Genetic differentiation of subspecies of the house mouse Mus musculus and their taxonomic relationships inferred from RAPD-PCR data

Genetic differentiation of six subspecies of the house mouse Mus musculus (Mus musculus musculus. M. m. domesticus, M. m. castaneus, M. m. gansuensis, M. m. wagneri, and M. m. ssp. (bactrianus?) was examined using RAPD-PCR analysis. In all, 373 loci of total length of about 530 kb were identified. Taxon-specific molecular markers were detected and the levels of genetic differences among the subspecies were estimated. Different degree of subspecific genetic differentiation was shown. The most similar subspecies pairs were M. m. castaneus--M. m. domesticus and M. m. musculus--M. m. gansuensis. In our phylogenetic reconstruction, M. m. wagnery proved to be most different from all the other subspecies. Genetic distances between it and other subspecies were two- to threefold higher than those between the "good"' species of the subgenus Mus (e.g., between M. m. musculus and M. spicilegus, M. musculus and M. abbottii). The estimates of genetic similarity and the taxonomic relationships between six house mouse subspecies inferred from RAPD partially conformed to the results based on cytogenetic and allozyme data. However, they were considerably different from phylogenetic reconstructions based on sequencing of the control mtDNA region, which reflects mutual inconsistency of different systems of inheritance.     

Evaluation of the Fertility of Ecologically Different Forms of House Mice and Hybrids of the Superspecies Complex <Emphasis Type="Italic">Mus musculus</Emphasis> sensu lato (Rodentia: Muridae)

In house mice from the superspecies complex Mus musculus s. l., the relative weight of their testicles is higher and the sperm quality is better for wildliving species than for synantropic species. It is shown that this pattern is observed at an intraspecific level as well, since the testicle weight index and sperm concentration were significantly higher in the hemi-synantropic subspecies Mus musculus wagneri and M. m. gansuensis as compared to the synantropic M. m. musculus in a number of comparisons. The heritability of these indices should be considered when interpreting the results of experimental crosses in house mice.     

Fine-Scale Phylogenetic Discordance across the House Mouse Genome

Population genetic theory predicts discordance in the true phylogeny of different genomic regions when studying recently diverged species. Despite this expectation, genome-wide discordance in young species groups has rarely been statistically quantified. The house mouse subspecies group provides a model system for examining phylogenetic discordance. House mouse subspecies are recently derived, suggesting that even if there has been a simple tree-like population history, gene trees could disagree with the population history due to incomplete lineage sorting. Subspecies of house mice also hybridize in nature, raising the possibility that recent introgression might lead to additional phylogenetic discordance. Single-locus approaches have revealed support for conflicting topologies, resulting in a subspecies tree often summarized as a polytomy. To analyze phylogenetic histories on a genomic scale, we applied a recently developed method, Bayesian concordance analysis, to dense SNP data from three closely related subspecies of house mice: Mus musculus musculus, M. m. castaneus, and M. m. domesticus. We documented substantial variation in phylogenetic history across the genome. Although each of the three possible topologies was strongly supported by a large number of loci, there was statistical evidence for a primary phylogenetic history in which M. m. musculus and M. m. castaneus are sister subspecies. These results underscore the importance of measuring phylogenetic discordance in other recently diverged groups using methods such as Bayesian concordance analysis, which are designed for this purpose.     

Phylogenetic relationships of intraspecific forms of the house mouse Mus musculus: Analysis of variability of the control region (D-loop) of mitochondrial DNA

Analysis of the control region of mitochondrial DNA (mtDNA) or D-loop of 96 house mice (Mus musculus) from Russia, Moldova, Armenia, Azerbaijan, Kazakhstan, and Turkmenistan has been used to reconstruct the phylogenetic relationships and phylogeographic patterns of intraspecific forms. New data on the phylogenetic structure of the house mouse are presented. Three phylogroups can be reliably distinguished in the eastern part of the M. musculus species range, the first one mainly comprising the haplotypes of mice from Transcaucasia (Armenia); the second one, the haplotypes of mice from Kazakhstan; and the third one, the haplotypes of mice from Siberia and some other regions. The morphological subspecies M. m. wagneri and M. m. gansuensis have proved to be genetically heterogeneous and did not form discrete phylogroups in the phylogenetic tree.     

