Spatio-temporal variation in the structure of a chromosomal polymorphism zone in the house mouse

Several long-term temporal analyses of the structure of Robertsonian (Rb) hybrid zones in the western house mouse, Mus musculus domesticus, have been performed. Nevertheless, the detection of gradual or very rapid variations in a zone may be overlooked when the time elapsed between periods of study is too long. The Barcelona chromosomal polymorphism zone of the house mouse covers about 5000, km(2) around the city of Barcelona and is surrounded by 40 chromosome telocentric populations. Seven different metacentrics and mice with diploid numbers between 27 and 40 chromosomes and several fusions in heterozygous state (from one to seven) have been reported. We compare the present (period 2008-2010) and past (period 1996-2000) structure of this zone before examining its dynamics in more detail. Results indicate that there is not a Rb race in this area, which is consistent with the proposal that this zone was probably originated in situ, under a primary intergradation scenario. The lack of individuals with more than five metacentrics in heterozygous state in the current period suggests that selection acted against such mice. By contrast, this situation did not occur for mice with fewer than five fusions in heterozygous condition. Changes in human activity may affect the dynamics of gene flow between subpopulations, thus altering the chromosomal composition of certain sites. Although these local variations may have modified the clinal trend for certain metacentrics, the general staggered structure of the zone has not varied significantly in a decade.     

Chromosomal Transmission Bias in Laboratory Hybrids between Wild Strains of the Two European Subspecies of House Mice

Laboratory crosses between wild strains of the two European house mouse subspecies Mus musculus domesticus (2n = 34) and M. m. musculus (2n = 40) were performed to analyze the selective processes involved in the non-introgression of centromeric regions of Robertsonian (Rb) fusions in the Danish hybrid zone. The chromosomal analysis of 226 backcross progeny from 22 reciprocal crosses showed that the segregation of the three Rb fusions present did not significantly differ from Mendelian expectations. However, a significant negative correlation was found between Rb transmission rates and the average litter sizes of the F(1) pairs. Among the different models of selection discussed, the most likely one supported the existence of two opposing selective factors resulting in an overall compensation of chromosomal types in the backcross progeny. A two-phase selective process involving embryo competition was postulated with non-Rb carriers being favored during pre-implantation but disadvantaged after implantation. Such balanced selective pressures acting on musculus non-Rb centromeres are compatible with the steep slope and off-centered position of the chromosomal cline observed in the Danish hybrid zone. These results suggested that these selective factors may be more related to centromere origin (musculus or domesticus) than to centromere structure (Rb or non-Rb).     

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.     

Inferences of selection and migration in the Danish house mouse hybrid zone

We analysed the patterns of allele frequency change for ten diagnostic autosomal allozyme loci in the hybrid zone between the house mouse subspecies Mus musculus domesticus and M. m. musculus in central Jutland. After determining the general orientation of the clines of allele frequencies, we analysed the cline shapes along the direction of maximum gradient. Eight of the ten clines are best described by steep central steps with coincident positions and an average width of 8.9 km (support limits 7.6–12.4) flanked by tails of introgression, indicating the existence of a barrier to gene flow and only weak selection on the loci studied. We derived estimates of migration from linkage disequilibrium in the centre of the zone, and by applying isolation by distance methods to microsatellite data from some of these populations. These give concordant estimates of σ =  0.5–0.8 km generation     . The barrier to gene flow is of the order of 20 km (support limits 14–28), and could be explained by selection of a few per cent at 43–120 underdominant loci that reduces the mean fitness in the central populations to 0.45. Some of the clines appear symmetrical, whereas others are strongly asymmetrical, and two loci appear to have escaped the central barrier to gene flow, reflecting the differential action of selection on different parts of the genome. Asymmetry is always in the direction of more introgression into musculus , indicating either a general progression of domesticus into the musculus territory, possibly mediated by differential behaviour, or past movement of the hybrid zone in the opposite direction, impeded by potential geographical barriers to migration in domesticus territory.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 593–616.     

Relative warp analysis of skull shape across the hybrid zone of the house mouse (Mus musculus) in Denmark.

The changes in skull shape that occur in the hybrid zone between the two European subspecies of the house mouse ( Mus musculus musculus and M. m. domesticus ) were studied by relative warp analyses. Landmarks observed on the ventral view of the skulls of 269 mice sampled in 18 localities in Denmark were analysed. Each population was also characterized by a hybrid index estimated from allozymic data. Although no clear pattern of within-population variability of shape could be established across the hybrid zone, the shape changes among the 18 populations were found to be correlated with the allozymic introgression but not with geographical location. Finally, the cline obtained for skull shape seems to be steeper than that corresponding to the average allozyme hybrid index which suggests that skull development could be slightly perturbed in hybrids.     

