[Hybridization between Mus musculus L. and Mus spretus Lataste under laboratory conditions (author's transl)]

Meiotic drive in house mice: mechanisms, consequences, and insights for human biology

Meiotic drive occurs when one allele at a heterozygous site cheats its way into a disproportionate share of functional gametes, violating Mendel's law of equal segregation. This genetic conflict typically imposes a fitness cost to individuals, often by disrupting the process of gametogenesis. The evolutionary impact of meiotic drive is substantial, and the phenomenon has been associated with infertility and reproductive isolation in a wide range of organisms. However, cases of meiotic drive in humans remain elusive, a finding that likely reflects the inherent challenges of detecting drive in our species rather than unique features of human genome biology. Here, we make the case that house mice (Mus musculus) present a powerful model system to investigate the mechanisms and consequences of meiotic drive and facilitate translational inferences about the scope and potential mechanisms of drive in humans. We first detail how different house mouse resources have been harnessed to identify cases of meiotic drive and the underlying mechanisms utilized to override Mendel's rules of inheritance. We then summarize the current state of knowledge of meiotic drive in the mouse genome. We profile known mechanisms leading to transmission bias at several established drive elements. We discuss how a detailed understanding of meiotic drive in mice can steer the search for drive elements in our own species. Lastly, we conclude with a prospective look into how new technologies and molecular tools can help resolve lingering mysteries about the prevalence and mechanisms of selfish DNA transmission in mammals.

Bidirectional introgression between Mus musculus domesticus and Mus spretus

Introgressed variants from other species can be an important source of genetic variation because they may arise rapidly, can include multiple mutations on a single haplotype, and have often been pretested by selection in the species of origin. Although introgressed alleles are generally deleterious, several studies have reported introgression as the source of adaptive alleles—including the rodenticide-resistant variant of Vkorc1 that introgressed from Mus spretus into European populations of Mus musculus domesticus. Here, we conducted bidirectional genome scans to characterize introgressed regions into one wild population of M. spretus from Spain and three wild populations of M. m. domesticus from France, Germany, and Iran. Despite the fact that these species show considerable intrinsic postzygotic reproductive isolation, introgression was observed in all individuals, including in the M. musculus reference genome (GRCm38). Mus spretus individuals had a greater proportion of introgression compared with M. m. domesticus, and within M. m. domesticus, the proportion of introgression decreased with geographic distance from the area of sympatry. Introgression was observed on all autosomes for both species, but not on the X-chromosome in M. m. domesticus, consistent with known X-linked hybrid sterility and inviability genes that have been mapped to the M. spretus X-chromosome. Tract lengths were generally short with a few outliers of up to 2.7 Mb. Interestingly, the longest introgressed tracts were in olfactory receptor regions, and introgressed tracts were significantly enriched for olfactory receptor genes in both species, suggesting that introgression may be a source of functional novelty even between species with high barriers to gene flow.

Whole exome sequencing of wild-derived inbred strains of mice improves power to link phenotype and genotype

The house mouse is a powerful model to dissect the genetic basis of phenotypic variation, and serves as a model to study human diseases. Despite a wealth of discoveries, most classical laboratory strains have captured only a small fraction of genetic variation known to segregate in their wild progenitors, and existing strains are often related to each other in complex ways. Inbred strains of mice independently derived from natural populations have the potential to increase power in genetic studies with the addition of novel genetic variation. Here, we perform exome-enrichment and high-throughput sequencing (~8× coverage) of 26 wild-derived strains known in the mouse research community as the "Montpellier strains." We identified 1.46 million SNPs in our dataset, approximately 19% of which have not been detected from other inbred strains. This novel genetic variation is expected to contribute to phenotypic variation, as they include 18,496 nonsynonymous variants and 262 early stop codons. Simulations demonstrate that the higher density of genetic variation in the Montpellier strains provides increased power for quantitative genetic studies. Inasmuch as the power to connect genotype to phenotype depends on genetic variation, it is important to incorporate these additional genetic strains into future research programs.

