Phylogeography of Chinese house mice (Mus musculus musculus/castaneus): distribution,routes of colonization and geographic regions of hybridization

House mice (Mus musculus) are human commensals and have served as a primary model in biomedical, ecological and evolutionary research. Although there is detailed knowledge of the biogeography of house mice in Europe, little is known of the history of house mice in China, despite the fact that China encompasses an enormous portion of their range. In the present study, 535 house mice caught from 29 localities in China were studied by sequencing the mitochondrial D‐loop and genotyping 10 nuclear microsatellite markers distributed on 10 chromosomes. Phylogenetic analyses revealed two evolutionary lineages corresponding to Mus musculus castaneus and Mus musculus musculus in the south and north, respectively, with the Yangtze River approximately representing the boundary. More detailed analyses combining published sequence data from mice sampled in neighbouring countries revealed the migration routes of the two subspecies into China: M. m. castaneus appeared to have migrated through a southern route (Yunnan and Guangxi), whereas M. m. musculus entered China from Kazakhstan through the north‐west border (Xinjiang). Bayesian analysis of mitochondrial sequences indicated rapid population expansions in both subspecies, approximately 4650–9300 and 7150–14 300 years ago for M. m. castaneus and M. m. musculus, respectively. Interestingly, the migration routes of Chinese house mice coincide with the colonization routes of modern humans into China, and the expansion times of house mice are consistent with the development of agriculture in southern and northern China, respectively. Finally, our study confirmed the existence of a hybrid zone between M. m. castaneus and M. m. musculus in China. Further study of this hybrid zone will provide a useful counterpart to the well‐studied hybrid zone between M. m. musculus and Mus musculus domesticus in central Europe.     

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.     

Genetic analysis of X-linked hybrid sterility in the house mouse

Hybrid sterility is a common postzygotic reproductive isolation mechanism that appears in the early stages of speciation of various organisms. Mus musculus musculus and Mus musculus domesticus represent two recently separated mouse subspecies particularly suitable for genetic studies of hybrid sterility. Here we show that the introgression of Chr X of M. m. musculus origin (PWD/Ph inbred strain, henceforth PWD) into the genetic background of the C57BL/6J (henceforth B6) inbred strain (predominantly of M. m. domesticus origin) causes male sterility. The X-linked hybrid sterility is associated with reduced testes weight, lower sperm count, and morphological abnormalities of sperm heads. The analysis of recombinant Chr Xs in sterile and fertile males as well as quantitative trait locus (QTL) analysis of several fertility parameters revealed an oligogenic nature of the X-linked hybrid sterility. The Hstx1 locus responsible for male sterility was mapped near DXMit119 in the central part of Chr X. To ensure full sterility, the PWD allele of Hstx1 has to be supported with the PWD allelic form of loci in at least one proximal and/or one distal region of Chr X. Mapping and cloning of Hstx1 and other genes responsible for sterility of B6–X^PWDY^B6 males could help to elucidate the special role of Chr X in hybrid sterility and consequently in speciation.     

Origin and radiation of the house mouse: mitochondrial DNA phylogeny

On the basis of patterns of allele frequency variation in nuclear genes (Din et al., in press) it has been proposed that the house mouse M. musculus originated in the northern Indian subcontinent, from where it radiated in several directions to form the well-described peripheral subspecies (M. m. domesticus, M. m. musculus and M. m. castaneus). Here we use a mitochondrial DNA (mtDNA) phylogeny to test this hypothesis and to analyse the historical and demographic events that have accompanied this differentiation. This marker also provides a powerful means to check for genetic continuity between the central and peripheral populations. We studied restriction site polymorphism of samples from India and the Middle East as well as samples from the rest of Eurasia and northern Africa. M. m. domesticus and M. m. musculus are both monophyletic for mtDNA and belong to the subspecies-specific mtDNA lineages that have been described previously. Average nucleotide diversity is low in M. m. musculus (0.2–5%). It is not only higher in M. m. domesticus (0.7–0.9%) but the distribution of pairwise divergence is wider, and the rate of evolution in this branch appears to be higher than in M. m. musculus. The nucleotide diversity found in M. m. castaneus (0.4%) is due to the existence of two rather divergent linages with little intralineage variation. These two lineages are part of a diversified bush of the phylogenetic tree that also comprises several previously undescribed branches and includes all samples from the northern Indian subcontinent and Iran. The degree of diversity found in each of the samples from this region is high (1.2–2.4%) although they come from small geographic areas. This agrees well with the idea that the origin of the radiation was in the northern Indian subcontinent. However, as neither haplotypes on the M. m. domesticus nor on the M. m. musculus branches were found in this region, there appear to be important phylogeographic discontinuities between this central region and these peripherial subspecies. On the basis of the present result and the nuclear data (Din et al., in press), we propose that M. musculus originated in the north of the indian subcontinent. Our calibration of the evolutionary rate of mtDNA in mice suggests that the mouse settlement in this region could be as old as 900 000 years. Possibly from there, a first radiation could have reach the Middle East and the Caspian Sea, where the M. m. domesticus and M. m. musculus lineages, respectively, would have started to differentiate a few hundred thousand years ago, and from where they could have colonised the peripheral part of their ranges only recently.M. m. castaneus appears from its mtDNA to be recent offshoot of the northern Indian population. This multiple and gradual radiation ultimately led to recent peripheral secondary contacts, such as the well-known European hybrid zone.     

