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.     

Fertility assessment in hybrids between monobrachially homologous Rb races of the house mouse from the island of Madeira: implications for modes of chromosomal evolution

The speciation model of divergence by monobrachially homologous fusions (that is, with one arm in common) benefits from a wide conceptual acceptance, because heterozygotes between populations carrying such fusions suffer from high levels of meiotic dysfunction. The same meiotic configurations can also be generated by WART (whole-arm reciprocal translocation), rearrangements that are known to occur in mammals. Estimating the disadvantage of heterozygotes carrying monobrachially homologous fusions is required to evaluate the relevance of this mode of chromosomal evolution in diversification and speciation. House mice are an excellent study models because chromosomal races exist carrying monobrachially homologous fusions, and WARTs have been documented in this species. The fertility of heterozygote mice carrying the smallest number of monobrachially homologous fusions (that is, a chain of four chromosomes, C4) was investigated in laboratory-bred hybrids between two parapatric chromosomal races from the island of Madeira. Meiotic nondisjunction analyses and histological sections of testes showed that aneuploidy (16.7%) and germ cell death (50.9%) rates reached significantly higher mean values in hybrids than in homozygotes. In females, however, the histological analysis of ovarian follicle parameters revealed no significant differences between hybrid and homozygous individuals. Overall, the reproductive assays indicated that these C4-carrying hybrids were not sterile but showed an approximately 50% decrease in fertility compared to homozygous parental mice. Implications for modes of chromosomal evolution involving monobrachially homologous fusions are discussed.     

Influence of physical environmental characteristics and anthropogenic factors on the position and structure of a contact zone between two chromosomal races of the house mouse on the island of Madeira (North Atlantic, Portugal)

Aim This study aimed to investigate if and how environmental characteristics (physical factors of the natural environment and the human impact on the landscape) influence the position and structure of a contact zone between two chromosomal races of the house mouse (Mus musculus domesticus Rutty 1712) from the island of Madeira. Location The western part of Madeira, a volcanic island in the North Atlantic. Methods Mice were sampled along a south/north‐western transect following the main road, in human‐modified outdoor habitats. Karyotypes of mice were determined using the yeast‐stimulated bone marrow cell method. Trapping sites were characterized in terms of their physical (altitude, temperature, precipitation and soil type) and habitat (human landscape use and occupancy) features. Demographic parameters of mouse populations, based on trapping‐with‐removal techniques, were also analysed (relative abundance, sex‐ratio, juvenile ratio and female fertility ratio), as well as body size (weight and length). Results Four chromosomal zones were identified on the basis of the frequency of two diagnostic rearrangements (Rb(6.7) in race E. Calheta and Rb(7.15) in race A. Cruz). E. Calheta was present in the two southern‐most zones, followed by the contact zone characterized by the presence of two inter‐racial hybrids and the co‐occurrence of mice belonging to the two races. The northern‐most part of the transect was occupied by A. Cruz. Environmental features differed leading us to split the transect into two parts. The southern part is characterized by lower altitude and precipitation, milder temperature, better soil quality supporting vegetable crops and vineyards, and more abundant and evenly distributed human habitats. This southern part is occupied by E. Calheta mice. The north‐western part presents characteristics opposite to those described above with cereals as the main cultivated crop, and it includes the contact zone as well as the zone inhabited by A. Cruz mice. The demographic parameters evaluated in this study did not differ significantly between chromosomal zones. Main conclusions This ecological survey highlights differences in climatic and edaphic features that have moulded the agricultural activities of humans, contributing to a differentiation of their spatial development, and hence the structure of potential habitats for mice. Results are interpreted within the source–sink framework of population dynamics, following which E. Calheta may function as a source and the areas where A. Cruz and the contact zone are located may function as a sink. Our study suggests that the position and chromosomal composition of the contact zone is influenced by the human component underlying broader environmental features. Similar characteristics were most likely present during the historical settlement of Madeira. They may have favoured the independent divergence of the two races and influenced the dynamics of the contact zone.     

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.     