Phylogenetic relationship and time of divergence of Mus terricolor with reference to other Mus species

Mitochondrial DNA control region of Mus terricolor, three aboriginal species M. spretus, M. macedonicus, M. spicilegus; the Asian lineage M. caroli, M. cervicolor, M. cookii; and the two house mice, M. musculus domesticus and M. m. castaneus were analysed to estimate the substitution rate, phylogenetic relationship and the probable time of divergence. Results showed that M. spretus, M. caroli and M. terricolor are highly diverged from each other (caroli/terricolor = 0.146, caroli/spretus = 0.147 and terricolor/spretus = 0.122), whereas M. spretus showed less divergence with two house mice species (0.070 and 0.071). Sequence divergence between M. terricolor and the Palearctic group were found to be ranging from 0.121 to 0.134. Phylogenetic analysis by minimum evolution, neighbour-joining, unweighed pair group method with arithmetic mean and maximum parsimony showed almost similar topology. Two major clusters were found, one included the Asian lineage, M. caroli, M. cookii and M. cervicolor and the other included the house mice M. m. domesticus, M. m. castaneus and the aboriginal mice M. macedonicus and M. spicilegus along with M. spretus, forming the Palearctic clade. M. terricolor was positioned between the Palearctic and Asian clades. Results showed that Palearctic-terricolor and the Asian lineages diverged 5.47 million years ago (Mya), while M. terricolor had split around 4.63 Mya from their ancestor. M. cervicolor, M. cookii and M. caroli diverged between 4.70 and 3.36 Mya, which indicates that M. terricolor and the Asian lineages evolved simultaneously. M. spretus is expected to have diverged nearly 2.9 Mya from their most recent common ancestor.

Interspecific introgressive origin of genomic diversity in the house mouse

We report on a genome-wide scan for introgression between the house mouse (Mus musculus domesticus) and the Algerian mouse (Mus spretus), using samples from the ranges of sympatry and allopatry in Africa and Europe. Our analysis reveals wide variability in introgression signatures along the genomes, as well as across the samples. We find that fewer than half of the autosomes in each genome harbor all detectable introgression, whereas the X chromosome has none. Further, European mice carry more M. spretus alleles than the sympatric African ones. Using the length distribution and sharing patterns of introgressed genomic tracts across the samples, we infer, first, that at least three distinct hybridization events involving M. spretus have occurred, one of which is ancient, and the other two are recent (one presumably due to warfarin rodenticide selection). Second, several of the inferred introgressed tracts contain genes that are likely to confer adaptive advantage. Third, introgressed tracts might contain driver genes that determine the evolutionary fate of those tracts. Further, functional analysis revealed introgressed genes that are essential to fitness, including the Vkorc1 gene, which is implicated in rodenticide resistance, and olfactory receptor genes. Our findings highlight the extent and role of introgression in nature and call for careful analysis and interpretation of house mouse data in evolutionary and genetic studies.

Increased glucocorticoid receptor expression and activity mediate the LPS resistance of SPRET/EI mice