The meek inherit the earth: less aggressive wild mice are more successful in challenging situations

Numerous studies have shown an association between aggressiveness and several other behavioural traits. For example, more aggressive animals were bold and active explorers tending to form persistent routines whereas less aggressive animals were shy, careful but more flexible. While the former are thought to be more successful under stable conditions the latter should have advantages in more dynamic situations. These differences can apply not only to individuals but also to populations, species or groups of species with important implications to species distributions and speciation rates. Here we utilized the Morris water task (MWT) to investigate how two subspecies, Mus musculus musculus and M. m. domesticus, known to differ in aggressiveness, cope with stressful situations. We found that less aggressive musculus males performed significantly better in solving the MWT than more aggressive domesticus males. This suggests that M. m. musculus is more flexible and could be more successful under stressful and/or dynamic situations typical of dispersal bouts. It seems plausible that this difference may have had an influence on the secondary contact between musculus and domesticus populations in the past and perhaps still can affect the dynamics of the European hybrid zone between the subspecies. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 310–319.     

Prdm9 Intersubspecific Interactions in Hybrid Male Sterility of House Mouse

The classical definition posits hybrid sterility as a phenomenon when two parental taxa each of which is fertile produce a hybrid that is sterile. The first hybrid sterility gene in vertebrates, Prdm9, coding for a histone methyltransferase, was identified in crosses between two laboratory mouse strains derived from Mus mus musculus and M. m. domesticus subspecies. The unique function of PRDM9 protein in the initiation of meiotic recombination led to the discovery of the basic molecular mechanism of hybrid sterility in laboratory crosses. However, the role of this protein as a component of reproductive barrier outside the laboratory model remained unclear. Here, we show that the Prdm9 allelic incompatibilities represent the primary cause of reduced fertility in intersubspecific hybrids between M. m. musculus and M. m. domesticus including 16 musculus and domesticus wild-derived strains. Disruption of fertility phenotypes correlated with the rate of failure of synapsis between homologous chromosomes in meiosis I and with early meiotic arrest. All phenotypes were restored to normal when the domesticus Prdm9^dom2 allele was substituted with the Prdm9^dom2H humanized variant. To conclude, our data show for the first time the male infertility of wild-derived musculus and domesticus subspecies F1 hybrids controlled by Prdm9 as the major hybrid sterility gene. The impairment of fertility surrogates, testes weight and sperm count, correlated with increasing difficulties of meiotic synapsis of homologous chromosomes and with meiotic arrest, which we suppose reflect the increasing asymmetry of PRDM9-dependent DNA double-strand breaks.     

