Genetic studies of house mice from the hybrid area of the Primorskii Region

Cytogenetic and allozyme differentiation of house mice Mus musculus L. was studied in 21 samples of Primorskii krai (region) from 1984 to 1998. The use of Q-H- and C-banding made it possible to reveal a high variation in the content and distribution of pericentromeric heterochromatin and high frequencies of marker autosomes 5, 7, 10, 12, 15, and 17-19 in the karyotypes of mice from some samples. The presence of marker alleles that are typical of the subspecies groups castaneus and musculus was shown for protein loci Idh, Sod-1, Aat-1, and Hbb. The data obtained indicate that the fauna of house mice of Primorye is characterized by subspecific mosaicism and hybrid origin. Apparently, subspecies M. m. castaneus, M. musculus ssp., M. M. homourus, and M. m. gansuensis contributed, to different degrees, to the formation of this fauna.     

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

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

Origin of the house mice (superspecies complex Mus musculus sensu lato) from the Transcaucasian region: A new look at dispersal routes and evolution

We analyzed our results and literature evidence on variability of nuclear protein genes in 39 populations of eight synanthropic and wild species of house mice (superspecies complexes Mus musculus and M. spicilegus) from Transcaucasia, Eastern and Western Europe, Near and Middle East, Central, South, and East Asia, and Cuba. These data were for the first time ever combined into a single database by unification of nomenclature of 21 loci examined by different authors in 39 populations. Analysis of geographical allele distribution have shown that populations of domestic Transcaucasian mice are close to Indo-Pakistani populations of form oriental of the species M. castaneus, which preserved a high level of ancestral polymorphism. We concluded that a very heterogeneous, rich gene pool of house mice from Transcaucasia could not develop only by secondary contacts of differentiated M. musculus s. str. and M. domesticus forms, since it is similar to the ancestral gene pool of the superspecies complexes M. musculus and M. spicilegus. In this context, unique characteristics of some Central Asian populations were examined; these populations may have served as a "transit station" in the dispersal of synanthropic house mice forms. We suggest that the Transcaucasian populations are genealogically closely related to an early Near East form of M. musculus, which, as M. domesticus and M. castaneus, split from the common ancestor and preserved nondifferentiated pool of ancestral alleles of protein genes. This hypothesis admits the involvement of differentiated species M. musculus s. str. and M. domesticus in the ultimate formation of the gene pool of Transcaucasian house mice. Apparently, these populations resulted from alternation and (or) "overlapping" of different evolutionary processes. A scenario suggesting that hybrid events having occurred in Transcaucasia at different times, were "superposed" on the gene pool of the ancient autochtonous population of house mice from this region seems most plausible. Analysis of allozyme variability in the modern Transcaucasian Mus populations could not always distinguish between ancestral polymorphism and hybridization consequences.     

Isoenzyme electrophoretic patterns in discus fish (Symphysodon aequifasciatus Pellegrin, 1904 and Symphysodon discus Heckel, 1840) from the Central Amazon

The discus is a very popular and expensive aquarium fish belonging to the family Cichlidae, genus Symphysodon, formed by three Amazon basin endemic species: Symphysodon aequifasciatus, S. discus and S. tarzoo. The taxonomic status of these fish is very controversial, with a paucity of molecular research on their population genetic structure and species identification. Information on molecular genetic markers, especially isoenzymes, in search of a better understanding of the population genetic structure and correct identification of fish species, has been receiving more attention when elaborating and implementing commercial fishery management programs. Aiming to contribute to a better understanding of the species taxonomic status, the present study describes the isoenzymatic patterns of 6 enzymes: esterase (Est - EC 3.1.1.1), lactate dehydrogenase (Ldh - EC 1.1.1.27), malate dehydrogenase (Mdh - EC 1.1.1.37), phosphoglucomutase (Pgm - EC 5.4.2.2), phosphoglucose isomerase (Pgi - EC 5.3.1.9), and super oxide dismutase (Sod - EC 1.15.1.1) extracted from skeletal muscle specimens and analyzed by starch gel electrophoresis. Monomorphic patterns, presumably controlled by 11 loci: Est-1, Est-2, Est-3, Ldh-1, Ldh-2, Mdh-1, Mdh-2, Pgi-1, Pgi-2, Pgm-1, and Sod-1 were fixed for the same alleles: Est-1(1), Est-2(1), Est-3(1), Ldh-1(1), Ldh-2(1), Mdh-1(1), Mdh-2(1), Pgi-1(1), Pgi-2(1), Pgm-1(1), and Sod-1(1), respectively, and detected in all 60 specimens examined (27 S. aequifasciatus from Manacapuru and 33 S. discus from Novo Air?o, Central Amazon). The failure in the present study to detect diagnostic loci, which could be very useful for differentiating S. aequifasciatus from S. discus species, and polymorphic loci, which could also be applied for possible identification and delimitation of their stocks, does not rule out the possibility of there existing in other isoenzyme gene loci to be analyzed in the future.     