High diversity of centric fusions with monobrachial homology in an area of chromosomal polymorphism of Mus musculus domesticus

One of the simplest models of chromosomal speciation is speciation by monobrachial centric fusion. This model is based on the assumption that a sterility barrier can develop between populations, in which fixed centric fusions show monobrachial homology, i.e. share only one chromosome arm. However, studies aimed at delineating intermediate stages of transition to reproductive isolation are lacking. In this paper, we describe a new area of chromosomal polymorphism in the house mouse, Mus musculus domesticus Schwarz and Schwarx, 1943, in Sicily (Italy). We trapped 79 mice at eighteen localities in an area of approximately 500 Km² surrounding the largest active European volcano, Mount Etna. Combining G‐banding and chromosome painting we identified twelve different Robertsonian (Rb) metacentrics. Considering the high number of Rb fusions, some of them shared with other documented areas, the presently studied area of chromosomal polymorphism is very likely to represent a mixture of allochthonous and autochthonous Rb fusions. The Rb(9.16) is the most widespread metacentric (overall frequency 0.80). Two Rb metacentrics, Rb(4.10) and Rb(5.6), have similar overall frequency, 0.29 and 0.37, respectively, and are narrowly co‐distributed in ten populations. Nine fusions – Rb(2.13), Rb(1.3), Rb(12.17), Rb(8.17), Rb(2.14), Rb(10.14), Rb(11.17), Rb(3.15), and Rb(11.14) – show a low frequency (0.04–0.01) and mostly non‐overlapping localization, but each of them shares monobrachial homology with at least one other metacentric. The overall geographical distribution of different Rb fusions seems to match an early stage of race formation. The eventual role of the presently studied hybrid zone in the context of chromosomal speciation by monobrachial centric fusions is discussed. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 722–731.     

Chromosomal variation in the house mouse

Although the standard karyotype of the western house mouse, Mus musculus domesticus , consists entirely of acrocentric chromosomes, there are 97 distinct 'populations' that are characterized by various combinations of metacentric chromosomes that have arisen by Robertsonian (Rb) fusions and whole-arm reciprocal translocations (WARTs). In this review we discuss the processes behind the origin and fixation of these rearrangements and then present a unified list of all known metacentric populations and evaluate their phylogenetic relationships. Eleven independent phylogeographical 'systems', each consisting of 2–25 metacentric populations, were identified in Scotland, Denmark, Northern Europe–Northern Switzerland, Southern Switzerland, Northern Italy, Croatia, Spain, Central–Southern Italy, Peloponnesus, Mainland Greece and Madeira. There are six isolated metacentric populations that do not belong to any of these systems. To generate phylogenies of the metacentric populations within each system, we determined those outcomes with the fewest steps regarding accumulation of metacentrics by Rb fusions, WARTs and zonal raciation and taking into account geographical proximity. These phylogenies should be viewed as working hypotheses that will be refined with further chromosomal and molecular data and improvements in methods of phylogenetic reconstruction. The list of metacentric populations and our phylogenies are also published electronically and can be accessed at http://www.studenec.ivb.cz/Projects/RobertsonianMice/ .  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 535–563.     

Genie differentiation in M. m. domesticus populations from Europe, the Middle East and North Africa: geographic patterns and colonization events

Geographic patterns of genie differentiation were investigated in the commensal house mouse subspecies, M. m. domesticus . The analysis by protein electrophoresis of 40 populations throughout Europe, the Middle East and North Africa indicated that genie differentiation was not highly structured on a macrogeographic scale. Mean genie distances between regions showed, however, that populations fell into three levels of differentiation: a low level in southern Europe within which interregional distances were no larger than intraregional ones, an intermediate level between southern European populations and North European and African ones, and finally, a higher level between all the latter and the Middle Eastern populations.
Gene flow estimates indicated that the homogeneity of southern European populations does not result from present high levels of gene flow, but more likely from a very recent ancestry. These data when argumented with the fossil records of mice from the Mediterranean Basin suggest a two-step colonization process, the most recent of which occurred very rapidly and resulted in the multiple founding of populations in southern Europe. The relationship of M. m. domesticus to other subspecies of mice is discussed in relation to introgression and taxonomy.
Microdifferentiation patterns with low levels of within population substructuring and of gene flow suggest that genie differentiation in the western European house mouse is largely determined by genetic drift and/or founder effects. Although historical factors are determinant in the large scale patterns of genie variation in commensal house mice, dispersal by man no longer seems to be a prominent feature moulding the genetic structure of M. m. domesticus .     