Counterselection on sex chromosomes in the Mus musculus European hybrid zone

The extent to which alleles can disperse across a hybrid zone depends on the selection they are subjected to in the hybrid genetic background or, for those that are selectively neutral, on their ability to escape from the unfavourable environment by recombination. Three markers spanning a 45 cM segment in the center of the X chromosome were used to investigate the degree to which selection against X chromosome linked genes helps to maintain the barrier to gene flow in the hybrid zone between Mus musculus domesticus and M. m. musculus in Denmark. The introgression of all the sex chromosome specific markers was more limited than that of the autosomal enzymes (Idh1, Amy, Gpd1, Pgm1, Es1, Es2, Mpi, Np1, Es10, Sod1) and the mitochondrial DNA. The cline for DXPas2, which is in the center of the X chromosome, is extremely steep and shows that certain genes located in this region are strongly selected against in the hybrid background. The clines of the other two X-linked markers, Hprt and DXPas1, and of the Y chromosome are not as abrupt and all three have similar asymmetric introgression patterns. Although the musculus variants appear to behave in much the same way as those of the autosomal genes, the domesticus variants do not introgress. The results show that X-linked and to a lesser extent Y-linked genes are more strongly selected against in the hybrid genome than the mitochondrial genome or the different autosomal loci. This suggests that co-adapted gene systems involving the sex chromosomes may play an important role in the hybrid breakdown between the two subspecies.     

Polymorphic HSRs in chromosome 1 of the two semispecies Mus musculus musculus and M. m. domesticus have a common origin in an ancestral population

HSRs (homogeneously staining regions) are the cytological correlates of DNA amplification. In the house mouse, Mus musculus, many populations are polymorphic for the presence or absence of HSRs on chromosome 1. In the semispecies M. m. domesticus the amplified DNA is present within one HSR, whereas in M. m. musculus chromosomes 1 with two HSRs are found. Hybridization of HSR-specific probes to Southern blots of HSR-carrying genomic DNAs from different localities and semispecies revealed similar complex band patterns. the remaining variation is restricted to sequences with a low degree of amplification. Variation is higher between semispecies than within one semispecies. It is assumed that HSRs are derived from one original amplification event and that unequal recombination is the mechanism underlying the length variation of HSRs present today in both semispecies. Evidence from G-banding and in situ hybridization shows that the two HSRs of M. m. musculus originated from a single HSR by means of a paracentric inversion, where one break-point was located within the single HSR and the second outside the HSR. As a consequence of the paracentric inversion the two HSRs of M. m. musculus are permanently linked together. Since exchange of genes between the two semispecies is restricted to a narrow hybrid zone the amplification that gave rise to the HSR most probably occurred prior to the divergence into the semispecies M. m. domesticus and M. m. musculus about 1 million years ago.by D. Schweizer     

ABRUPT CLINE FOR SEX CHROMOSOMES IN A HYBRID ZONE BETWEEN TWO SPECIES OF MICE

We compared the patterns of movement of sex chromosomal and autosomal loci along a 160 km transect across a zone of hybridization between M. domesticus and M. musculus in southern Germany and western Austria using seven genetic markers. These included one Y-specific DNA sequence (YB10), two X-specific loci (DXWas68 and DXWas31), and four autosomal isozyme loci (Es-10, Es-1, Mpi-1, and Np-1). Random effects logistic regression analysis enabled us to examine the relationship between M. domesticus allele frequency and geographic distance from the western edge of the hybrid zone and allowed statistical evaluation of differences in cline midpoint and width among loci. More limited movement was observed for all three sex chromosomal markers across the zone compared with three of the four autosomal markers. If differential movement reflects fitness differences of specific alleles (or alleles at closely linked loci) on a hybrid background, then alleles that move to a limited extent across a hybrid zone may contribute to hybrid breakdown between two species. The limited flow of both X- and Y-specific alleles suggest that sex chromosomes have played an important role in Mus speciation.     

The location of the<Emphasis Type="Italic">Mus musculus/M. domesticus</Emphasis> hybrid zone in the Balkans: clues from morphology

Three morphological characters were used to depict the position of the hybrid zone between two species of house mice,M. musculus Linnaeus, 1758 andM. domesticus Schwarz et Schwarz, 1943, across a vast area covering countries of the former Yugoslavia, Albania, Bulgaria and Greece. Quantitative approach based on a morphological index (MI), resembling the hybrid index widely used in allozyme-based genetic studies, was used. The zone crosses Slovenia south of the Sava River, and then follows the Dinaric Mts to Montenegro and northern Albania. Contrary to many previously published results, the zone was found to run parallel with northern borders of Albania and the former Yugoslavian Macedonia, about 150 km north of the Greek border, thus giving its course rather “shallow” appearance at this part of the Balkan Peninsula.     