Effect of Robertsonian translocations on sperm head form in the house mouse

Sperm morphology reflects a long process of adaptation to external conditions and the barriers encountered before ova fertilization can take place; however, not all morphological variation found in gametes can be explained by the effects of these selective forces, as the genetic component may also contribute to the establishment of different gametic features. In north‐eastern Spain, there is a wide Robertsonian system of Mus musculus domesticus, where individuals with 2n ranging from 27 to 40 chromosomes have been described. To elucidate the effect of the karyotype on sperm head form, a comparative analysis between different chromosomal groups of mice from this zone was carried out. Sperm heads from eight St (2n = 40) and 24 Rb (2n = 30–39) males were processed for scanning electron microscopy and analysed using geometric morphometric techniques. Canonical variate analyses showed substantial shape differences between St and Rb mice in the ventral spur region and between Rb groups in the post‐acrosomal region. Significant differences in sperm head size were also detected between chromosomal groups. Structural disorders related to spermatogenesis, genetic alterations, and epistatic interactions among loci are probably involved in the relationship between the phenotypic variation of the sperm head and Rb translocations. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 878–889.     

Robertsonian translocations in free-living populations of the house mouse in Belgium

Mice with Robertsonian (Rb) translocations have been discovered in some 45 populations in Europe. In Belgium, we have observed either Rb(4.12) in a heterozygous or homozygous state (2 n = 39 and 38 respectively), or Rb(4.12) homozygous with a heterozygous Rb(5.10), or both fusions in a homozygous state (2 n = 37 and 36, respectively). Some locations are highly polymorphic and hétérozygotes are found frequently. These observations suggest that the Rb population in Belgium could be of recent origin and probably is the result of introgression from Alsace (France) or Germany.     

The Robertsonian phenomenon in the house mouse: mutation,meiosis and speciation

Many different chromosomal races with reduced chromosome number due to the presence of Robertsonian fusion metacentrics have been described in western Europe and northern Africa, within the distribution area of the western house mouse Mus musculus domesticus. This subspecies of house mouse has become the ideal model for studies to elucidate the processes of chromosome mutation and fixation that lead to the formation of chromosomal races and for studies on the impact of chromosome heterozygosities on reproductive isolation and speciation. In this review, we briefly describe the history of the discovery of the first and subsequent metacentric races in house mice; then, we focus on the molecular composition of the centromeric regions involved in chromosome fusion to examine the molecular characteristics that may explain the great variability of the karyotype that house mice show. The influence that metacentrics exert on the nuclear architecture of the male meiocytes and the consequences on meiotic progression are described to illustrate the impact that chromosomal heterozygosities exert on fertility of house mice—of relevance to reproductive isolation and speciation. The evolutionary significance of the Robertsonian phenomenon in the house mouse is discussed in the final section of this review.     

Origin and radiation of the house mouse: mitochondrial DNA phylogeny

On the basis of patterns of allele frequency variation in nuclear genes (Din et al., in press) it has been proposed that the house mouse M. musculus originated in the northern Indian subcontinent, from where it radiated in several directions to form the well-described peripheral subspecies (M. m. domesticus, M. m. musculus and M. m. castaneus). Here we use a mitochondrial DNA (mtDNA) phylogeny to test this hypothesis and to analyse the historical and demographic events that have accompanied this differentiation. This marker also provides a powerful means to check for genetic continuity between the central and peripheral populations. We studied restriction site polymorphism of samples from India and the Middle East as well as samples from the rest of Eurasia and northern Africa. M. m. domesticus and M. m. musculus are both monophyletic for mtDNA and belong to the subspecies-specific mtDNA lineages that have been described previously. Average nucleotide diversity is low in M. m. musculus (0.2–5%). It is not only higher in M. m. domesticus (0.7–0.9%) but the distribution of pairwise divergence is wider, and the rate of evolution in this branch appears to be higher than in M. m. musculus. The nucleotide diversity found in M. m. castaneus (0.4%) is due to the existence of two rather divergent linages with little intralineage variation. These two lineages are part of a diversified bush of the phylogenetic tree that also comprises several previously undescribed branches and includes all samples from the northern Indian subcontinent and Iran. The degree of diversity found in each of the samples from this region is high (1.2–2.4%) although they come from small geographic areas. This agrees well with the idea that the origin of the radiation was in the northern Indian subcontinent. However, as neither haplotypes on the M. m. domesticus nor on the M. m. musculus branches were found in this region, there appear to be important phylogeographic discontinuities between this central region and these peripherial subspecies. On the basis of the present result and the nuclear data (Din et al., in press), we propose that M. musculus originated in the north of the indian subcontinent. Our calibration of the evolutionary rate of mtDNA in mice suggests that the mouse settlement in this region could be as old as 900 000 years. Possibly from there, a first radiation could have reach the Middle East and the Caspian Sea, where the M. m. domesticus and M. m. musculus lineages, respectively, would have started to differentiate a few hundred thousand years ago, and from where they could have colonised the peripheral part of their ranges only recently.M. m. castaneus appears from its mtDNA to be recent offshoot of the northern Indian population. This multiple and gradual radiation ultimately led to recent peripheral secondary contacts, such as the well-known European hybrid zone.     