EXTREME KARYOTYPIC VARIATION IN A MUS MUSCULUS DOMESTICUS HYBRID ZONE: THE TOBACCO MOUSE STORY REVISITED

The Robertsonian fusion is a common chromosomal mutation among mammal species and is especially prevalent in the West European house mouse, Mus musculus domesticus. More than 40 races of the house mouse exist in Europe, including the famous “tobacco mouse” (Poschiavo race) of Val Poschiavo, Switzerland. Documented here is the discovery of an extreme case of karyotypic variation in the neighboring Upper Valtellina, Italy. In a 20-km stretch of the valley, 32 karyotypes were observed, including five chromosomal races and 27 hybrid types. One previously unknown race is reported, the “Mid Valtellina” race, with a diploid number of 2n = 24 and the Robertsonian fusions Rb(1.3), Rb(4.6), Rb(5.15), Rb(7.18), Rb(8.12), Rb(9.14), Rb(11.13), and Rb(16.17). The Poschiavo race (2n = 26), Upper Valtellina race (2n = 24), Lower Valtellina race (2n = 22) and all-acrocentric race (2n = 40) were also present. The races form a patchy distribution, which we term a “mottled hybrid zone.” Geographical position, isolation, extinction, recolonization, and selection against hybrids are all believed to be instrumental in the origin and evolution of this complex system. Previous studies of house mice from Upper Valtellina indicated that two of the races in the valley (the Upper Valtellina and Poschiavo races) may have speciated in the village of Migiondo. We discuss the possibility that there may have been a reinforcement event in this village.     

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.     

Robertsonian house mouse populations in Tunisia: A karyological and biochemical study

A new population of Mus musculus domesticus showing Robertsonian translocations (2n = 22) has been identified in Tunisia. Mice carried 9 pairs of Robertsonian translocations: Rb(1.11), Rb(2.16), Rb(3.12), Rb(4.6), Rb(5.14), Rb(7.18), Rb(8.9), Rb(10.17) and Rb(13.15). Five of the nine translocations are not known to occur elsewhere. In two populations only 2n = 22 mice were present, whereas in a third one a mixture of karyotypes (2n = 22, 38, 39, 40) was found. The slight electrophoretic differentiation observed for other Robertsonian populations as well as the hybrid nature of the karyotypically heterogeneous populations are confirmed. Allele frequency differences between the 2n = 22 and 2n = 40 mice in the latter population show that local electrophoretic differentiation exists and allow to compare chromosome flow and gene flow. The presence of a chromosome-1 polymorphism for additional fragments of heterochromatin present in the Robertsoman population provides a convenient marker of recombinational events in the hybrid zone.     

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.     

Genic differentiation and origin of Robertsonian populations of the house mouse (Mus musculus domesticus Rutty)

This paper examines the relation between chromosomal and nuclear-gene divergence in 28 wild populations of the house mouse semi-species, Mus musculus domesticus, in Western Europe and North Africa. Besides describing the karyotypes of 15 of these populations and comparing them to those of 13 populations for which such information was already known, it reports the results of an electrophoretic survey of proteins encoded by 34 nuclear loci in all 28 populations. Karyotypic variation in this taxon involves only centric (or Robertsonian) fusions which often differ in arm combination and number between chromosomal races. The electrophoretic analysis showed that the amount of genic variation within Robertsonian (Rb) populations was similar to that for all-acrocentric populations, i.e. bearing the standard karyotype. Moreover, divergence between the two types of populations was extremely low. These results imply that centric fusions in mice have not modified either the level or the nature of genic variability. The genetic similarity between Rb and all-acrocentric populations is not attributed to the persistence of gene flow, since multiple fusions cause marked reproductive isolation. Rather, we attribute this extreme similarity to the very recent origin of chromosomal races in Europe. Furthermore, genic diversity measures suggest that geographically separated Rb populations have in situ and independent origins. Thus, Rb translocations are probably not unique events, but originated repeatedly. Two models are presented to explain how the rapid fixation of a series of chromosomal rearrangements can occur in a population without lowering variability in the nuclear genes. The first model assumes that chromosomal mutation rates are between 10(-3) and 10(-4) and that populations underwent a series of transient bottlenecks in which the effective population size did not fall below 35. In the second model, genic variability is restored following severe bottlenecks, through gene flow and recombination.     

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.     