Experimental hybridization and an evaluation of the fertility of some forms of the house mouse supraspecies complex <Emphasis Type="Italic">Mus musculus</Emphasis> (Rodentia,Muridae)

The degree of development of the mechanisms of postcopulatory isolation was evaluated on the basis of experimental hybridization of representatives of three subspecies of M. musculus (M. m. musculus, M. m. wagneri, and M. m. gansuensis) and remote populations of the subspecies M. m. musculus. Experimental crosses between the different subspecies and populations indicated the presence of initial stages of postcopulatory reproductive isolation between some forms of house mice. In a number of crosses conducted between different populations and subspecies of M. musculus, asymmetry was observed. In one variant of mating, M. m. musculus (male) × M. m. wagneri (female), a reduced intensity of breeding and nonviability of pups were observed. A decrease in the intensity of reproduction was found in all variants of crosses that used male M. m. musculus from the city of Ishim. These data are assumed to confirm the previous assumption about the hybrid origin of mice inhabiting that city. The results confirm a significant level of divergence of the subspecies M. m. musculus and M. m. wagneri. Thus, initial stages both of post- and precopulatory isolation mechanisms between M. m. wagneri and M. m. musculus were shown.     

Ultrasonic Vocalizations of Male Mice Differ among Species and Females Show Assortative Preferences for Male Calls

Male house mice (Mus musculus) emit ultrasonic vocalizations (USVs) during courtship, which attract females, and we aimed to test whether females use these vocalizations for species or subspecies recognition of potential mates. We recorded courtship USVs of males from different Mus species, Mus musculus subspecies, and populations (F1 offspring of wild-caught Mus musculus musculus, Mus musculus domesticus (and F1 hybrid crosses), and Mus spicilegus), and we conducted playback experiments to measure female preferences for male USVs. Male vocalizations contained at least seven distinct syllable types, whose frequency of occurrence varied among species, subspecies, and populations. Detailed analyses of multiple common syllable types indicated that Mus musculus and Mus spicilegus could be discriminated based on spectral and temporal characteristics of their vocalizations, and populations of Mus musculus were also distinctive regardless of the classification model used. Females were able to discriminate USVs from different species, and showed assortative preferences for conspecific males. We found no evidence that females discriminate USVs of males from a different subspecies or separate populations of the same species, even though our spectral analyses identified acoustic features that differ between species, subspecies, and populations of the same species. Our results provide the first comparison of USVs between Mus species or between Mus musculus subspecies, and the first evidence that male USVs potentially facilitate species recognition.     

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.     

Evidence for the presence of two sympatric species of mice (genus Mus L.) in southern France based on biochemical genetics

Populations of mice established outdoors as well as indoors have been investigated at 24 loci using starch gel electrophoresis. Two reproductively isolated groups are recognized, one of which is referable to a house mouse subspecies, Mus musculus brevirostris, and the other to a different species, Mus spretus, contrary to the view of Schwarz and Schwarz that only one species of Mus is present in the Mediterranean Basin. The genetic distance between these two groups is larger than between any pair of investigated subspecies of M. musculus. M. m. brevirostris is biochemically almost indistinguishable from M. m. domesticus. On the other hand, M. spretus exhibits several allelic variants unknown or at most very infrequent in M. musculus, as for instance at the lactate dehydrogenase B-chain locus.This work was supported by research grants from the Centre National de la Recherche Scientifique (E.R.A. No. 261) and the Ecole Pratique des Hautes Etudes.     