SPRET/Ei mice are extremely resistant to acute LPS-induced lethal inflammation when compared with C57BL/6. We found that in vivo SPRET/Ei mice exhibit strongly reduced expression levels of cytokines and chemokines. To investigate the role of the potent anti-inflammatory glucocorticoid receptor (GR) in the SPRET/Ei phenotype, mice were treated with the GR antagonist RU486 or bilateral adrenalectomy. Under such conditions, both C57BL/6 and SPRET/Ei mice were strongly sensitized to LPS, and the differences in LPS response between SPRET/Ei and C57BL/6 mice were completely gone. These results underscore the central role of GR in the LPS hyporesponsiveness of SPRET/Ei mice. Compared with C57BL/6, SPRET/Ei mice were found to express higher GR levels, which were reflected in increased GR transactivation. Using a backcross mapping strategy, we demonstrate that the high GR transcription levels are linked to the Nr3c1 (GR) locus on chromosome 18 itself. Unexpectedly, SPRET/Ei mice exhibit a basal overactivation of the hypothalamic-pituitary-adrenal axis, namely strongly increased corticosterone levels, ACTH levels, and adrenocortical size. As a consequence of the excess of circulating glucocorticoids (GCs), levels of hepatic gluconeogenic enzymes are increased, and insulin secretion from pancreatic β-cells is impaired, both of which result in hyperglycemia and glucose intolerance in SPRET/Ei mice. We conclude that SPRET/Ei mice are unique as they display an unusual combination of elevated GR expression and increased endogenous GC levels. Hence, these mice provide a new and powerful tool for the study of GR- and GC-mediated mechanisms, including immune repressive functions, neuroendocrine regulation, insulin secretion, and carbohydrate metabolism.

Thirty years of Mus spretus: a promising future

Extensive genetic polymorphisms in Mus spretus have ensured its widespread use in many areas of genetics. With the recent increase in the number of single nucleotide polymorphisms available for laboratory mouse strains, M. spretus is becoming less appealing, in particular for genetic mapping. Although M. spretus mice are aggressive and poor breeders, they have a bright future because they provide phenotypes unobserved in laboratory strains, and tools are available for modifying their genome and dissecting the genetic architecture of complex traits. Furthermore, they provide information on fundamental genetic questions, such as the details of evolution of genomes and speciation. Here, we examine the use of M. spretus from these perspectives. The impending completion of the M. spretus genome sequence will synergize these advantages.

Dynamic evolution of V1R putative pheromone receptors between Mus musculus and Mus spretus

Background

The mammalian vomeronasal organ (VNO) expresses two G-protein coupled receptor gene families that mediate pheromone responses, the V1R and V2R receptor genes. In rodents, there are ~150 V1R genes comprising 12 subfamilies organized in gene clusters at multiple chromosomal locations. Previously, we showed that several of these subfamilies had been extensively modulated by gene duplications, deletions, and gene conversions around the time of the evolutionary split of the mouse and rat lineages, consistent with the hypothesis that V1R repertoires might be involved in reinforcing speciation events. Here, we generated genome sequence for one large cluster containing two V1R subfamilies in Mus spretus, a closely related and sympatric species to Mus musculus, and investigated evolutionary change in these repertoires along the two mouse lineages.

Results

We describe a comparison of spretus and musculus with respect to genome organization and synteny, as well as V1R gene content and phylogeny, with reference to previous observations made between mouse and rat. Unlike the mouse-rat comparisons, synteny seems to be largely conserved between the two mouse species. Disruption of local synteny is generally associated with differences in repeat content, although these differences appear to arise more from deletion than new integrations. Even though unambiguous V1R orthology is evident, we observe dynamic modulation of the functional repertoires, with two of seven V1Rb and one of eleven V1Ra genes lost in spretus, two V1Ra genes becoming pseudogenes in musculus, two additional orthologous pairs apparently subject to strong adaptive selection, and another divergent orthologous pair that apparently was subjected to gene conversion.

Conclusion

Therefore, eight of the 18 (~44%) presumptive V1Ra/V1Rb genes in the musculus-spretus ancestor appear to have undergone functional modulation since these two species diverged. As compared to the rat-mouse split, where modulation is evident by independent expansions of these two V1R subfamilies, divergence between musculus and spretus has arisen more by mutations within coding sequences. These results support the hypothesis that adaptive changes in functional V1R repertoires contribute to the delineation of very closely related species.

