studied restriction fragment length polymorphism (RFLP) and DNA sequence variations in mouse mitochondrial (mt) DNA throughout the British Isles (328 mice from 105 localities, including previously published data). There is a major mtDNA lineage revealed by both RFLP and sequence analyses, which is restricted to the northern and western peripheries of the British Isles, and also occurs in Norway. This distribution of the 'Orkney' lineage fits well with the sphere of influence of the Norwegian Vikings and was probably generated through inadvertent transport by them. To form viable populations, house mice would have required large human settlements such as the Norwegian Vikings founded. The other parts of the British Isles (essentially most of mainland Britain) are characterized by house mice with different mtDNA sequences, some of which are also found in Germany, and which probably reflect both Iron Age movements of people and mice and earlier development of large human settlements. MtDNA studies on house mice have the potential to reveal novel aspects of human history.

Population genetic structure of two ecologically distinct multimammate rats: the commensal Mastomys natalensis and the wild Mastomys erythroleucus in southeastern Senegal

Using the same set of microsatellite markers, we compared the population genetic structure of two Mastomys species, one being exclusively commensal in southeastern Senegal, and the other being continuously distributed outside villages in this region. Both species were sampled in the same landscape context and at the same spatial scale. According to the expectations based on the degree of habitat patchiness (which is higher for commensal populations in this rural area), genetic diversity was lower and genetic differentiation was higher in commensal populations of Mastomys natalensis than in wild populations of Mastomys erythroleucus. Contrasting estimates of effective dispersal and current migration rates corroborates previous data on differences in social structure between the two species. Isolation-by-distance analyses showed that human-mediated dispersal is not a major factor explaining the pattern of genetic differentiation for M. natalensis, and that gene flow is high and random between M. erythroleucus populations at the spatial scale considered.

Patterns of genic diversity and structure in a species undergoing rapid chromosomal radiation: an allozyme analysis of house mice from the Madeira archipelago

The chromosomal radiation of the house mouse in the island of Madeira most likely involved a human-mediated colonization event followed by within-island geographical isolation and recurrent episodes of genetic drift. The genetic signature of such processes was assessed by an allozyme analysis of the chromosomal races from Madeira. No trace of a decrease in diversity was observed suggesting the possibility of large founder or bottleneck sizes, multiple introductions and/or a high post-colonization expansion rate. The Madeira populations were more closely related to those of Portugal than to other continental regions, in agreement with the documented human colonization of the island. Such a Portuguese origin contrasts with a study indicating a north European source of the mitochondrial haplotypes present in the Madeira mice. This apparent discrepancy may be resolved if not one but two colonization events took place, an initial north European introduction followed by a later one from Portugal. Asymmetrical reproduction between these mice would have resulted in a maternal north European signature with a nuclear Portuguese genome. The extensive chromosomal divergence of the races in Madeira is expected to contribute to their genic divergence. However, there was no significant correlation between chromosomal and allozyme distances. This low apparent chromosomal impact on genic differentiation may be related to the short time since the onset of karyotypic divergence, as the strength of the chromosomal barrier will become significant only at later stages.

Evidence for the existence of two distinct species: Psammomys obesus and Psammomys vexillaris within the sand rats (Rodentia, Gerbillinae), reservoirs of cutaneous leishmaniasis in Tunisia

A thorough taxonomic knowledge about putative animal reservoirs of transmissible diseases is an absolute prerequisite to any ecological investigation and epidemiological survey of zoonoses. Indeed, accurate identification of these reservoirs is essential for predicting species-specific population outbreaks and therefore to develop accurate ecological control strategies. The systematic status of sand rats (genus Psammomys) remains unclear despite the pivotal role of these rodents in the epidemiology of Zoonotic Cutaneous Leishmaniasis (ZCL) disease as sand rats are the main known reservoir hosts of the protozoan parasite Leishmania major. In the present work, we expose morphological, biochemical, genetic and cytogenetic evidence supporting the identification of at least two cryptic species within the genus Psammomys in Tunisia. First, significant morphometric differences were observed and were correlated associated with external features and biogeographic origins. Second, differences in patterns of two isoenzymic systems (Glutamate Oxaloacetate Transaminase (GOT) and 6-PhosphoGluconate Dehydrogenase (6PGD)) were found, which makes it possible to amount these isoenzyme characters to two diagnostic loci. Third, based on the mitochondrial cytochrome b (cyt b) gene, a high magnitude of genetic distance (13.89%) was also observed. Fourth, cytogenetic analysis showed that these two populations groups differ in their diploid chromosome numbers, i.e. 2N=46 versus 2N=48. We consider that all these variations are enough important to be considered as demonstrative and we propose that these two lineages should be considered as two distinct species that we refer to the fat sand rat Psammomys obesus Cretzschmar, 1828 and the thin sand rat Psammomys vexillaris Thomas, 1925. Implications of such results on the eco-epidemiology of ZCL in Tunisia are discussed.