A TEST OF THE NEUTRAL MODEL OF EXPRESSION CHANGE IN NATURAL POPULATIONS OF HOUSE MOUSE SUBSPECIES

Changes in expression of genes are thought to contribute significantly to evolutionary divergence. To study the relative role of selection and neutrality in shaping expression changes, we analyzed 24 genes in three different tissues of the house mouse ( Mus musculus ). Samples from two natural populations of the subspecies M. m. domesticus and M. m. musculus were investigated using quantitative PCR assays and sequencing of the upstream region. We have developed an approach to quantify expression polymorphism within such populations and to disentangle technical from biological variation in the data. We found a correlation between expression polymorphism within populations and divergence between populations. Furthermore, we found a correlation between expression polymorphism and sequence polymorphism of the respective genes. These data are most easily interpreted within a framework of a predominantly neutral model of gene expression change, where only a fraction of the changes may have been driven by positive selection. Although most genes investigated were expressed in all three tissues analyzed, significant changes of expression levels occurred predominantly in a single tissue only. This adds to the notion that enhancer-specific effects or transregulatory effects can modulate the evolution of gene expression in a tissue-specific way.     

Studies of mitochondrial DNA,allozyme and morphometric variation in a house mouse hybrid zone

An unusual chromosomal hybrid zone of the house mouse, Mus musculus domesticus, exists in Upper Valtellina, Northern Italy, consisting of four Robertsonian (Rb) races and the standard (all-acrocentric, or 2n = 40) race, all hybridizing freely within 10 km2. The hybrid zone in Valtellina provides an excellent opportunity to study the role of Rb fusions in reproductive isolation and speciation. This hybrid zone has already been well studied for the distribution of Rb fusions and the fertility of hybrids, but in order to understand the dynamics of the zone, a basic understanding of the origin and genetic similarity of the chromosomal races is necessary. This paper presents the results of three different methods of measuring genetic differentiation: multivariate analysis of morphological traits and analyses of allozyme variation and mitochondrial DNA sequences. The standard race is clearly distinguishable from the three Rb races by all three methods, but the Rb races are not distinguishable from one another. This provides strong evidence for our previous suggestions that the well-established Rb races in Valtellina are closely related, and that the standard race was introduced into the valley more recently from a distant source. The fact that the Rb races are indistinguishable is also consistent with our hypothesis that a within-village speciation event involving two of the races (Hauffe & Searle, 1992) was a recent occurrence. The low level of allozyme heterozygosity among the Rb races suggests that these populations are the products of at least one bottleneck. The present article substantially extends earlier studies and provides the first detailed morphological and molecular analysis of this complex hybrid zone.     

Probable origin of the Robertsonian phenomena in domestic mice in Tunisia

The Robertsonian phenomenon in house mice (Mus musculus domesticus) from Tunisia consists in the presence of only one 22-chromosome Robertsonian race (22Rb) carrying the maximum number of fusions observed until now. The 22Rb populations exclusively occupy urban centers in the Eastern-Central region of Tunisia where standard population with 40-all acrocentric chromosomes (40Std) occur in surrounding neighborhoods and rural environments. In addition to the habitat partition, allozyme and mitochondrial DNA analyses showed that the 22Rb populations were genetically differentiated from the 40Std ones. This differentiation mostly stemmed from an important decrease in genetic variability in the 22Rb populations from the Sahel towns. The extent of morphological, ecological and genetical divergence observed between these chromosomal races in Tunisia is in agreement with the predictions of the chromosomal speciation model of White which advocates that karyotypic differentiation between taxa can lead to their reproductive isolation and independent evolution. Such a process is verified if the Rb process in Tunisia results from local differentiation which is supported by both the genetic and morphological data. However, the hypothesis of an origin by introduction of these mice from another region of Tunisia or from another country cannot be totally dismissed. In this study, an allozymic analysis of mice (22Rb and 40Std) from the geographically distant city of Kairouan was performed. Results showed that 22Rb and 40Std mice from Kairouan shared the same high degree of variability, and were not genetically differentiated. This contrasts with the results registered in the two chromosomal races in the Sahel towns. Such data argue in favor of a local differentiation of the Robertsonian process in Tunisia and suggest that the decrease in variability of the structural nuclear genes in the Sahel 22Rb populations can be related to an introduction from Kairouan into a Sahel locality resulting in a founder effect or followed by a severe bottleneck prior to its dispersion throughout the Sahel region.     

There is no heterotic effect upon developmental stability in the ventral side of the skull within the house mouse hybrid zone

Fluctuating asymmetry (FA) of a body is expected to be related to an organism's developmental instability. We studied patterns of FA in the ventral side of the skull along a transect across the central-European portion of the hybrid zone between two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus. We found that FA was not significantly different between introgression classes relative to differences between localities within those classes and that the within-class differences were much larger than differences between individual-signed asymmetries within localities. However, if year was added to the same analysis as another factor, FA was not significantly different among localities within the same introgression class. When individual asymmetries were plotted against individual hybrid indices, hybrids appeared more asymmetric than individuals from outside of the zone. Thus contrary to previous studies, we did not find lower FA indicating heterotic effect in hybrids for the traits studied. It is suggested that the impact of hybridization on FA in the ventral side of the mouse skull is negligible or overwhelmed by other factors.