Genetic differentiation and habitat partition of Robertsonian house mouse populations (Mus musculus domesticus) of Tunisia

The karyological analysis of Tunisian populations of house mice revealed the existence of a Robertsonian (Rb) chromosomal race carrying nine pairs of metacentric chromosomes in central Tunisia. Divergence estimates showed that they are genetically differentiated from local all-acrocentric populations and have a reduced level of genic variability. The Rb populations are restricted to urban habitats, whereas all-acrocentric populations occur in rural areas. Contact zones between these two types of habitat yield chromosomally polymorphic populations. Analysis of gene flow indicates that it is reduced and limited to populations bordering the contact areas. The reduced genic variability and patchy distribution exhibited by the Tunisian Rb mice do not agree with results from previous studies of the European Rb populations. Two hypotheses are presented to account for this discrepancy based on local differentiation versus introduction of the Tunisian Rb race.     

Is chromosomal speciation occurring in house mice in Tunisia?

Two chromosomal races of house mice are present in Tunisia, one represented by mice carrying the 40-acrocentric standard karyotype and the other by a Robertsonian race (2re = 22) homozygous for nine centric fusions (Rb). A comparative summary on allozyme divergence, geographical distribution and level of reproductive isolation in the Tunisian and European Rb races is presented, to which new data on mitochondrial DNA and morphological divergence are added. The Tunisian 22Rb race revealed unique features not matched by the European chromosomal races, such as a decrease in allozymic variability, a higher level of genetic and morphological differentiation and a mosaic geographical distribution. The mtDNA analysis argued in favour of a local origin of the chromosomal divergence suggesting that the higher level of differentiation between the Tunisian races resulted from the older age of the 22Rb race and/or from a severe botdeneck. The decrease in fertility of chromosomal hybrids between the Tunisian races was compatible with the limited genetic introgression between diem. Furthermore, data on the restricted distribution of hybrid populations suggested that premating reproductive barriers may be evolving. The Tunisian 22Rb race is thus an appropriate model to investigate a chromosomally-mediated speciation event.     

Chromosomal rearrangements and evolution of recombination: comparison of chiasma distribution patterns in standard and robertsonian populations of the house mouse

The effects of chromosomal rearrangements on recombination rates were tested by the analysis of chiasma distribution patterns in wild house mice. Males and females of two chromosomal races from Tunisia differing by nine pairs of Robertsonian (Rb) fusions (standard all-acrocentric, 2N = 40 and 2N = 22) were studied. A significant decrease in chiasma number (CN) was observed in Rb mice compared to standard ones for both sexes. The difference in CN was due to a reduction in the number of proximal chiasmata and was associated with an overall more distal redistribution. These features were related to distance of chiasmata to the centromere, suggesting that the centromere effect was more pronounced in Rb fusions than in acrocentric chromosomes. These modifications were interpreted in terms of structural meiotic constraints, although genic factors were likely involved in patterning the observed differences between sexes within races. Thus, the change in chromosomal structure in Rb mice was associated with a generalized decrease in recombination due to a reduction in diploid number, a lower CN, and a decrease in the efficiency of recombination. The effects of such modifications on patterns of genic diversity are discussed in the light of models of evolution of recombination.     

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.     

New Robertsonian populations of the house mouse (Mus musculus domesticus) from north of the Alps]

New Robertsonian (Rb) populations of the house mouse (Mus musculus domesticus) carrying different combinations of centric fusions are reported in France, Switzerland, and Germany. In Alsace (France), the diploid number varied from 2n = 38 to 34; four fusions were present, with Rb(4.12) homozygous in all populations whereas Rb(5.10), Rb(5.7), or Rb(10.14) were found to be segregating. In Switzerland, only all-acrocentric mice (2n = 40) were present in Bern while Rb(5.7) and Rb(9.16) occurred in Basel. In the Konstanz locality from southern Germany, all the mice were homozygous for nine Rb fusions: Rb(1.18), Rb(2.5), Rb(3.6), Rb(4.12), Rb(7.15), Rb(8.17), Rb(9.14), Rb(10.11), and Rb(13.16). The phylogenetic relationship of these new Rb populations with those already known is discussed.     