Linkage of faded gene (fe) to chromosome 6 of the mouse

Linkage tests on the faded gene were carried out with some coat color and biochemical markers, It was shown that the faded locus was not closely linked to the following loci: Idh-1 (chromosome 1), a (2), Car 2 (3), Mup-1 (4), Pgm-1 (5), Hbb (7), Gpi-1 (7), Es-1 (8), Trf (9), Es-3 (11), s (14), Sod-1 (16) and Ce-2 (17). The mutant locus showed linkage with Ggc on chromosome 6.     

Two erythrocytic lactate dehydrogenase variants in the house mouse, Mus musculus

Two erythrocytic LDH variants have been found in the house mouse, Mus musculus. One, involving the presence and absence of the next to the slowest electrophoretically separable isozyme, is controlled by a regulatory locus which may be similar to Ldr-1 (Shows and Ruddle, 1968). The second appears to be indirectly controlled by the Hbb locus and may be attributed to the accumulation of a storage product in the lysates. Ldr-1 is polymorphic in natural populations of the house mouse.     

Hybrid origin of Japanese mice "Mus musculus molossinus": evidence from restriction analysis of mitochondrial DNA

The Japanese mouse, Mus musculus molossinus, has long been considered an independent subspecies of the house mouse. A survey of restriction- site haplotypes of mitochondrial DNA (mtDNA) showed that Japanese mice have two main maternal lineages. The most common haplotype is closely related to the mtDNA of the European subspecies M. m. musculus. The other common haplotype and two minor ones are closely related to each other and to the mtDNA of an Asiatic subspecies, M. m. castaneus. Two other rare variants are probably the result of recent contamination by European M. m. domesticus. The musculus type of mtDNA is found in the southern two-thirds of Japan, whereas the common castaneus type is found in the northern third and the minor variants are found sporadically throughout Japan. The castaneus mtDNA lineage had a few minor variants, whereas the musculus lineage was completely monomorphic. By contrast, the native population of M. m. castaneus and the Chinese and Korean musculus populations were highly polymorphic. These results suggest that M. m. molossinus is a hybrid between ancestral colonies, possibly very small, of M. m. musculus and M. m. castaneus, rather than an independent subspecies.      

Genetic variability and the origin of house mouse from the territory of russia and neighboring countries

Genetic differentiation and gene geographic variation of house mouse from the territory of Russia and neighboring countries was examined based on the allozyme analysis of samples from natural, semisynanthropic, and obligate synanthropic populations. The results of analysis of genetic differentiation, performed using 22 interpreted loci, as well as the data on gene geographic variation of four allozyme markers (Idh-1, Sod-1, Aat-1, and hemoglobin) validated the hypothesis on rapid mice expansion from the south of Eastern Europe to the Pacific coast of Asia. It was demonstrated that moving eastwards led to the formation currently expanding zones of hybridization between the “northern” M. musculus group and the “Central Asian” M. wagneri group in Siberia, and with the M. m. castaneus group in the south of the Russian Far East. The allozyme data were compared with the data of molecular genetic and karyological analyses performed using the same experimental material. The phenomenon of hybrid zones of the house mouse from Eurasia is discussed.     

The diverse origins of New Zealand house mice

Molecular markers and morphological characters can help infer the colonization history of organisms. A combination of mitochondrial (mt) D-loop DNA sequences, nuclear DNA data, external measurements and skull characteristics shows that house mice (Mus musculus) in New Zealand and its outlying islands are descended from very diverse sources. The predominant genome is Mus musculus domesticus (from western Europe), but Mus musculus musculus (from central Europe) and Mus musculus castaneus (from southern Asia) are also represented genetically. These subspecies have hybridized to produce combinations of musculus and domesticus nuclear DNA coupled with domesticus mtDNA, and castaneus or musculus mtDNA with domesticus nuclear DNA. The majority of the mice with domesticus mtDNA that we sampled had D-loop sequences identical to two haplotypes common in Britain. This is consistent with long-term British-New Zealand cultural linkages. The origins of the castaneus mtDNA sequences widespread in New Zealand are less easy to identify.     