The mitochondrial genome sequence of Mus terricolor: comparison with Mus musculus domesticus and implications for xenomitochondrial mouse modeling

Knowledge of the mitochondrial DNA (mtDNA) sequence of divergent murine species is critical from both a phylogenetic perspective and in understanding nuclear-mitochondrial interactions, particularly as the latter influences our xenocybrid models of mitochondrial disease. To this end, the sequence of the mitochondrial genome of the murine species Mus terricolor (formerly Mus dunni) is reported and compared with the published sequence for the common laboratory mouse Mus musculus domesticus strain C57BL/6J. These species are of interest because xenomitochondrial cybrid mice were created that harbor M. terricolor mtDNA in a M. m. domesticus nuclear background. Although the total of 1763 nucleotide substitutions represents striking heterogeneity, the majority of these are silent, leading to highly conserved protein sequences with only 159 amino acid differences. Moreover, 58% of these amino acid differences represented conservative substitutions. All of the tRNA genes and rRNA genes have homology of 91% or greater. The control region shows the greatest heterogeneity, as expected, with 85% homology overall. Regions of 100% homology were found for Conserved Sequence Block I, Conserved Sequence Block III and the L-strand origin of replication. Complex I genes showed the greatest degree of difference among protein-coding genes with amino acid homology of 91-97% among the seven mitochondrial genes. Complexes III and IV genes show high homology ranging from 98-100%. From these data, complex I differences appear most critical for the viability of M. m. domesticus: M. terricolor cybrids. Moreover, the sequence information reported here should be useful in identifying critical regions for mitochondrial transfer between species, for furthering the understanding of mitochondrial dynamics and pathology in transmitochondrial organisms, and for the study of Mus genus origins.

Breaking the species barrier: derivation of germline-competent embryonic stem cells from Mus spretus x C57BL/6 hybrids

Embryonic stem (ES) cells, which can differentiate into almost all types of cells, have been derived from the house mouse Mus musculus, rat, rabbit, humans, and other species. Transmission of the genotype to the offspring of chimeras has been achieved only with M. musculus ES cells, limiting targeted mutagenesis using ES cells to this species. Mus spretus, which exhibits many genetic polymorphisms with M. musculus, displays dominant resistance to cancer and inflammation, making derived inbred strains very useful in positional cloning and interspecies mapping. We show here for the first time the derivation of ES cells from hybrid blastocysts, obtained by the mating of two different species, namely Mus musculus and Mus spretus, and their use for the generation of chimeric mice that transmit the Mus spretus genotype and phenotype to the offspring. These hybrid ES cells allow the genetic manipulation of Mus spretus, as an alternative to Mus musculus.

Characterization of recombinant nonecotropic murine leukemia viruses from the wild mouse species Mus spretus

The wild mouse species most closely related to the common laboratory strains contain proviral env genes of the xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs). To determine if the polytropic proviruses of Mus spretus contain functional genes, we inoculated neonates with Moloney MLV (MoMLV) or amphotropic MLV (A-MLV) and screened for viral recombinants with altered host ranges. Thymus and spleen cells from MoMLV-inoculated mice were plated on Mus dunni cells and mink cells, since these cells do not support the replication of MoMLV, and cells from A-MLV-inoculated mice were plated on ferret cells. All MoMLV-inoculated mice produced ecotropic viruses that resembled their MoMLV progenitor, although some isolates, unlike MoMLV, grew to high titers in M. dunni cells. All of the MoMLV-inoculated mice also produced nonecotropic virus that was infectious for mink cells. Sequencing of three MoMLV- and two A-MLV-derived nonecotropic recombinants confirmed that these viruses contained substantial substitutions that included the regions of env encoding the surface (SU) protein and the 5' end of the transmembrane (TM) protein. The 5' recombination breakpoint for one of the A-MLV recombinants was identified in RNase H. The M. spretus-derived env substitutions were nearly identical to the corresponding regions in prototypical laboratory mouse polytropic proviruses, but the wild mouse infectious viruses had a more restricted host range. The M. spretus proviruses contributing to these recombinants were also sequenced. The seven sequenced proviruses were 99% identical to one another and to the recombinants; only two of the seven had obvious fatal defects. We conclude that the M. spretus proviruses are likely to be recent germ line acquisitions and that they contain functional genes that can contribute to the production of replication-competent virus.