Variation in fluctuating asymmetry levels across a Robertsonian polymorphic zone of the house mouse

Fluctuating asymmetry (FA) levels were assessed within the Barcelona Robertsonian polymorphic zone of Mus musculus domesticus as a measure of developmental instability (DI). This zone is characterized by populations with a reduced diploid number (2n = 27–39) surrounded by others with standard karyotype (2n = 40). Mice were distributed into four classes according to mean diploid number at each collecting site, and differences in their FA levels were studied. Three different FA indices were calculated on interlandmark distances from two major morphogenetic units of the mandible: the teeth and muscular areas. Three kinds of analysis were performed: trait‐by‐trait, considering the two regions separately and pooling all traits. In univariate analyses few differences were detected between classes. Nevertheless, when characters were pooled, differences in FA values were found between one Robertsonian group and standard mice, and between several Robertsonian classes. Teeth area traits showed higher FA levels in Rb I mice, i.e. those geographically close to the standard populations. However, FA levels decreased in more introgressed Robertsonian groups and showed significant differences with Rb I. Muscular area traits showed lower DI levels in the Robertsonian mice, especially those with an intermediate mean diploid number (Rb II). When all traits were pooled, FA levels increased in Rb I, whilst the other Robertsonian groups showed similar, or even lower, FA values to those of the standard mice. The higher FA levels found in Rb I suggest an increase in the disruption of genetic coadaptation. Furthermore, in Rb I the presence of metacentrics is more recent than in more introgressed populations; we therefore suggest that Rb I have had less time to restore genetic coadaptation. In addition, the teeth region of the mandible seems to be more sensitive to genomic stress than the muscular region. In the light of these results we suggest that hybrids should be separated into groups and that mandible morphogenetic units be differentiated when using FA in hybrid zones to analyse DI.     

Rb(7.17), a rare Robertsonian fusion in wild populations of the house mouse

Robertsonian (Rb) translocation is the largest source of chromosomal diversity in the western European house mouse (Mus musculus domesticus). Recently, the fusion Rb(7.17) was found in the chromosomal polymorphic zone of this subspecies in the north-east of the Iberian Peninsula. This fusion has not been reported in any other European population. Here we give data on the distribution and frequency of this mutation in this region. Results revealed that Rb(7.17) is restricted to a small geographic area, and that, in comparison with other fusions in this polymorphic zone, it occurs at low frequencies. We suggest some possible explanations for the distribution of this translocation.     

Behavioural discrimination among chromosomal races of the house mouse (Mus musculus domesticus)

This work is concerned with the extent of behavioural discrimination between three chromosomal races of the house mouse (the standard 40-chromosome race and a 32- and 36-chromosome races) found in the vicinity of a hybrid zone in northern Scotland. Mice were investigated for several elements of their social behaviour. Within-population dyadic encounters did not show consistent behavioural differences attributable to karyotype among five populations (two standard race, two 36-chromosome race, one 32-chromosome race). Between-population dyadic encounters revealed significant differences between three populations. The standard population examined appeared to be the most “open” to foreigners, the 32-chromosome population the most “closed” while the 36-chromosome mice displayed an intermediate response. Differences in behaviour displayed during between-population as compared to within-population dyadic encounters revealed the occurrence of behavioural discrimination between populations. The implication of these results on the dynamics of the hybrid zone are discussed.     