Genetic diversity of the house mouse <Emphasis Type="Italic">Mus musculus</Emphasis> and geographic distribution of its subspecies-specific RAPD markers on the territory of Russia

Genetic diversity and geographic distribution of taxon-specific RAPD markers was examined in ten local populations of the house mouse Mus musculus (n = 42). The house mice were generally characterized by moderate genetic variation: polymorphism P ₉₉ = 60%, P ₉₅ = 32.57%; heterozygosity H = 0.12; the observed allele number n _a = 1.6; the effective allele number n _e = 1.18; the within-population differentiation ?_s = 0.388; and Shannon index I = 0.19. The degree of genetic isolation of individual local populations was greatly variable. The genetic subdivision index G _st varied from 0.162 to 0.770 at the gene flow of Nm = 2.58?0.149, while the among-population distances D _N varied from 0.026 to 0.178. The largest part of the genetic diversity was found among the populations (H _T = 0.125), while the within-population diversity was twice lower (H _S = 0.06). The samples examined were well discriminated relative to the sets of RAPD markers. The character distribution pattern provided conditional subdivision of the mice into the “western” and the “eastern” groups with the putative boarder along the Baikal Lake. The first group was characterized by the prevalence of the markers typical of M. m. musculus and M. m. domesticus. The second group was characterized by the prevalence of the markers typical of M. m. musculus, M. m. gansuensis, M. m. castaneus, M. m. domesticus, and M. m. wagneri. The genotype of the nominative subspecies M. m. musculus was background for all populations. In the populations examined some of earlier described subspecies-specific molecular markers were found at different frequencies, pointing to the involvement of several subspecies of M. musculus in the process of hybridization.     

Divergence of odorant signals within and between the two European subspecies of the house mouse

The olfactory acuity of mice allows them to discriminate odorsof conspecifics differing by a few genes. This acuity is usedin habituation procedures where investigation of novel odorsby the mouse can be translated into relative difference or similaritybetween the stimuli. This study adapts these behavioral proceduresto address suprapopulation divergence among urinary odors inthe house mouse. Specifically, we investigate geographical patternsof odor divergence within and between 2 subspecies of the housemouse, Mus musculus musculus and Mus musculus domesticus, whichdiverged in allopatry and met secondarily in Europe where theyhybridize. Based on M. m. musculus perception, our study suggeststhat odors of the 2 subspecies differ in both allopatric andcontact zone populations and that divergence is more markedin the latter. Our earlier studies documented mate preferenceand signal divergence between the 2 subspecies. Hence, we considerthe role of the urinary odors as mating signals. We discusshow signal divergence between the 2 subspecies may relate toreproductive character displacement. This study validates theuse of habituation procedures to reconstruct geographical patternsof odorant signal divergence, providing a strong methodologicalplatform to address reproductive character displacement affectingcryptic mating signals in mammals.     

The telocentric tandem repeat at the p-arm is not conserved in Mus musculus subspecies

Mouse chromosomes, with the exception of the Y chromosome, are telocentric. The telomere at the p-arm is separated from the centromere by the tL1 sequence and TLC tandem repeats. A previous report showed that the TLC array was also conserved in other strains of the subgenus Mus. These results suggest that the TLC arrays promote the stable evolutionary maintenance of a telocentric karyotype in the subgenus Mus. In this study, we investigated the degree of conservation of TLC arrays among a variety of wild-derived inbred strains, all of which are descendants of wild mice captured in several areas of the world. Genomic PCR analysis indicates that the sequential order of telomere-tL1 is highly conserved in all strains, whereas tL1-TLC is not. Next, Southern blot analysis of DNAs isolated from a panel of mouse subspecies showed both Mus musculus domesticus and Mus musculus castaneus subspecies possess TLC arrays. Unexpectedly, this repeat appears to be lost in almost all Mus musculus musculus and Mus musculus molossinus subspecies, which show a clear geographic divide. These results indicate that either other unknown sequences were replaced by the TLC repeat or almost all M. m. musculus and M. m. molossinus subspecies do not have any sequence between the telomere and minor satellites. Our observation suggests that the TLC array might be evolutionarily unstable and not essential for murine chromosomal conformation. This is the first example of the subspecies-specific large genome alterations in mice.     

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.     