[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.

Hybrid sterility in the mouse

The sterility of hybrids between various mouse species follows Haldane's rule by affecting only the males. Hitherto, five hybrid sterility (Hst) loci have been identified in the mouse genome. Haldane's rule holds also for chromosomal sterility in all studied mammalian species, including man. The males heterozygous for various male-sterile chromosomal rearrangements show the same meiotic phenotype: partially asynaptic, rearranged autosomes associate with the X Chromosome. Thus, the X-Chromosome inactivation in the male meiosis might function as a checkpoint leading to the meiotic arrest in sterile hybrids. Chromosomal and gene hybrid sterility might be interrelated because the attributes of chromosomal sterility, such as X-autosome associations, occur also in interspecies hybrids with genic sterility.

An X-chromosome linked locus contributes to abnormal placental development in mouse interspecific hybrid

Interspecific hybridization between closely related species is commonly associated with decreased fertility or viability of F1 hybrids. Thus, in mouse interspecific hybrids, several different hybrid sterility genes that impair gametogenesis of the male hybrids have been described. We describe a novel effect in hybrids between different mouse species that manifests itself in abnormal growth of the placenta. Opposite phenotypes, that is, placental hypotrophy versus hypertrophy, are observed in reciprocal crosses and backcrosses. The severity of the phenotype, which is mainly caused by abnormal development of the spongiotrophoblast, is influenced by the sex of the conceptus. In general, placental hypertrophy is associated with increased fetal growth. Hypotrophy of the placenta frequently leads to growth impairment or death of the fetus. One of the major genetic determinants of placental growth maps to the proximal part of the mouse X chromosome.

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.

Genetic linkage analysis of the murine developmental mutant velvet coat (Ve) and the distal chromosome 15 developmental genes Hox-3.1, Rar-g, Wnt-1, and Krt-2

We have identified restriction fragment length polymorphisms between Mus musculus and Mus spretus for the Chromosome 15 loci Hox-3, Wnt-1, Krt-2, Rar-g, and Ly-6. We followed the inheritance of these alleles in interspecific genetic test crosses between velvet coat (Ve) heterozygotes and M. spretus. The results suggest a gene order and recombination distances (in cM) of Ly-6-22-Wnt-1-2-Ve/Krt-2/Rar-g-3-Hox-3. No recombination was found between Ve, Krt-2, and Rar-g. The data also provide evidence for the hypothesis of a large-scale genomic duplication involving homologous gene pairs on mouse Chromosomes 15 and 11.

LINE-1 repetitive DNA probes for species-specific cloning from Mus spretus and Mus domesticus genomes

Mus domesticus and Mus spretus mice are closely related subspecies. For genetic investigations involving hybrid mice, we have developed a set of species-specific oligonucleotide probes based on the detection of LINE-1 sequence differences. LINE-1 is a repetitive DNA family whose many members are interspersed among the genes. In this study, library screening experiments were used to fully characterize the species specificity of four M. domesticus LINE-1 probes and three M. spretus LINE-1 probes. It was found that the nucleotide differences detected by the probes define large, species-specific subfamilies. We show that collaborative use of such probes can be employed to selectively detect thousands of species-specific library clones. Consequently, these probes could be exploited to monitor and access almost any given species-specific region of interest within hybrid genomes.