Evolution of a long-range repeat family in Chromosome 1 of the genus Mus

Copy numbers and variation of a clustered long-range repeat family on Chromosome (Chr) 1 have been studied in different species of the genus Mus. The repeat sequence was present in all, as inferred from cross-hybridization with probes derived from the Mus musculus repeat family. Copy numbers determined by dot blot hybridization were very low, from three to six per haploid genome in M. caroli, M. cervicolor, and M. cookii. These species form one branch of the phylogenetic tree in the genus Mus. In the other group of phylogenetically related species—M. spicilegus, M. spretus, M. musculus and M. macedonicus—copy numbers ranged from 6 to 1810 per haploid genome. The repeat cluster is cytogenetically visible as a fine C-band in M. macedonicus and as a C-band positive homogeneously staining region (HSR) in several populations of M. m. domesticus and M. m. musculus. When cytogenetically visible, the clusters contained from 179 to 1810 repeats. Intragenomic restriction fragment length polymorphisms (RFLPs), which reflect sequence variation among different copies of the long-range repeat family, increased with higher copy numbers. The high similarity of the RFLP pattern among genomes with C-band positive regions in Chr 1 of M. m. musculus, M. m. domesticus, and M. macedonicus points to a close evolutionary relationship of their Chr 1 repeat families.     

Asymmetric size and shape variation in the Central European transect across the house mouse hybrid zone

We studied asymmetric variation of the mandible in the Central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. Within introgression classes, defined by the share of diagnostic allozymes, we quantified the directional and fluctuating component of asymmetric variation, as well as skewness and kurtosis of individual asymmetry distributions. Furthermore, in the same manner we re‐analysed asymmetric variation of the ventral side of the skull. According to the quadratic polynomial model, the mandible shape‐fluctuating asymmetry, but not size‐fluctuating asymmetry, was significantly decreased in the centre of the hybrid zone (with a minimum predicted for a hybrid index of 0.41). On the contrary, the skull shape‐fluctuating asymmetry non‐monotonically increased towards the musculus side of the hybrid zone (with a peak predicted for a hybrid index of 0.86). Thus, the impact of hybridization on fluctuating asymmetry is trait‐specific in this portion of the house mouse hybrid zone. The only general feature of asymmetric variation we observed was the shift towards the platykurtosis of asymmetry distributions in the centre of the hybrid zone. Taken together, we suggest genetic variability for right–left asymmetries to be generally increased, but the developmental instability of mandible shape to be decreased, by hybridization. We hypothesize the decrease of developmental instability to be caused by overdominant effects on developmental dynamics rather than by increased heterozygosity. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 13–27.     

A Candidate Subspecies Discrimination System Involving a Vomeronasal Receptor Gene with Different Alleles Fixed in M. m. domesticus and M. m. musculus

Assortative mating, a potentially efficient prezygotic reproductive barrier, may prevent loss of genetic potential by avoiding the production of unfit hybrids (i.e., because of hybrid infertility or hybrid breakdown) that occur at regions of secondary contact between incipient species. In the case of the mouse hybrid zone, where two subspecies of Mus musculus (M. m. domesticus and M. m. musculus) meet and exchange genes to a limited extent, assortative mating requires a means of subspecies recognition. We based the work reported here on the hypothesis that, if there is a pheromone sufficiently diverged between M. m. domesticus and M. m. musculus to mediate subspecies recognition, then that process must also require a specific receptor(s), also sufficiently diverged between the subspecies, to receive the signal and elicit an assortative mating response. We studied the mouse V1R genes, which encode a large family of receptors in the vomeronasal organ (VNO), by screening Perlegen SNP data and identified one, Vmn1r67, with 24 fixed SNP differences most of which (15/24) are nonsynonymous nucleotide substitutions between M. m. domesticus and M. m. musculus. We observed substantial linkage disequilibrium (LD) between Vmn1r67 and Abpa27, a mouse salivary androgen-binding protein gene that encodes a proteinaceous pheromone (ABP) capable of mediating assortative mating, perhaps in conjunction with its bound small lipophilic ligand. The LD we observed is likely a case of association rather than residual physical linkage from a very recent selective sweep, because an intervening gene, Vmn1r71, shows significant intra(sub)specific polymorphism but no inter(sub)specific divergence in its nucleotide sequence. We discuss alternative explanations of these observations, for example that Abpa27 and Vmn1r67 are coevolving as signal and receptor to reinforce subspecies hybridization barriers or that the unusually divergent Vmn1r67 allele was not a product of fast positive selection, but was derived from an introgressed allele, possibly from Mus spretus.     