DEVELOPMENTAL INSTABILITY IN WILD CHROMOSOMAL HYBRIDS OF THE HOUSE MOUSE

In wild populations of the house mouse from Tunisia, fluctuating asymmetry and character size of tooth traits were compared between chromosomal races (2n = 40, all acrocentric standard karyotype, and 2n = 22, with nine fixed Robertsonian fusions) and their natural hybrids. Developmental stability was impaired in hybrids compared to both parental groups. Because genetic divergence measured by allozyme markers was low, genomic incompatibilities were not expected between the chromosomal races. This suggests that differentiation of gene systems specifically involved in development may have occurred between the chromosomal races. Support for the latter was found in the study of character size which showed that the 2n = 22 mice had smaller teeth than either the hybrid or the standard mice. The study of Tunisian chromosomal races thus shows that chromosomal evolution may lead to important changes in coadapted gene systems without involving extensive genic differentiation.     

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.     

The non-random occurrence of Robertsonian fusion in the house mouse

Chromosomal rearrangements such as Robertsonian (Rb) fusions constitute a major phenomenon in the evolution of genome organization in a wide range of organisms. Although proximate mechanisms for the formation of Rb fusion are now well identified, the evolutionary forces that drive chromosomal evolution remain poorly understood. In the house mouse, numerous chromosomal races occur in nature, each defined by a unique combination of Rb fusions. Among the 106 different Rb fusions that were reported from natural populations, the low involvement of chromosome 19 in Rb fusions is striking, prompting the question of the randomness of chromosomal involvement in Rb fusions. We uncover a significant quadratic relationship between chromosome size and probability of fusing, which has never previously been in this species. It appears that fusions involving chromosome 19 are not particularly infrequent, given the expected low fusion probability associated with the chromosome's size. The results are discussed, assessing selective processes or constraints that may operate on chromosome size.     

Mitochondrial DNA Variation and the Evolution of Robertsonian Chromosomal Races of House Mice, Mus Domesticus

The house mouse, Mus domesticus, includes many distinct Robertsonian (Rb) chromosomal races with diploid numbers from 2n = 22 to 2n = 38. Although these races are highly differentiated karyotypically, they are otherwise indistinguishable from standard karyotype (i.e., 2n = 40) mice, and consequently their evolutionary histories are not well understood. We have examined mitochondrial DNA (mtDNA) sequence variation from the control region and the ND3 gene region among 56 M. domesticus from Western Europe, including 15 Rb populations and 13 standard karyotype populations, and two individuals of the sister species, Mus musculus. mtDNA exhibited an average sequence divergence of 0.84% within M. domesticus and 3.4% between M. domesticus and M. musculus. The transition/transversion bias for the regions sequenced is 5.7:1, and the overall rate of sequence evolution is approximately 10% divergence per million years. The amount of mtDNA variation was as great among different Rb races as among different populations of standard karyotype mice, suggesting that different Rb races do not derive from a single recent maternal lineage. Phylogenetic analysis of the mtDNA sequences resulted in a parsimony tree which contained six major clades. Each of these clades contained both Rb and standard karyotype mice, consistent with the hypothesis that Rb races have arisen independently multiple times. Discordance between phylogeny and geography was attributable to ancestral polymorphism as a consequence of the recent colonization of Western Europe by mice. Two major mtDNA lineages were geographically localized and contained both Rb and standard karyotype mice. The age of these lineages suggests that mice have moved into Europe only within the last 10,000 years and that Rb populations in different geographic regions arose during this time.     