Systematic identification of LINE-1 repetitive DNA sequence differences having species specificity between Mus spretus and Mus domesticus

LINE-1 is a family of repetitive DNA sequences interspersed among mammalian genes. In the mouse haploid genome there are about 100,000 LINE-1 copies. We asked if the subspecies Mus spretus and Mus domesticus have developed species-specific LINE-1 subfamilies. Sequences from 14 M. spretus LINE-1 elements were obtained and compared to M. domesticus LINE-1 sequences. Using a molecular phylogenetic tree we identified several differences shared among a subset of young repeats in one or the other species as candidates for species-specific LINE-1 variants. Species specificity was tested using oligonucleotide probes complementary to each putative species-specific variant. When hybridized to genomic DNAs, single-variant probes detected an expanded number of elements in the expected mouse. In the other species these probes detected a smaller number of matches consistent with the average rate of random divergence among LINE-1 elements. It was further found that the combination of two species-specific sequence differences in the same probe reduced the detection background in the wrong species below our detection limit.

Linkage analysis of the murine mos proto-oncogene on chromosome 4

A linkage analysis of the murine Mos gene, which codes for the c-mos proto-oncogene, was performed in 88 backcross progeny of an interspecies cross of laboratory mice and Mus spretus. Linkage was tested for four different genes on mouse chromosome 4: Aco-1, Mup-1, b, and Ifb. The gene order (from centromere) with intervening percentage recombination is Mos-15.9 (+/- 3.9)-Aco-1-5.6 (+/- 2.4)-Mup-1-3.4 (+/- 1.9)-b-5.6 (+/- 2.4)-Ifb. These results confirm the previous assignment of Mos to chromosome 4 on the basis of segregation in somatic cell hybrids (D. Swan et al., 1982, J. Virol. 44: 752-754) and show furthermore that Mos and the Ifa/Ifb clusters are not tightly linked as a group of intronless genes, but are separated by a map distance of 30.6 +/- 4.9 recombination units. The linkage data obtained in the present study place Mos in a region compatible with the physical map (D. W. Threadgill and J. E. Womack, 1988, Genomics 3: 82-86).

Serological survey of complement factor H in common laboratory and wild mice: a new third allotype

Antigenic specificities of complement factor H from mice were studied serologically. In addition to previously reported allotypes, referred to as H.1 and H.2, a new allotype of complement factor H, H.3, was identified in the BFM/2Ms strain derived from European wild mice. Using three different alloantisera raised against the various mouse factor H allotype, a serological survey of the common laboratory strains and wild-derived strains of Mus musculus and its relatives, Mus spretus, Mus spretoides, and Mus spicilegus was carried out. All of the common laboratory strains examined in this survey had the H.1 allotype except for STR/N which had H.2. The geographical distributions of factor H allotypes in M. musculus were specific to the subspecies. Mice derived from Mus musculus domesticus and Mus musculus castaneus had the H.1 allotype. Mice derived from M. m. musculus, Mus musculus bactrianus, and Mus musculus molossinus had the H.2 allotype. Only BFM/2Ms and BFM/1Mpl strains derived from M. m. domesticus had the novel H.3 allotype. Sera of mice from strains derived from M. spretoides and M. spicilegus cross-reacted with H.2-specific antiserum, and those from M. spretus cross-reacted with H.3-specific antiserum.

Fructose bisphosphatase isozymes of the mouse. I. Inheritance

Electrophoretically detectable polymorphisms of fructose bisphosphatase (EC 3.1.3.11) have been found in the mouse. One polymorphism, found among inbred strains of Mus musculus and feral animals, affects the isozymes found in the muscle and in most other tissues examined but is not expressed in kidney, liver, or testis. These tissues have other electrophoretically distinct isozymes which are monomorphic in Mus musculus but are present as a different electromorph in the sympatric species Mus spretus. Breeding data have established that the genetic control of the muscle enzyme is expressed by an autosomal structural locus Fbp-1 which is distinct from that expressing the liver, kidney, and testis enzyme, Fbp-2. The organ-specific expression of the two loci suggests possible functional differences between the two products.