Genetic analysis of genome-scale recombination rate evolution in house mice

The rate of meiotic recombination varies markedly between species and among individuals. Classical genetic experiments demonstrated a heritable component to population variation in recombination rate, and specific sequence variants that contribute to recombination rate differences between individuals have recently been identified. Despite these advances, the genetic basis of species divergence in recombination rate remains unexplored. Using a cytological assay that allows direct in situ imaging of recombination events in spermatocytes, we report a large (∼30%) difference in global recombination rate between males of two closely related house mouse subspecies (Mus musculus musculus and M. m. castaneus). To characterize the genetic basis of this recombination rate divergence, we generated an F2 panel of inter-subspecific hybrid males (n = 276) from an intercross between wild-derived inbred strains CAST/EiJ (M. m. castaneus) and PWD/PhJ (M. m. musculus). We uncover considerable heritable variation for recombination rate among males from this mapping population. Much of the F2 variance for recombination rate and a substantial portion of the difference in recombination rate between the parental strains is explained by eight moderate- to large-effect quantitative trait loci, including two transgressive loci on the X chromosome. In contrast to the rapid evolution observed in males, female CAST/EiJ and PWD/PhJ animals show minimal divergence in recombination rate (∼5%). The existence of loci on the X chromosome suggests a genetic mechanism to explain this male-biased evolution. Our results provide an initial map of the genetic changes underlying subspecies differences in genome-scale recombination rate and underscore the power of the house mouse system for understanding the evolution of this trait.     

Do changes in gene expression contribute to sexual isolation and reinforcement in the house mouse?

Expression divergence, rather than sequence divergence, has been shown to be important in speciation, particularly in the early stages of divergence of traits involved in reproductive isolation. In the two European subspecies of house mice, Mus musculus musculus and Mus musculus domesticus, earlier studies have demonstrated olfactory‐based assortative mate preference in populations close to their hybrid zone. It has been suggested that this behaviour evolved following the recent secondary contact between the two taxa (~3,000 years ago) in response to selection against hybridization. To test for a role of changes in gene expression in the observed behavioural shift, we conducted a RNA sequencing experiment on mouse vomeronasal organs. Key candidate genes for pheromone‐based subspecies recognition, the vomeronasal receptors, are expressed in these organs. Overall patterns of gene expression varied significantly between samples from the two subspecies, with a large number of differentially expressed genes between the two taxa. In contrast, only ~200 genes were found repeatedly differentially expressed between populations within M. m. musculus that did or did not display assortative mate preferences (close to or more distant from the hybrid zone, respectively), with an overrepresentation of genes belonging to vomeronasal receptor family 2. These receptors are known to play a key role in recognition of chemical cues that handle information about genetic identity. Interestingly, four of five of these differentially expressed receptors belong to the same phylogenetic cluster, suggesting specialization of a group of closely related receptors in the recognition of odorant signals that may allow subspecies recognition and assortative mating.     

A molecular characterization of the charismatic Faroe house mouse

Faroe house mice are a ‘classic’ system of rapid and dramatic morphological divergence highlighted by J. S. Huxley during the development of the Modern Synthesis. In the present study, we characterize these charismatic mice using modern molecular techniques, examining specimens from all Faroe islands occupied by mice. The aims were to classify the mice within the modern house mouse taxonomy (i.e. as either Mus musculus domesticus or Mus musculus musculus) using four molecular markers and a morphological feature, and to examine the genetic diversity and possible routes of colonization using mitochondrial (mt) control region DNA sequences and microsatellite data (15 loci). Mice on the most remote islands were characterized as M. m. domesticus and exhibited exceptionally low genetic diversity, whereas those on better connected islands were more genetically diverse and had both M. m. musculus and M. m. domesticus genetic elements, including one population which was morphologically M. m. musculus‐like. The mtDNA data indicate that the majority of the mice had their origins in south‐western Norway (or possibly southern Denmark/northern Germany), and probably arrived with the Vikings, earlier than suggested by Huxley. The M. m. musculus genetic component appears to derive from recent mouse immigration from Denmark. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 471–482.     