Limited Introgression between Rock-Wallabies with Extensive Chromosomal Rearrangements

Chromosome rearrangements can result in the rapid evolution of hybrid incompatibilities. Robertsonian fusions, particularly those with monobrachial homology, can drive reproductive isolation amongst recently diverged taxa. The recent radiation of rock-wallabies (genus Petrogale) is an important model to explore the role of Robertsonian fusions in speciation. Here, we pursue that goal using an extensive sampling of populations and genomes of Petrogale from north-eastern Australia. In contrast to previous assessments using mitochondrial DNA or nuclear microsatellite loci, genomic data are able to separate the most closely related species and to resolve their divergence histories. Both phylogenetic and population genetic analyses indicate introgression between two species that differ by a single Robertsonian fusion. Based on the available data, there is also evidence for introgression between two species which share complex chromosomal rearrangements. However, the remaining results show no consistent signature of introgression amongst species pairs and where evident, indicate generally low introgression overall. X-linked loci have elevated divergence compared with autosomal loci indicating a potential role for genic evolution to produce reproductive isolation in concert with chromosome change. Our results highlight the value of genome scale data in evaluating the role of Robertsonian fusions and structural variation in divergence, speciation, and patterns of molecular evolution.     

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.     

Morphological, genetic, and chromosomal variation at a small spatial scale within a mosaic hybrid zone of the grasshopper Dichroplus pratensis Bruner (Acrididae)

Hybrid zones are regions where genetically different populations meet and mate, resulting in offspring of mixed characteristics. In organisms with limited dispersal, such as melanopline grasshoppers, hybrid zones can occur at small spatial scales (i.e., <500 m). We assessed levels of morphological, chromosomal, and molecular variability in adult males of the grasshopper Dichroplus pratensis Bruner (N = 137 males, 188 females) collected at 12 sites within a mosaic hybrid zone in a heterogeneous environment in Sierra de la Ventana, Argentina. In this hybrid zone, 2 Robertsonian chromosomal races, polymorphic for different centric fusions, meet (the "Northern race" at low altitudes and the "Southern race" at higher altitudes), forming hybrids that show monobrachial homologies during meiosis. High morphometric variation in 6 traits was revealed among grasshoppers of both sexes, with male body size positively and significantly correlated with increasing altitude. Frequency of Robertsonian fusions characteristic of the Southern race increased significantly with altitude. Moreover, fusion frequencies covaried between samples. Considerable genetic variation was revealed by random amplification of polymorphic DNA markers, with heterozygosity ranging from 0.3477 to 0.3745. Insects from low-altitude and high-altitude populations showed significant genetic differentiation, as indicated by F(ST) values. The proposed model for D. pratensis, involving the generation and maintenance by chromosomal fusions, of gene complexes adaptive in different environments, could explain the observed clinal patterns within the contact zone.     

Chromosomal phylogeny of Robertsonian races of the house mouse on the island of Madeira: testing between alternative mutational processes

The ancestral karyotype of the house mouse (Mus musculus) consists of 40 acrocentric chromosomes, but numerous races exist within the domesticus subspecies characterized by different metacentric chromosomes formed by the joining at the centromere of two acrocentrics. An exemplary case is present on the island of Madeira where six highly divergent chromosomal races have accumulated different combinations of 20 metacentrics in 500-1000 years. Chromosomal cladistic phylogenies were performed to test the relative performance of Robertsonian (Rb) fusions, Rb fissions and whole-arm reciprocal translocations (WARTs) in resolving relationships between the chromosomal races. The different trees yielded roughly similar topologies, but varied in the number of steps and branch support. The analyses using Rb fusions/fissions as characters resulted in poorly supported trees requiring six to eight homoplasious events. Allowance for WARTs considerably increased nodal support and yielded the most parsimonious trees since homoplasy was reduced to a single event. The WART-based trees required five to nine WARTs and 12 to 16 Rb fusions. These analyses provide support for the role of WARTs in generating the extensive chromosomal diversification observed in house mice. The repeated occurrence of Rb fusions and WARTs highlights the contribution of centromere-related rearrangements to accelerated rates of chromosomal change in the house mouse.     