Mus spretus et M. musculus (Rodentia, Mammalia) en zone méditerranéenne: différenciation biométrique et morphologique: application à des fossiles marocains pléistocènes / Mus spretus and M. musculus (Rodentia, Mammalia) in the Mediterranean zone: biometric and morphological differentiation: application to Pleistocene Moroccan fossils

La présente étude porte sur la taille et la forme des dents et sur la morphologie crânienne d'individus en provenance d'Europe sud-occidentale et d'Afrique du Nord caractérisés génétiquement comme appartenant aux espèces Mus spretus et Mus musculus domesticus . Une révision des caractères diagnostiques des dents et du crâne utilisés pour la détermination de l'appartenance spécifique des spécimens a été faite. La révision porte sur 17 caractères métriques et 14 caractères morphologiques. Une application à un matériel fossile inédit d'âge pléistocène supérieur du Maroc a ensuite été tentée. La détermination de ces restes fossiles représentés par des molaires inférieures de souris du gisement de Doukkala II, près de Rabat, permet de conclure à la présence d'une souris très proche de l'actuelle M. spretus . La présence dans le Pléistocène supérieur d'une souris, différente de la souris domestique, est ainsi démontrée dans le nord de l'Afrique, l'arrivée de la souris domestique dans ce secteur restant à préciser.

Allozymic polymorphism among 14 populations of the house mouse, Mus musculus domesticus, from Greece

Nineteen loci from 239 individuals of the house mouse Mus musculus domesticus (Rodentia, Muridae) were analyzed by means of thin layer electrophoresis. The mice were collected from 14 localities of Greece mainly confined to the area of NW Peloponnese, where a Robertsonian (Rb) system is observed. The individuals were chromosomally characterized by nine diploid numbers, the 2n = 24, 26, 27, 28, 29, 30, 31, 32, and 40. The statistic elaboration revealed that all 14 populations studied were not characterized by cohesive demic structure and high inbreed levels while the gene flow among them has resulted in low levels of genetic differentiation. The resulting values for Nei's genetic distance corresponded to distances known for the level of geographical populations of M. musculus. Wagner's cladogram for the phylogenetic relations between the populations studied implied that it is the diploid number, rather than the geographical factor, that characterizes or dominates each population, which mainly influences the phylogenetic relationships.

Population differentiation and gene flow revealed by microsatellite DNA markers in the house mouse (Mus musculus castaneus) in Taiwan

We analyzed population subdivision and gene flow of the Southeast Asian house mouse (Mus musculus castaneus) in Taiwan by using six microsatellite DNA markers. Seven populations of the house mouse (187 individuals), including one from Fukien Province in southeastern China, which is separated from Taiwan by the Taiwan Strait, were analyzed in this study. The overall polymorphic level at the six loci was high (He = 0.76) although individual populations varied in their levels of heterozygosity (He = 0.35-0.83). For the populations within Taiwan, there was no evidence of isolation by distance and the level of gene flow was not (inversely) correlated to geographic distances. Gene flow was estimated to be higher across the Taiwan Strait than within the island of Taiwan. These observations of gene flow cannot be understood unless in the context of the historical human settlements and agricultural expansion, and the commensal habits of the species. We also discussed the causes of population subdivision and genetic variation among populations in terms of ecological characteristics of the house mouse in Taiwan.

[Natural hybridization between 2 sympatric species of mice, Mus musculus domesticus L. and Mus spretus Lataste]

Using protein loci and DNA markers, we show by a multilocus genetic analysis that certain populations of the two sympatric mouse species Mus musculus domesticus and Mus spretus show clear signs of partial introgression. Given the sterility of F1 males and the known partial genetic incompatibilities between the genomes of the two species, our finding does not invalidate the biological species complex, but allows to think that very limited genetic exchanges remain possible even long after the divergence of taxa. This may have some consequences on the dynamics of certain kinds of invasive or advantageous DNAs like transposable elements or pathogen resistance genes.