Transgressive segregation in a behavioural trait? Explorative strategies in two house mouse subspecies and their hybrids

Hybrid zones between genetically diverged populations are widespread among animals and plants. Their dynamics usually depend on selection against admixture and dispersal of parental forms in the zone. Although indirect estimates of selection have been the target of many studies, dispersal has been neglected. In this study we carried out open field experiments to test whether males of two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus, differ in their propensity to disperse and in their character of exploration. We tested wild‐caught males and males of two wild‐derived inbred strains. In addition, we examined reciprocal F₁ crosses to test the prediction that these hybrids display intermediate behaviours. We revealed that M. m. musculus males were less hesitant to enter the experimental arena than were M. m. domesticus males, but once inside the arena their movements were more timid. F₁ males differed from both parental strains, with longer latencies to enter the arena, but explored the arena in a similar fashion as the M. m. domesticus males, thus displaying transgressive behavioural phenotypes. These results contribute to our knowledge of behavioural divergence between the mouse subspecies, and add a new facet to the study of speciation. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●●, ●●–●●.     

Patterns of morphological evolution in the mandible of the house mouse Mus musculus (Rodentia: Muridae)

The worldwide distributed house mouse, Mus musculus, is subdivided into at least three lineages, Mus musculus musculus, Mus musculus domesticus, and Mus musculus castaneus. The subspecies occur parapatrically in a region considered to be the cradle of the species in Southern Asia (‘central region’), as well as in the rest of the world (‘peripheral region’). The morphological evolution of this species in a phylogeographical context is studied using a landmark‐based approach on mandible morphology of different populations of the three lineages. The morphological variation increases from central to peripheral regions at the population and subspecific levels, confirming a centrifugal sub‐speciation within this species. Furthermore, the outgroup comparison with sister species suggests that M. musculus musculus and populations of all subspecies inhabiting the Iranian plateau have retained a more ancestral mandible morphology, suggesting that this region may represent one of the relevant places of the origin of the species. Mus musculus castaneus, both from central and peripheral regions, is morphologically the most variable and divergent subspecies. Finally, the results obtained in the present study suggest that the independent evolution to commensalism in the three lineages is not accompanied by a convergence detectable on jaw morphology. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 635–647.     

Polymerase chain reaction multiplexing of microsatellites and single nucleotide polymorphism markers for quantitative trait loci mapping of wild house mice

We tested 96 microsatellites and 10 single nucleotide polymorphisms for their allelic distribution in two subspecies of the house mouse, Mus musculus musculus and M. m. domesticus. Sixty‐two microsatellites discriminated strain‐specific differences among nine wild‐derived ‘musculus’ and ‘domesticus’ and three ‘classical’ laboratory strains. For efficient genotyping, we optimized multiplex conditions using five microsatellites per polymerase chain reaction. All 10 single nucleotide polymorphisms were also optimized for simultaneous analysis in one reaction using SNaPshot multiplex. The uniform distribution of markers on autosomes and on the X chromosome makes these panels potentially useful tools for quantitative trait loci mapping of wild house mice.     

Introgression of nuclear and mitochondrial DNA markers of Mus musculus musculus to aboriginal populations of wild mice from Central Asia (M. m. wagneri) and South Siberia (M. m. gansuensis)

Variability of the nucleotide sequences of the second intron of the b1-chain of hemoglobin (Hbb-b1) and complete control region of mitochondrial DNA (D-loop) was studied in aboriginal and synanthropic populations of M. m. wagneri from Central Asia and M. m. gansuensis from South Siberia. A difference in the frequency of the Hbb^w1 hemoglobin variant for natural and urban populations of mice was shown. All mice from natural habitats of studied areas have musculus type of mtDNA. Apparently, the substitution of taxon-specific mitochondrial haplotypes of wagneri, and gansuensis might occur due to the absorbing hybridization with nominate subspecies musculus, which is consistent with the results on nuclear DNA (Hbb-b1 gene) obtained in this work. Two differentiated haplogroups among aboriginal subspecies wagneri (d = 0.01), one of which included house mice from Turkmenistan, were discovered for the first time. This may indicate mtDNA introgression from synanthropic forms of Turkmenistan into natural populations of Kazakhstan mice. The type of mtDNA typical for the castaneus subspecies was detected in two individuals from the natural habitat of Kazakhstan and Turkmenistan; it had not been encountered in Central Asia before. It has been suggested that the gene flow of nuclear and mitochondrial genomes in microevolution processes in M. musculus is directed from the synanthropic forms towards wild populations.