Testing for selection on the androgen-binding protein in the Danish mouse hybrid zone

The three peripheral subspecies of the house mouse Mus musculus have fixed specific variants of the androgen-binding protein (ABP) that have been proposed to be part of a recognition mechanism that could participate in sexual isolation between the subspecies. We tested for selection on Abpa by characterizing the pattern of Abpa introgression across a transect of the hybrid zone between M. m. musculus and M. m. domesticus in Jutland. On the musculus side, the cline for Abpa resembled that of a nearly neutral allozyme more than that of strongly selected X and Y chromosome markers. However, the high central step which displaces the tail of introgression of Abpa to higher frequencies was best accounted for by linkage to a locus under strong selection against hybrids. Still, we cannot exclude that this pattern results from weak selection against Abpa in the tail of introgression, which would be compatible with an assortative choice mechanism. On the domesticus side there was little introgression close to the hybrid zone, presumably due to a geographical barrier to migration. However, substantial frequencies of musculus alleles occurred further away, suggesting mixed colonization patterns as well as flow across the hybrid zone.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 447–459.     

Genetic analysis of autosomal and X-linked markers across a mouse hybrid zone

In this paper, we present results of the first comprehensive study of the introgression of both autosomal and sex-chromosome markers across the central European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and M. m. domesticus. More than 1800 individuals sampled from 105 sites were analyzed with a set of allozyme loci (hopefully representing neutral or nearly neutral markers) and X-linked loci (which are assumed to be under selection). The zone center is best modeled as a single straight line independent of fine-scale local geographic or climatic conditions, being maintained by a balance between dispersal and selection against hybrids. The width (w) of the multilocus autosomal cline was estimated as 9.6 km whereas the estimate for the compound X-chromosome cline was about 4.6 km only. As the former estimate is comparable to that of the Danish portion of the zone (assumed to be much younger than the central European one), zone width does not appear to be related to its age. The strength (B) of the central barrier was estimated as about 20 km; with dispersal (sigma) of about 1 km/gen(1/2), this means effective selection (s*) is approximately 0.06-0.09 for autosomal loci and about 0.25 for X-linked loci. The number of loci under selection was estimated as N= 56-99 for autosomes and about 380 for X-linked loci. Finally, we highlight some potential pitfalls in hybrid zone analyses and in comparisons of different transects. We suggest that conclusions about parts of the mouse genome involved in reproductive isolation and speciation should be drawn with caution and that analytical approaches always providing some estimates should not be used without due care regarding the support or confidence of such estimates, especially if conclusions are based on the difference between these estimates. Finally, we recommend that analysis in two-dimensional space, dense sampling, and rigorous treatment of data, including inspection of likelihood profiles, are essential for hybrid zone studies.     

Mitochondrial DNA in the hybrid zone between Mus musculus musculus and Mus musculus domesticus: a comparison of two transects

We studied mtDNA introgression across the contact zone between Mus musculus musculus and M. m. domesticus in two independent transects in the Czech Republic and Bavaria, Germany. A total of 1270 mice from 98 localities in the Czech transect and 456 mice from 41 localities in the Bavarian transect were examined for presence or absence of a Bam HI restriction site in the mt-Nd1 gene. Using this simple mtDNA marker, variants that belonged to the M. m. domesticus lineage (presence of restriction site) could be unequivocally distinguished from those belonging to the M. m. musculus lineage (absence of restriction site). The extent of introgression of mtDNA, three autosomal allozymes and the X chromosome was compared. The introgression of X markers was more limited than was that of the allozymes and mtDNA. In the Czech transect, the centre for the mtDNA cline was shifted about 3.6 km to the west relative to the X chromosome cline, with asymmetric introgression from M. m. musculus to M. m. domesticus . Interestingly, in the Bavarian transect, the centre of the mtDNA cline was shifted about 10.9 km to the east relative to the X chromosome cline, with asymmetric introgression from M. m. domesticus to M. m. musculus, opposite in direction to that observed in the Czech transect.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 363–378.     