Investigation of genetic linkage between myosin and actin genes using an interspecific mouse back-cross

The introduction of cloned probes to follow the segregation of DNA restriction fragment length polymorphisms (RFLPs) has led to a revival of mendelian genetics in attempts to map the human genome. In the mouse, however, it has often proved difficult to detect an RFLP with a DNA probe between different inbred strains of the laboratory mouse. To circumvent this problem, we have used two species, Mus musculus domesticus and Mus spretus which interact as sympatric species but can be interbred under laboratory conditions. Because of the relative evolutionary distance between these species, they exhibit polymorphism at many more loci than do different strains of the usual M. m. domesticus laboratory mouse. This is also observed at the DNA level when the sizes of restriction fragments encoding a specific gene are compared. We have used these RFLPs between M. m. domesticus and M. spretus to follow the segregation of genes encoding different isoforms of myosin alkali light chains in the backcross progeny between these species and to compare this with that of other contractile protein genes. No linkage between these genes was observed.

Novel carbonic anhydrase (Car-2) allele in Spanish mice

A unique electrophoretic form of carbonic anhydrase is characteristic of some laboratory-maintained mice of the wild mouse species Mus spretus. This isozyme has been characterized by cellulose acetate electrophoresis and by isoelectric focusing. It is proposed that this isozyme be called CAR-2C and that its encoding allele be designated Car-2c. Fertile hybrids of Mus spretus and C57BL/6J (Car-2a) show both CAR-2A and CAR-2C bands of approximately equal intensity. The CAR-2C isozyme is readily identified by electrophoresis on 75-mm cellulose acetate strips because it migrates significantly faster than the isozymes of inbred mice, the CAR-2A and CAR-2B that do not separate from one another under standard conditions. Isoelectric focusing cleanly resolves all three of these CAR-2 forms. Mus hortulanus, although closely related to Mus spretus in other biochemical-genetic characteristics, has a CAR-2-homologous isozyme that is distinctly different from the CAR-2C of Mus spretus and from the isozymes of the common inbred strains.

Biochemical diversity and evolution in the genus Mus

Thirteen biochemical groups of wild mice from Europe, Asia, and Africa belonging to the genus Mus are analyzed at 22-42 protein loci. Phylogenetic trees are proposed and patterns of biochemical evolution are discussed, as well as the possible contribution of wild mice to the genetic diversity of laboratory stocks.

Novel class of mouse mammary tumor virus-related DNA sequences found in all species of Mus, including mice lacking the virus proviral genome

Mice in breeding colonies of feral Mus musculus brevirostris (Azrou, Morocco), M. m. musculus (Studenec, Czechoslovakia), and M. m. molossinus (Fukuoka, Japan) were found to lack the mouse mammary tumor virus (MMTV-alpha) proviral genome in their germ line. MMTV-alpha proviral genomes have been found in all inbred strains of M. musculus by using high-stringency nucleic acid hybridization conditions. We conclude that feral mice in these colonies are heterozygous for a limited number of MMTV-alpha proviral genomes and that those lacking them arose as a result of random chromosomal segregation. All mice in another breeding colony of feral M. m. musculus (Sladeckovce, Czechoslovakia) lack MMTV proviral genes. By relaxing the conditions of nucleic acid hybridization, MMTV-related sequences (designated MMTV-beta) were detected in restricted cellular DNA from MMTV-negative mice and all other inbred strains and feral species of the genus Mus. The apparent ubiquity of the MMTV-beta DNA sequences in the genus Mus and the lack of variation in the pattern of restriction fragments containing these sequences within a species distinguishes them from MMTV-alpha. These results suggest that the MMTV-beta DNA sequences either are the evolutionary progenitors of the infectious MMTV genome or represent an accumulation of evolutionarily divergent MMTV-alpha insertions into the mouse germ line.

Lactate dehydrogenase polymorphism in Mus musculus L. and Mus spretus Lataste

First variation at the Ldh-A locus and a new allele at the Ldh-B locus are reported in a M. musculus population dimorphic at the Ldh-A locus and in a M. spretus population trimorphic at the Ldh-B locus.