Frequency and distribution of t-haplotypes in the Southeast Asian house mouse (Mus musculus castaneus) in Taiwan

t-haplotypes are a meiotic drive system found on the 17th chromosome of the house mouse (Mus musculus). They can be found in wild populations of all four genetically differentiated subspecies. The drive phenomenon is male-specific, such that heterozygous males (+/t) show non-Mendelian transmission and transmit the t-chromosome to > 90% of their offspring. So far the most comprehensive studies on the frequencies of t-haplotypes in natural populations have been on just one of the subspecies (M. musculus domesticus). We applied molecular methods to accurately screen t-haplotypes in a large number of populations of a second subspecies (M. musculus castaneus) distributed in Taiwan. We found that the overall t-haplotype frequency is low in M. m. castaneus (0.108), and the chromosomes are patchily distributed among its populations, closely resembling the situation found in M. m. domesticus. Further, we found the frequencies of t-haplotypes in our sample did not differ in relation to the sex or age of mice. This resemblance in the frequency and distribution among populations of the two distinct subspecies suggests that similar general mechanisms might be responsible for the low frequencies in both subspecies.     

Mosaic genealogy of the Mus musculus genome revealed by 21 nuclear genes from its three subspecies

Patterns of genetic variation provide insight into the evolutionary history of a species. Mouse (Mus musculus) is a good model for this purpose. Here we present the analysis of genealogies of the 21 nuclear loci and one mitochondrial DNA region in M. musculus based on our nucleotide sequences of nine inbred strains from three M. musculus subspecies (musculus, domesticus, and castaneus) and one M. spicilegus strain as an outgroup. The mitochondrial DNA gene genealogy of those strains confirmed the introgression pattern of one musculus strain. When all the nuclear DNA data were concatenated to produce a phylogenetic tree of nine strains, musculus and domesticus strains formed monophyletic clusters with each other, while the two castaneus strains were paraphyletic. When each DNA region was treated independently, the phylogenetic networks revealed an unnegligibly high level of subspecies admixture and the mosaic nature of their genome. Estimation of ancestral and derived population sizes and migration rates suggests the effects of ancestral polymorphism and gene flow on the pattern of genetic variation of the current subspecies. Gene genealogies of Fut4 and Dfy loci also suggested existence of the gene flow between M. musculus and M. spicilegus or other distant species.     

Genetics and evolution of hybrid male sterility in house mice

Comparative genetic mapping provides insights into the evolution of the reproductive barriers that separate closely related species. This approach has been used to document the accumulation of reproductive incompatibilities over time, but has only been applied to a few taxa. House mice offer a powerful system to reconstruct the evolution of reproductive isolation between multiple subspecies pairs. However, studies of the primary reproductive barrier in house mice-hybrid male sterility-have been restricted to a single subspecies pair: Mus musculus musculus and Mus musculus domesticus. To provide a more complete characterization of reproductive isolation in house mice, we conducted an F(2) intercross between wild-derived inbred strains from Mus musculus castaneus and M. m. domesticus. We identified autosomal and X-linked QTL associated with a range of hybrid male sterility phenotypes, including testis weight, sperm density, and sperm morphology. The pseudoautosomal region (PAR) was strongly associated with hybrid sterility phenotypes when heterozygous. We compared QTL found in this cross with QTL identified in a previous F(2) intercross between M. m. musculus and M. m. domesticus and found three shared autosomal QTL. Most QTL were not shared, demonstrating that the genetic basis of hybrid male sterility largely differs between these closely related subspecies pairs. These results lay the groundwork for identifying genes responsible for the early stages of speciation in house mice.     

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.     

Isolation and gene flow: inferring the speciation history of European house mice

Inferring the history of isolation and gene flow during species differentiation can inform us on the processes underlying their formation. Following their recent expansion in Europe, two subspecies of the house mouse (Mus musculus domesticus and Mus musculus musculus) have formed a hybrid zone maintained by hybrid incompatibilities and possibly behavioural reinforcement, offering a good model of incipient speciation. We reconstruct the history of their divergence using an approximate Bayesian computation framework and sequence variation at 57 autosomal loci. We find support for a long isolation period preceding the advent of gene flow around 200,000 generations ago, much before the formation of the European hybrid zone a few thousand years ago. The duration of the allopatric episode appears long enough (74% of divergence time) to explain the accumulation of many post-zygotic incompatibilities expressed in the present hybrid zone. The ancient contact inferred could have played a role in mating behaviour divergence and laid the ground for further reinforcement. We suggest that both subspecies originally colonized the Middle East from the northern Indian subcontinent, domesticus settling on the shores of the Persian Gulf and musculus on those of the Caspian Sea. Range expansions during interglacials would have induced secondary contacts, presumably in Iran, where they must have also interacted with Mus musculus castaneus. Future studies should incorporate this possibility, and we point to Iran and its surroundings as a hot spot for house mouse diversity and speciation studies.     