Assessment of mate preference in the house mouse with reference to investigations on assortative mating

This study analysed patterns of mate choice based on different types of laboratory test, and points out the advantages of combined methods to assess mate preference. We compared dyadic encounters and two-way choice tests involving a pair of mice or their urinary signature. Preference was assessed in males and females of the two European subspecies of the house mouse Mus musculus that share a secondary hybrid zone. A preference was deduced from directional choice or behavioural discrimination in favour of a mouse. Our results show discrepancies between the outcomes of the different types of test, which we discuss in terms of the quality of the stimuli involved, and of physical access to the information carried by those stimuli. Despite variation, our results indicate that M. m. domesticus did not show a preference, but could discriminate between stimuli of the two subspecies and tended to direct sexual behaviour assortatively. In contrast, both male and female M. m. musculus showed positive discrimination and preference for potential mates and signals carried by their own subspecies. Additionally, our results confirm that subspecies informative signals are present in mouse urine and suggest that they may be molecules of low volatility, necessitating contact for preference to be displayed.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 461–471.     

Assessing multilocus introgression patterns: a case study on the mouse X chromosome in central Europe

Multilocus hybrid zone (HZ) studies predate genomics by decades. The power of early methods is becoming apparent and now large datasets are commonplace. Relating introgression along a chromosome to evolutionary process is challenging: although reduced introgression regions may indicate speciation genes, this pattern may be obscured by asymmetric introgression of linked invasive genes. Further, HZ movement may form salients and leave islands in its wake. Barton's concordance was proposed 24 years ago for assessing introgression where geographic patterns are complex. The geographic axis of introgression is replaced with the hybrid index. We compare this, a recently proposed genomic clines approach, and two-dimensional (2D) geographic analyses, for 24 X chromosome loci of 2873 mice from the central-European house mouse HZ. In 2D, 14 loci show linear contact, seven precisely matching previous studies. Four show introgression islands to the east of the zone, suggesting past westward zone movement, two show westward salients. Barton's concordance both recovers and refines this information. A region of reduced introgression on the central X is supported, despite X centromere-proximal male-biased westward introgression matching a westward 2D geographic salient. Genomic clines results are consistent regarding introgression asymmetries, but otherwise more difficult to interpret. Evidence for genetic conflict is discussed.     

The genomics of speciation: investigating the molecular correlates of X chromosome introgression across the hybrid zone between Mus domesticus and Mus musculus

Understanding the genetic details of reproductive isolation is a key goal in the study of speciation. Hybrid zones, geographical regions where two species meet and exchange genes, can provide insight into the genetic basis of reproductive isolation. This is especially true in species with mapped molecular markers because patterns of gene flow can be compared among different genomic regions. Even greater insight can be obtained in species with complete genome sequences because gene identity, gene number and other features of interest can be assessed for genomic regions with different patterns of introgression. Here, we review recent studies on the well-characterized hybrid zone between Mus domesticus and M. musculus , including a detailed survey of patterns of introgression for 13 markers on the X chromosome. We then compare levels of introgression for these 13 regions to a number of genomic attributes inferred from the complete sequence of the X chromosome, with two purposes. First, we identify candidate genes for reproductive isolation by finding genes that map to an X-linked region of reduced introgression and that are only expressed in the male germ line or that show high rates of protein evolution in comparison with rat. Second, we ask whether patterns of gene flow are correlated with recombination rate, gene density, base composition, CpG island density, mutation rate and the rate of protein evolution, as might be expected if many genes contribute to reproductive isolation. We identify seven candidate genes for reproductive isolation between M. domesticus and M. musculus , and our analyses reveal no general correlations between levels of introgression and other measured sequence characteristics. These results underline the utility of the house mouse as a model system for the study of speciation.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 523–534.     

Limited mitochondrial DNA introgression in a Bombina hybrid zone

We analysed the variation at mitochondrial DNA (mtDNA) and six allozyme loci in two transects across a hybrid zone between the fire-bellied toads Bombina bombina and Bombina variegata in southern Poland. The mtDNA cline was narrower than allozyme clines in one transect (Przemyśl, β = 0.435, w  = 3.42 km) and shifted to the B .  bombina side in both (near Kraków by 1.32 km, α = 0.437, and near Przemyśl by 0.97 km, α = 0.319). Cytonuclear associations were weak. Narrowed mtDNA clines could be a by-product of female demography and lowered effective population size of mtDNA. Alternatively, restricted mtDNA introgression is a likely result of negative epistasis in recombinants augmented by environment-dependent selection because divergence of Bombina mitochondrial and nuclear genomes is large. The shift of the mtDNA cline, contrary to expectations from initial mating preferences and fecundity differences between the species, suggests that on the B .  bombina side of the zone hybrid females with B .  variegata mtDNA have a higher chance of leaving progeny.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 295–306.     