Population genetic analysis of Colombian Trypanosoma cruzi isolates revealed by enzyme electrophoretic profiles

Although Colombia presents an enormous biological diversity, few studies have been conducted on the population genetics of Trypanosoma cruzi. This study was carried out with 23 Colombian stocks of this protozoa analyzed for 13 isoenzymatic loci. The Hardy-Weinberg equilibrium, the genetic diversity and heterogeneity, the genetic relationships and the possible spatial structure of these 23 Colombian stocks of T. cruzi were estimated. The majority of results obtained are in agreement with a clonal population structure. Nevertheless, two aspects expected in a clonal structure were not discovered in the Colombian T. cruzi stocks. There was an absence of given zymodemes over-represented from a geographical point of view and the presumed temporal stabilizing selective phenomena was not observed either in the Colombian stocks sampled several times through the years of the study. Some hypotheses are discussed in order to explain the results found.

Geographical patterns of genetic subdivision in the cellar spider Pholcus phalangioides (Araneae)

Geographical patterns of gene flow and drift were analysed in the commensal cellar spider Pholcus phalangioides to get insight into the causes affecting genetic variation in this species strictly associated with man. Our sampling consisted of 23 subpopulations collected over five urban regions in central Europe (distances ranged from 920 km to sites within the same building complex). Five variable allozyme loci showed significant interpopulation subdivision (theta=0.146) and isolation by distance over the area studied. On a regional scale (up to 70 km) significant differentiation was found, but the genetic pattern did not correlate with distance. Moreover, significant two-locus disequilibria were detected and a recent reduction in the effective population size was indicated within six sites. These results suggest that in P. phalangioides a high potential of dispersal and strong effects of drift within small, demographically unstable mating units seem to cause significant, but unpredictable genetic differentiation patterns at lower geographical scales. Our study documents strong effects of drift in a strictly commensal species outside the murine rodents.

Population genetic structure in a Robertsonian race of house mice: evidence from microsatellite polymorphism

Genetic evidence was assessed for inbreeding and population subdivision in a Robertsonian fusion (Rb) race of the western European form of house mouse, Mus musculus domesticus, in central Belgium. Inbreeding, and the factors responsible for subdivision (genetic drift and extinction-recolonization) can theoretically influence the fixation of underdominant Rb variants. The data consisted of allele frequencies of eight microsatellite loci and of the Rb(4.12) and Rb(5.10) chromosomes. Six populations were sampled once, and a seventh was sampled successively over 3 years. No evidence for inbreeding within populations was found. Levels of between-population subdivision were high (theta = 0.15-0.39), and showed no association with either karyotype or geographical distance over 8-60 km. In addition, low values of effective size were found in the successively sampled population (Ne = 5-20). Cases of significant two-locus disequilibria were associated with the most closely linked pair of microsatellite loci (r = 0.15): also consistent with small effective sizes. These results suggest that both the lack of inbreeding, and the combined effects of genetic drift and extinction-recolonization, may promote Rb polymorphism in M. m. domesticus.

Genetic distinctiveness of a village population of house mice: relevance to speciation and chromosomal evolution

A population of house mice, Mus musculus domesticus, from the village of Migiondo was found to be genetically distinct from nearby populations in Upper Valtellina (Italian Alps). At the supernatant malic enzyme locus, Mod1, the only alleles found in Migiondo (c and n2) were virtually absent from the other populations in the valley, which were characterized by allele a. The extraordinary genetic distinctiveness of the Migiondo population is apparently the result of genetic drift, perhaps coupled with a founder event, and attests to the existence of nearly impenetrable geographic barriers around the village isolating it from other settlements only a few hundred metres away. The Mod1 features of the house mice in Migiondo are reminiscent of the characteristics of house mice on maritime islands. The genetic confirmation of the geographic isolation of Migiondo is of interest because there is evidence that this village may have been the site of recent speciation and extinction events. The data are also of significance given the phenomenal chromosomal variation in house mice from the vicinity of the Alps. It has frequently been proposed that genetic drift/founder events are of importance in the fixation of chromosomal rearrangements; this study provides the first direct evidence for their occurrence in alpine mouse populations.

Chromosomal and molecular divergence in the Indian pygmy field mice Mus booduga-terricolor lineage of the subgenus Mus

Mus booduga and Mus terricolor both have 2n = 40. Unlike M. booduga, with all acrocentric chromosomes, M. terricolor invariably has large submetacentric X and acrocentric Y due to an increase of heterochromatin. In contrast to the conservative karyotype of the co-existing sibling species booduga, three chromosome types of terricolor are found in different populations and their divergent karyotypes have autosomal heterochromatin variations established in the homozygous condition. The average genetic distance determined from electrophoretic study of 20 protein loci ranges from lowest (D = 0.106) between chromosome types I & II to highest (D = 0.185) between types II & III. In terricolor, booduga and M. m. tytleri high mean values of variations per locus (range A = 1.604 to 1.928) and heterozygosity per individual per locus (range H = 0.180 to 0.336) have been observed. Sequence divergence of 0.39 to 1.2%, calculated from restriction profiles of mtDNA, shows that the terricolor chromosome types have diverged recently. Hybridizations between type I females and type III males gave a preponderance of males in the F1 with varying degrees of sterility. The 'terricolor complex' is an interesting system for critical probing for the role of heterochromatin in the process of speciation. MtDNA, protein loci and AT-rich musculus-related major and minor satellite DNA data indicate that progenitors of the booduga-terricolor lineage might have evolved simultaneously with the caroli-cookii-cervicolor lineage in the evolution of the subgenus Mus.