Genetic polymorphism of the sixth component of complement (C6) in mice

Immunofixation after isoelectric focusing revealed two forms of mouse C6, C6A and C6M, both of which consist of two major protein bands and one or more acidic minor bands. They were distinguishable by their different isoelectric point (pI) ranges: C6M has more acidic pI ranges (pH < 6.2) than C6A (pH < 6.3). C6A was found in common inbred mice of Mus musculus domesticus, while C6M was found in inbred and wild mice of M. m. molossinus (Japanese wild mice, an Asian subspecies). Breeding experiments showed that these two forms of C6 were controlled by a single codominant autosomal locus. We propose the designation C-6 for this locus with two alleles, C-6 ^a and C-6 ^m , which encode for C6A and C6M, respectively. Linkage analysis indicated that the locus is not closely linked to the following loci: Idh-1, agouti, Amy-1, brown, Gpd-1, Mup-1, Pgm-2, Pgm-1, albino, Hbb, Es-1, Mod-1, Sep-1, Es-3, Igh-1, beige, Es-10, Sod-1, and C-3.     

A new haplotype of the beta-globin gene complex, Hbbw1, in Chinese wild mouse

A new pattern was observed in the electrophoretic survey of the hemoglobin beta chain (Hbb) in Chinese wild mice, Mus musculus. The electrophoretic mobility of the major component of the new Hbb was identical to that of Hbbs on cellulose acetate plate, although it was almost identical to that of Hbbd or Hbbp on acrylamide gel. This suggests that the major component of Chinese Hbb has a unique primary structure. A minor component of the new Hbb was completely different from that of the other three Hbb haplotypes well known. These results indicate that the Hbb-b1 and Hbb-b2 of the new Hbb haplotype, assigned Hbbw1, are unique genes in their molecular structure. So far, Hbbw1 has been observed in northwestern China.     

A pronounced evolutionary shift of the pseudoautosomal region boundary in house mice

The pseudoautosomal region (PAR) is essential for the accurate pairing and segregation of the X and Y chromosomes during meiosis. Despite its functional significance, the PAR shows substantial evolutionary divergence in structure and sequence between mammalian species. An instructive example of PAR evolution is the house mouse Mus musculus domesticus (represented by the C57BL/6J strain), which has the smallest PAR among those that have been mapped. In C57BL/6J, the PAR boundary is located just ~700 kb from the distal end of the X chromosome, whereas the boundary is found at a more proximal position in Mus spretus, a species that diverged from house mice 2-4 million years ago. In this study we used a combination of genetic and physical mapping to document a pronounced shift in the PAR boundary in a second house mouse subspecies, Mus musculus castaneus (represented by the CAST/EiJ strain), ~430 kb proximal of the M. m. domesticus boundary. We demonstrate molecular evolutionary consequences of this shift, including a marked lineage-specific increase in sequence divergence within Mid1, a gene that resides entirely within the M. m. castaneus PAR but straddles the boundary in other subspecies. Our results extend observations of structural divergence in the PAR to closely related subspecies, pointing to major evolutionary changes in this functionally important genomic region over a short time period.     

Polymorphism and inheritance of serum esterases and beta-globulins in the paradise fish (Macropodus opercularis; Anabantidae)

Electrophoretic variants of serum esterases and beta-globulins in two subspecies of paradise fish (Macropodus opercularis) were studied. Four esterase loci (Est-1, Est-2, Est-3 and Est-4), a single transferrin (Tf) and another major beta-globulin locus (Bg) were identified by segregational analysis. Est-3 seems to be a monomorphic locus. Three alleles of Est-1, two of Est-2, two of Est-4, four of Tf and two alleles of Bg were found in the laboratory population. None of these loci were closely linked. Electrophoretic patterns of F1 hybrids confirmed the monomeric structures of each of the studied proteins. Allelic segregation at the Tf and Bg loci was normal in F2 and backcross populations. In crosses of the two Macropodus subspecies there were deviations from Mendelian ratios because of missing recombinant esterase phenotypes. Each of these would have been homozygous Est-2f/f. We suppose that Est-2f/f causes lethality in the early phase of development, except in the Est-1c/c, Est-2f/f combination characteristic of the parental subspecies M.o. concolor.     

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

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

Genetics of hydroxyacid oxidase isozymes in the mouse: localisation of Hao-2 on linkage group XVI.

Electrophoretic variants of individual isozymes of L-alpha-hydroxyacid oxidase (HAOX-B4) and amylase (AMY-A) in an Asian subspecies of mouse, Mus musculus castaneus, have been used to localise the gene encoding the HAOX B subunit. The structural gene loci for these isozymes (Hao-2 and Amy-1) are apparently linked (4.9 +/- 2.4 per cent recombinants) in this organism, which places Hao-2 on linkage group XVI, since previous studies by Eicher and co-workers (1976) have localised Amy-1 on this chromosome.