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.     

The house mouse: a model and motor for evolutionary understanding

Commensal house mice have spread from their probable origin in north India, differentiating into a number of forms described variously as species, semispecies or subspecies. The different taxa can breed together and exchange genes but they retain their distinctiveness (although Mus ( musculus ) molossinus of Japan seems to be the result of a complete fusion between M.  ( m .)  musculus and M.  ( m .)  castaneus ). The most widespread form is M.  ( m. )  domesticus , which has successfully colonized every continent as a commensal, albeit with varying contributions from other Mus genomes. It has also been domesticated as the laboratory mouse. This means that the same genome is exposed to a wide variety of environments and gives tremendous opportunities for exploring the operation of different evolutionary mechanisms. Mice have accompanied evolutionary understanding from early Darwinian days – confirming Mendelian ratios and showing they applied in mammals, providing data on rates of evolution, and representing examples of dominance modification, differential survival, competition and other indicators of a struggle for existence. However, they have an unfulfilled potential to drive as well as to illuminate evolutionary theory – by revealing more about, for example, the interactions between gene flow and social determinants, constraints on introgression and physiological adjustments. This potential can be explored through our ever-deepening knowledge of the genome and molecular mechanisms, and by the application of new techniques, but its most effective agent will always be the cross-disciplinary synergy of visionary scientists like Julian Huxley, Charles Elton and Louis Thaler.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 335–347.     

Chromosomal introgression in house mice from the hybrid zone between M. m. domesticus and M. m. musculus in Denmark

The recent discovery of Robertsonian (Rb) translocations in Danish mice from the hybrid zone between Mus musculus musculus and M. m. domesticus stimulated the chromosomal analysis of populations along a north-south transect through this zone. G-Banding identified the Rb fusions as Rb(3.8), Rb(2.5) and Rb(6.9). The cytogenetic results show that there is a gradual decrease in the number of fusions as one proceeds north, the translocations abruptly ending in populations from the centre of the hybrid zone determined by seven diagnostic allozymic markers. These results indicate that Rb fusions are present only in domesticus or predominantly domesticus-genotype mice and that they do not introgress into M. m. musculus . To test if genie incompatibilities between the musculus genetic background and Rb fusions were involved in the systematic elimination of the latter, predominantly musculus mice from the hybrid zone were crossed with Rb domesticus mice carrying Rb(3.8). The karyotypic analysis of the progeny showed no distortion of the transmission ratio of this fusion.
The chromosomal and allozymic analysis of these mice further indicates that (i) recombination is not suppressed between metacentrics and their acrocentric homologues and (ii) specific domesticus chromosomal segments are tolerated in the musculus genomes whereas the Rb centromeres are not.     

Genetic analysis of the hybridization zone between two subspecies Mus musculus domesticus and Mus musculus musculus in Bulgaria

The hybrid zone between the two subspecies of mice Mus musculus domesticus and Mus musculus musculus, which has been studied extensively in Denmark, crosses Europe to the Black Sea through the Alps and the Balkans. Two hundred and seventy-nine animals were captured in 22 localities along a transect across the Balkans. The animals were characterized for seven diagnostic nuclear loci by protein electrophoresis and by restriction pattern analysis of their mitochondrial DNA. The nuclear data show a sharp transition between the two subspecies, most of the variations in allele frequencies (from 0.9 to 0.1) occurring within a 36-km section of the transect. The introgression varies from one locus to the other and is more pronounced, in terms of distance, in M. m. musculus territory. Mitochondrial DNA introgression is important but occurs in one direction only, i.e. from M. m. musculus to M. m. domesticus, while a cytoplasmic transfer from M. m. domesticus to M. m. musculus has been reported. A previous study showed that no Y chromosome introgression occurs. The different behaviour of these three types of markers could be due to the interaction between selection against hybrid genomes and meiotic recombination. Objectively, it would appear that the genes that can introgress are neutral or nearly so and have been separated from deleterious genes they were linked to by recombination. This could explain the differential introgression between autosomal loci. The mitochondrial and Y chromosomes undergo no or very little recombination and each is transmitted as a whole. Their degree of introgression is thus indicative of the intensity of selection resulting from the amount of functional differentiation between the two taxa, which seems to be strong for the Y chromosome and weak for mitochondrial DNA. We propose that the asymmetry of nuclear introgression is due to different population structures. As M. m. musculus is relatively less structured, the rapid spreading of introgressed genes would be favoured. Such a scheme, however, can hardly account for the unidirectionality of the mitochondrial flow, which could be due to sex-dependent behaviour.     