Seeking signatures of reinforcement at the genetic level: a hitchhiking mapping and candidate gene approach in the house mouse

Reinforcement is the process by which prezygotic isolation is strengthened as a response to selection against hybridization. Most empirical support for reinforcement comes from the observation of its possible phenotypic signature: an accentuated degree of prezygotic isolation in the hybrid zone as compared to allopatry. Here, we implemented a novel approach to this question by seeking for the signature of reinforcement at the genetic level. In the house mouse, selection against hybrids and enhanced olfactory‐based assortative mate preferences are observed in a hybrid zone between the two European subspecies Mus musculus musculus and M. m. domesticus, suggesting a possible recent reinforcement event. To test for the genetic signature of reinforcing selection and identify genes involved in sexual isolation, we adopted a hitchhiking mapping approach targeting genomic regions containing candidate genes for assortative mating in mice. We densely scanned these genomic regions in hybrid zone and allopatric samples using a large number of fast evolving microsatellite loci that allow the detection of recent selection events. We found a handful of loci showing the expected pattern of significant reduction in variability in populations close to the hybrid zone, showing assortative odour preference in mate choice experiments as compared to populations further away and displaying no such preference. These loci lie close to genes that we pinpoint as testable candidates for further investigation.     

Genetic mapping of two DNA markers,D16Ros1 and D16Ros2, flanking the mutation site in the chakragati mouse,a transgenic insertional mutant

We present here the genetic mapping of two novel loci, D16Ros1 and D16Ros2, to mouse Chromosome (Chr) 16. The probes for these loci were genomic framents isolated from the chakragati mouse, a behavioural mutant resulting from insertional mutagenesis during the course of making transgenic mice. D16Ros1 and D16Ros2 were first mapped by recombinant inbred (RI) strain analysis and subsequently by the analysis of 145 progeny of two interspecific backcrosses between Mus domesticus and Mus spretus. These progeny had been typed for the centromere and this allowed mapping of D16Ros1 and D16Ros2 relative to the centromere. The other markers included in this study were Prm-1, Gap43 and Sod-1. The genetic map generated spanned 47.5 cM from the centromere to Sod-1, the most distal marker mapped here. The linkage data presented here should prove useful in mapping other loci relative to the centromere of Chr 16.     

Clonal and atypical Toxoplasma strain differences in virulence vary with mouse sub-species

The severe virulence of Toxoplasma gondii in classical laboratory inbred mouse strains contradicts the hypothesis that house mice (Mus musculus) are the most important intermediate hosts for its transmission and evolution because death of the mouse before parasite transmission equals death of the parasite. However, the classical laboratory inbred mouse strains (Mus musculus domesticus), commonly used to test Toxoplasma strain differences in virulence, do not capture the genetic diversity within Mus musculus. Thus, it is possible that Toxoplasma strains that are severely virulent in laboratory inbred mice are avirulent in some other mouse sub-species. Here, we present insight into the responses of individual mouse strains, representing strains of the genetically divergent Mus musculus musculus, Mus musculus castaneus and Mus musculus domesticus, to infection with individual clonal and atypical Toxoplasma strains. We observed that, unlike M. m. domesticus, M. m. musculus and M. m. castaneus are resistant to the clonal Toxoplasma strains. For M. m. musculus, we show that this is due to a locus on chromosome 11 that includes the genes that encode the interferon gamma (IFNG)-inducible immunity-related GTPases (Irgs) that can kill the parasite by localising and subsequently vesiculating the parasitophorous vacuole membrane. However, despite the localization of known effector Irgs to the Toxoplasma parasitophorous vacuole membrane, we observed that some atypical Toxoplasma strains are virulent in all the mouse strains tested. The virulence of these atypical strains in M. m. musculus could not be attributed to individual rhoptry protein 5 (ROP5) alleles, a secreted parasite pseudokinase that antagonises the canonical effector Irgs and is indispensable for parasite virulence in laboratory inbred mice (M. m. domesticus). We conclude that murine resistance to Toxoplasma is modulated by complex interactions between host and parasite genotypes and may be independent of known effector Irgs on murine chromosome 11.     