Chromosomal rearrangements,phenotypic variation and modularity: a case study from a contact zone between house mouse Robertsonian races in Central Italy

The Western European house mouse, Mus musculus domesticus, is well‐known for the high frequency of Robertsonian fusions that have rapidly produced more than 50 karyotipic races, making it an ideal model for studying the mechanisms of chromosomal speciation. The mouse mandible is one of the traits studied most intensively to investigate the effect of Robertsonian fusions on phenotypic variation within and between populations. This complex bone structure has also been widely used to study the level of integration between different morphogenetic units. Here, with the aim of testing the effect of different karyotypic assets on the morphology of the mouse mandible and on its level of modularity, we performed morphometric analyses of mice from a contact area between two highly metacentric races in Central Italy. We found no difference in size, while the mandible shape was found to be different between the two Robertsonian races, even after accounting for the genetic relationships among individuals and geographic proximity. Our results support the existence of two modules that indicate a certain degree of evolutionary independence, but no difference in the strength of modularity between chromosomal races. Moreover, the ascending ramus showed more pronounced interpopulation/race phenotypic differences than the alveolar region, an effect that could be associated to their different polygenic architecture. This study suggests that chromosomal rearrangements play a role in the house mouse phenotypic divergence, and that the two modules of the mouse mandible are differentially affected by environmental factors and genetic makeup.     

Developmental stability in house mice heterozygous for single Robertsonian fusions

Fluctuating asymmetry (FA) of tooth traits has been reported to be increased in Down syndrome patients as well as hybrids between chromosomal races of the house mouse differing in several Robertsonian (Rb) fusions. Developmental stability, assessed by FA, is thus thought to be impaired by spontaneous chromosomal abnormality or by chromosomal heterozygosity. Although the effect of a single fusion on developmental stability could theoretically be expected, it has never been documented. Crosses involving two chromosomal races of the house mouse diverging for one Rb fusion were performed to assess developmental stability in parental homozygous races as well as in their hybrids. Moreover, the occurrence of a spontaneous chromosomal mutation (WART type-b) allowed us to study the instantaneous effect of such a translocation on developmental stability. No difference in fluctuating asymmetry levels was detected among the groups considered in this study. This result suggested that a single stable or spontaneous balanced structural rearrangement did not inherently disturb developmental stability. In addition, the differential effect on developmental stability of one versus many heterozygous Rb fusions highlights the role of their quantitative accumulation in the disruption of coadaptation in chromosomal hybrids.     

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.     

Geometric morphometrics on Greek house mouse populations (Mus musculus domesticus) with Robertsonian and all-acrocentric chromosomal arrangements

This work aims to give the first comprehensive morphometric analysis of intraspecific variation for the different populations of the western house mouse (Mus musculus domesticus), in the Robertsonian (Rb) system of the N-NW Peloponnisos. Furthermore, we study all-acrocentric karyotype populations (2n = 40) of the species coming from several localities of Greece. We apply 2D shape analysis, i.e. landmark analysis and Elliptic Fourier Analysis, on the dorsal and ventral side of skull and the occlusal view of the first upper molar (M¹), respectively. Although significant genetic divergence between typical and Rb populations and even ongoing speciation processes have been reported for this species, this was not the case for the Greek populations studied. However, our analyses herein reveal morphologically differentiated chromosome groups in N-NW Peloponnisos Rb system and a clear geographical discrimination of the all-acrocentric (2n = 40) populations for all characters studied. We suggest that in all-acrocentric (2n = 40) karyotype mice the geographical distance drives their differentiation while within the Rb system of N-NW Peloponnisos, karyotype is the key factor that acts on their phenotypic variation.     

Fixation of metacentric chromosomes in populations of the common shrew Sorex araneus L. from populations of Eastern Europe

In this review, we discuss the processes of fixation of Robertsonian chromosome fusions in populations of the common shrew Sorex araneus L. Various Robertsonian fusions, accumulating in populations, create an illusion of large chromosomal rearrangements, reciprocal translocations of complete chromosome arms. The use of these rearrangements for phylogenetic reconstructions results in false conclusions. Robertsonian fusions accumulate in populations at such stages of the species evolution, when large open or subdivided populations prevail (populations of warm periods of Pleistocene and many present-day populations) and are fixed in small isolated populations and glacial refugia. The formation of monomorphic chromosome races requires a long time, several glaciation epochs during the whole Pleistocene.