Steep clines within a highly permeable genome across a hybrid zone between two subspecies of the European rabbit

Maintenance of genetic distinction in the face of gene flow is an important aspect of the speciation process. Here, we provide a detailed spatial and genetic characterization of a hybrid zone between two subspecies of the European rabbit. We examined patterns of allele frequency change for 22 markers located on the autosomes, X‐chromosome, Y‐chromosome and mtDNA in 1078 individuals sampled across the hybrid zone. While some loci revealed extremely wide clines (w ≥ 300 km) relative to an estimated dispersal of 1.95–4.22 km/generation, others showed abrupt transitions (w ≈ 10 km), indicating localized genomic regions of strong selection against introgression. The subset of loci showing steep clines had largely coincident centers and stepped changes in allele frequency that did not co‐localize with any physical barrier or ecotone, suggesting that the rabbit hybrid zone is a tension zone. The steepest clines were for X‐ and Y‐chromosome markers. Our results are consistent with previous inference based on DNA sequence variation of individuals sampled in allopatry in suggesting that a large proportion of each genome has escaped the overall barrier to gene flow in the middle of the hybrid zone. These results imply an old history of hybridization and high effective gene flow and anticipate that isolation factors should often localize to small genomic regions.     

GENETIC ANALYSIS OF A HYBRID ZONE BETWEEN THE FIRE-BELLIED TOADS,BOMBINA BOMBINA AND B. VARIEGATA,NEAR CRACOW IN SOUTHERN POLAND

The fire-bellied toads Bombina bombina and B. variegata differ extensively in biochemistry, morphology, and behavior. We use a survey of five diagnostic enzyme loci across the hybrid zone near Cracow in Southern Poland to estimate the dispersal rate, selection pressures, and numbers of loci which maintain this zone. The enzyme clines coincide closely with each other and with morphological and mitochondrial DNA clines. Although the zone lies on a broad transition between environments suitable for bombina and variegata, the close concordance of diverse characters, together with increased aberrations and mortality in hybrids, suggest that the zone is maintained largely by selection against hybrids. There are strong “linkage disequilibria” between each pair of (unlinked) enzyme loci (R? = 0.129 [2-unit support limits: 0.119–0.139]). These are probably caused by gene flow into the zone, and they give an estimate of dispersal (σ = 890 [790–940] m gen^?½). The clines are sharply stepped, with most of the change occurring within 6.15 (5.45–6.45) km, but with long tails of introgression on either side. This implies that the effective selection pressure on each enzyme marker (due largely to disequilibrium with other loci) is s* = 0.17 (0.159–0.181) at the center but that the selection acting directly on the enzyme loci is weak or zero (s_e < 0.0038). The stepped pattern implies a barrier to gene flow of 220 (48–415) km. This would substantially delay neutral introgression but would have little effect on advantageous alleles; the two taxa need not evolve independently. Strong selection is needed to maintain such a barrier: hybrid populations must have their mean fitness reduced by a factor of 0.65 (0.60–0.77). This selection must be spread over a large number of loci to account for the concordant patterns and the observed cline widths (N = 300 [80–2,000]).     

Morphological,genetic and behavioural analyses of a hybrid zone between the ground beetles Carabus lewisianus and C. albrechti (Coleoptera,Carabidae): asymmetrical introgression caused by movement of the zone?

We analysed the putative hybrid zone between the ground beetles Carabus lewisianus Breuning and C. albrechti Morawitz from the Kanto and Tanzawa Mountains in central Honshu, Japan, using morphological, genetic, and behavioural data. Canonical discriminant analysis of three external and five genital morphological characters revealed an apparent morphological gap, suggesting restricted sympatry in the contact zone. RFLP and molecular phylogenetic analyses of the mitochondrial gene ND5 revealed that haplotypes originating from C. lewisianus introgressed extensively into C. albrechti . Fitting tanh curves to morphological (external and genital) and genetic characters showed differential cline widths and centres, suggesting stronger selection for genital morphology with narrower cline widths, and extensive introgression of the nuclear genes responsible for external characters relative to those controlling genital morphology into the range of C. albrechti . Linkage disequilibria between the morphological characters did not differ from zero, suggesting sufficient recombination occurred in the zone but the effect of small sample sizes was not negligible. Mate choice trials demonstrated that males showed a weak but not significant preference for conspecific mates. We interpret these patterns as evidence for (1) partial reproductive isolation presumably by postmating processes; (2) selection against intermediate genital morphologies, and (3) independent responses of the characters to different evolutionary forces that may cause movement of genital clines and introgression of the mitochondrial gene and external morphology. The movement of the boundaries may have resulted in the particular species distribution patterns in the study area.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 79–94.     