Detection of recombinant haplotypes in wild mice (Mus musculus) provides new insights into the origin of Japanese mice

Japanese house mice (Mus musculus molossinus) are thought to be a hybrid lineage derived from two prehistoric immigrants, the subspecies M. m. musculus of northern Eurasia and M. m. castaneus of South Asia. Mice of the western European subspecies M. m. domesticus have been detected in Japanese ports and airports only. We examined haplotype structuring of a 200 kb stretch on chromosome 8 for 59 mice from throughout Eurasia, determining short segments (≈ 370–600 bp) of eight nuclear genes (Fanca, Spire2, Tcf25, Mc1r, Tubb3, Def8, Afg3l1 and Dbndd1) which are intermittently arranged in this order. Where possible we identified the subspecies origin for individual gene alleles and then designated haplotypes for concatenated alleles. We recovered 11 haplotypes among 19 Japanese mice examined, identified either as ‘intact’ haplotypes derived from the subspecies musculus (57.9%), domesticus (7.9%), and castaneus (2.6%), or as ‘recombinant’ haplotypes (31.6%). We also detected recombinant haplotypes unique to Sakhalin. The complex nature of the recombinant haplotypes suggests ancient introduction of all three subspecies components into the peripheral part of Eurasia or complicated genomic admixture before the movement from source areas. ‘Intact’domesticus and castaneus haplotypes in other Japanese wild mice imply ongoing stowaway introductions. The method has general utility for assessing the history of genetic admixture and for disclosing ongoing genetic contamination.     

The uncharacterized gene 1700093K21Rik and flanking regions are correlated with reproductive isolation in the house mouse,Mus musculus

Reproductive barriers exist between the house mouse subspecies, Mus musculus musculus and M. m. domesticus, members of the Mus musculus species complex, primarily as a result of hybrid male infertility, and a hybrid zone exists where their ranges intersect in Europe. Using single nucleotide polymorphisms (SNPs) diagnostic for the two taxa, the extent of introgression across the genome was previously compared in these hybrid populations. Sixty-nine of 1316 autosomal SNPs exhibited reduced introgression in two hybrid zone transects suggesting maladaptive interactions among certain loci. One of these markers is within a region on chromosome 11 that, in other studies, has been associated with hybrid male sterility of these subspecies. We assessed sequence variation in a 20 Mb region on chromosome 11 flanking this marker, and observed its inclusion within a roughly 150 kb stretch of DNA showing elevated sequence differentiation between the two subspecies. Four genes are associated with this genomic subregion, with two entirely encompassed. One of the two genes, the uncharacterized 1700093K21Rik gene, displays distinguishing features consistent with a potential role in reproductive isolation between these subspecies. Along with its expression specifically within spermatogenic cells, we present various sequence analyses that demonstrate a high rate of molecular evolution of this gene, as well as identify a subspecies amino acid variant resulting in a structural difference. Taken together, the data suggest a role for this gene in reproductive isolation.     

Location of a contact zone between Mus musculus domesticus and M. m. domesticus with M. m. castaneus mtDNA in southern New Zealand

The house mouse, Mus musculus, was first introduced into New Zealand in significant numbers in the early to mid nineteenth century, with genomic components from different sources of the three subspecies M. m. domesticus, M. m. musculus and M. m. castaneus. M. m. domesticus is now widely distributed in New Zealand, with genomic and morphological evidence of M. m. musculus in a few scattered locations. M. m. domesticus/M. m. castaneus hybrids are dominant in the southern third of the South Island. We anticipated that there should be a definable southern contact zone between pure M. m. domesticus and M. m. domesticus/M. m. castaneus hybrids. We tested this hypothesis by screening 170 DNA samples from mice collected in the southern South Island, using a PCR technique which rapidly distinguishes the mitochondrial genomes of the three subspecies.All mice sampled from in or north of Lincoln (43.63° S) had only M. m. domesticus mtDNA, whereas all those from or further south than Hook (44.68° S) had M. m. castaneus mtDNA. Between the two sites, mice carrying mtDNA of both subspecies were found, sometimes in the same building. On present data, this contact zone extends approximately 50 km north to south and some 30 km inland. Classical tests with three nuclear DNA markers confirmed earlier work showing that the nuclear genomes of all mice appeared to be predominantly domesticus-like.We conclude that if purebred M. m. castaneus mice did originally reach New Zealand, extensive backcrossing with M. m. domesticus has made the castaneus nuclear genome virtually undetectable with the tests that we employ.