The nature of gene evolution on the mammalian Y chromosome: lessons from Sry

With the exception of a small region, heteromorphic sex chromosomes of mammals do not undergo recombination in male meiosis. As a result, the majority of the Y chromosome is clonally transmitted through paternal lineages. Numerous phenomena, including the Hill-Robertson effect, Muller's ratchet, genetic hitch-hiking, and male-driven molecular evolution, are associated with the special transmission properties of the Y chromosome, and can potentially explain the tempo and pattern of gene evolution on the mammalian Y. We explore these phenomena in light of comparative data from the Y-linked sex-determining locus, Sry. Sry exhibits rapid amino acid divergence between species and little to no variation within species. We find no evidence for directional selection acting on this locus. The pattern of evolution between species is consistent with the Hill-Robertson effect and Muller's ratchet. Lack of variation in Sry within species may reflect genetic hitch-hiking, however, we cannot exclude the confounding effects of small effective population size of Y chromosomes. We find no support for male-driven molecular evolution for Sry in Old World mice and rats. However, a more appropriate test of this hypothesis would be to compare the evolution of Sry to the X-linked Sox3 gene in these same species. Clearly, more comparative studies of Sry and other Y-linked loci are needed to characterize the effects of Y chromosome transmission on the evolution of Y-linked sequences.

Population subdivision and gene flow in Danish house mice

Genetic subdivision in local populations of the European house mice, Mus musculus domesticus and M. m. musculus, was analysed to study patterns of gene flow. The data consisted of frequencies of microsatellite alleles in 16 samples (250 individuals) from a total of 11 sites in Jutland, which included successive samples from three sites. Sequences of the control region of mitochondrial DNA in three successive samples from one site were also analysed. Microsatellite genotype frequencies within samples were close to Hardy-Weinberg expectations. Levels of microsatellite differentiation among samples (theta = 0.05-0.21) corresponded to limited gene flow at migration-drift equilibrium (Nm = 1-5). Weak isolation by distance for microsatellites in M.m. musculus suggested that gene flow tends to occur among neighbouring sites. Estimates of effective population size over a few generations were much lower than those corresponding to the long periods needed for arrival at mutation-drift equilibrium. This suggested that subpopulations had been influenced by gene flow since formation, or had originated recently from genetically diverse founders.

The origin of common laboratory mice

The house mouse is one of the model organisms in genetics and more than 400 inbred strains have been established. However, many of the strains are related and their ancestry can be traced back to European fancy mice inbred in the 1920s. Recent molecular studies corroborate the early historical records that assert that Japanese fancy mice were introduced into European stocks and thus contributed to the development of "old" inbred strains. Consequently, many inbred strains have genomic DNA derived from more than one subspecies of Mus musculus. The subspecific hybrid origin of common inbred strains has important bearings on the interpretation of genetic data, and the limitations that history imposes upon the currently available strains make it necessary to establish new inbred strains representing specific wild populations.

Polymorphism, localization and geographical transfer of mitochondrial DNA in Mus musculus domesticus (Irish house mice)

The analysis of mtDNA restriction fragments from Irish house mice revealed much polymorphism, both within and between populations. Many phenotypes showed geographical localization and there was a strong correlation between geographical distance and genetic divergence. Populations, which are discontinuous and limited to buildings or their vicinity, are apparently the result of short-range migration. Transport by man, with whom the species is closely associated, appears to have negligible impact. There is some evidence of the influence of topographical features on migration and consequent genetic interchange.

Life history and bioeconomy of the house mouse

1. More is known about the western European house mouse, Mus (musculus) domesticus than any other non-human mammal. If laboratory and field information is combined, an extremely valuable understanding of the species' bioeconomy could be obtained. 2. The seven stages of mouse life-history are surveyed (up to birth, nest life, sex life, social structure, population statics and stability, senescence, and death), and the interactions between the changing phenotype and the environment are described. 3. These interactions can be used to build up a model of the opportunities and compromises which result in the fitness of individual mice. It is not yet possible to quantify such a model, but this should in principle be achievable.