The role of salivary androgen-binding protein in reproductive isolation between two subspecies of house mouse: Mus musculus musculus and Mus musculus domesticus

Previous behavioural studies using inbred lines have suggested that the gene ( Abpa ) for the alpha subunit of salivary androgen-binding protein (ABP) plays a role in prezygotic isolation between house mouse Mus musculus subspecies. We tested this hypothesis in animals from wild allopatric (121 individuals from four samples) and parapatric (320 animals from 15 samples) populations sampled on the Czech–Bavarian transect across the hybrid zone between M. m. domesticus and M. m. musculus . The study did not reveal a consistent statistically significant trend of homosubspecific preferences in individual allopatric and parapatric populations. Nonetheless, the whole pattern of preference was skewed toward homosubspecific preference mostly on the M. m. musculus side of the hybrid zone. The pattern of homosubspecific preferences was stronger for the time spent sniffing than it was for the first choice of the signal (the ratio of homosubspecific vs. heterosubspecific preferences for both sexes was 6 : 2 in allopatric and 21 : 9 in parapatric populations, while the same rates were 4 : 4 and 16 : 14 for the first choice). To the extent that Y-maze tests reflect preference under wild conditions, we suggest that this slight preference may not in itself be sufficient to impede gene flow between the two subspecies and thus act as a reproductive barrier. ABP most probably participates in a complex system of subspecies-specific recognition in the hybrid zone, but the picture is far too complex at this time to allow a conclusive evaluation of the importance of this role.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 349–361.     

Postzygotic isolation between the two European subspecies of the house mouse: estimates from fertility patterns in wild and laboratory-bred hybrids

We assessed the fertility (reproductive success, litter size, testis weight, spermatocyte-to-spermatid ratio) of F₁s and backcrosses between different wild-derived outbred and inbred strains of two mouse subspecies, Mus musculus domesticus and M. m. musculus . A significant proportion of the F₁ females between the outbred crosses did not reproduce, suggesting that female infertility was present. As the spermatocyte-to-spermatid ratio was correlated with testis weight, the latter was used to attribute a sterile vs. fertile phenotype to all males. Segregation proportions in the backcrosses of F₁ females yielded 11 (inbred) to 17% (outbred) sterile males, suggesting the contribution of two to three major genetic factors to hybrid male sterility. Only one direction of cross between the inbred strains produced sterile F₁ males, indicating that one factor was borne by the musculus X-chromosome. No such differences were observed between reciprocal crosses in the outbred strains. The involvement of the X chromosome in male sterility thus could not be assessed, but its contribution appears likely given the limited introgression of X-linked markers through the hybrid zone between the subspecies. However, we observed no sterile phenotypes in wild males from the hybrid zone, although testis weight tended to decrease in the centre of the transect.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 379–393.     

Morphology, growth and reproduction in the Australian house mouse: differential effects of moderate temperatures

The house mouse ( Mus musculus domesticus ) was introduced into Australia two centuries ago and is now succeeding in a wide range of habitats and climatic regions. To explore how mice exploit such extreme environments, we compared growth rate, morphology and reproductive success of animals reared under differing thermal regimes (13 °C 'cool', 22 °C 'moderate' and 30 °C 'warm') in laboratory mice derived from wild stock. 'Warm' group young were smaller and grew more slowly than those from other groups. At 6 weeks of age, body mass was less in 'warm' than in 'cool' treatment individuals; and liver mass/body mass also was less in 'warm' than in 'cool' treatment individuals. Paired kidney mass/body mass and paired adrenal mass/body mass were less in 'warm' than in 'cool' and 'moderate' treatment mice. Low heritability values indicate that these effects were from the temperature treatments rather than genetic influences. Irrespective of temperature treatment, females were more likely to produce a litter from post-partum matings if they were experienced, rather than young or reproductively naïve, and also bore more young from post-partum matings. These observations contribute to understanding of the sudden plague activities of mice in some parts of Australia and also their sparse distribution in the interior of the continent.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 21–30.