Habitat use and potential interactions between the house mouse and lesser white-toothed shrew on an island undergoing habitat restoration

To forecast the potential impact of plant community and dry-stone wall restoration on an insular population of the lesser white-toothed shrewCrocidura suaveolens Pallas, 1811, shrew and house mouseMus musculus Linnaeus, 1758 abundances were assessed in 3 anthropogenic habitats on Béniguet Island, Brittany, France, by a standardised annual trapping system checked yearly for 9 years and in 6 “natural” habitats by trap-lines. The standardised trapping system showed that abundances of both species fluctuated synchronously for nine years, suggesting that interactions between the species had little impact if any on abundances. Mice were trapped in all habitats, but shrews only in “stone” habitats except for rare occurrences in one damp depression. Ruderal habitat was rarely used by either species. Data suggest that on Béniguet Island: (1)M. musculus is associated with anthropogenic habitats but is not as strictly tied to them as at nearby continental sites; (2)C. suaveolens is synanthropic, as has been reported in continental northern France; (3) shingle beaches and seashore food resources are particularly important forC. suaveolens; (4) the relationship betweenC. suaveolens andM. musculus could not be determined by the experiments and, if it exists at all, appears to be more competitive than predatory. Grassland restoration is unlikely to affect shrew populations. Dry-stone wall restoration may temporarily affect shrews but should ultimately benefit them.     

Chromosomal dynamics of nucleolar organizer regions (NORs) in the house mouse: micro-evolutionary insights

Variation in the number and chromosomal location of nucleolar organizer regions (NORs) was studied in the house mouse, Mus musculus (2n=40). From an origin in Western Asia, this species colonized the Middle East, Europe and Asia. This expansion was accompanied by diversification into five subspecies. NOR diversity was revealed by fluorescence in situ hybridization using 18S and 28S probes on specimens spanning Asia to Western Europe. The results showed that the house mouse genome possessed a large number of NOR-bearing autosomes and a surprisingly high rate of polymorphism for the presence/absence of rRNA genes on all these chromosomes. All NOR sites were adjacent to the centromere except for two that were telomeric. Subspecific differentiation established from the NOR frequency data was concordant with the overall pattern of radiation proposed from molecular studies, but highlighted several discrepancies that need to be further addressed. NOR diversity in M. musculus consisted of a large number of polymorphic NORs that were common to at least two subspecies, and a smaller number of NORs that were unique to one subspecies. The most parsimonious scenario argues in favor of a subspecific differentiation by lineage sorting of ancestral NOR polymorphisms; only the unique NORs would have appeared by inter-chromosomal transposition, except for the two telomeric ones that may have originated by hybridization with another species. Such a scenario provides an alternative view from the one prevailing in most systematic and phylogenetic analyses that NORs have a high transposition rate due to concerted evolution of rRNA genes.     

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.     

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.     

Chromosomal polymorphism in the house mouse (Mus domesticus) of Greece and Yugoslavia

A total of 88 wild mice from the Dalmatian coast of Yugoslavia (35 animals), and Peloponnesus (30 animals) and Thebes (23 animals) on mainland Greece were karyotyped. In all but five animals Robertsonian translocations were found. Mice from the Dalmatian region were homozygous for translocations Rb(5.15), Rb(6.12), Rb(8.17), Rb(9.13), and Rb(10.14); they were homo-or heterozygous for the translocation Rb(1.11). Some of them lacked the Rb(1.11) translocation altogether so that the diploid numbers in the Yugoslavian mice were 2n=28, 29, 30, or 40. The mice from the vicinity of Olympia in northwestern Peloponnesus were homozygous for eight Robertsonian translocations: Rb(1.3), Rb(2.5), Rb(4.6), Rb(8.12), Rb(9.16), Rb(10.14), Rb(11.17), and Rb(13.15). Their diploid chromosome number was therefore 2n=24. Mice from the vicinity of Patras in northwest Peloponnesus carried all except the first three of these eight translocations; their chromosome number was 2n=30. Finally, the mice from Thebes were homozygous for translocations Rb(2.15), Rb(4.14), Rb(5.12), and Rb(10.13). They were homo- or heterozygous for Rb(6.9), Rb(8.17), and Rb(1.11); some mice lacked the Tb(1.11) translocation altogether. The translocations Rb(6.9)40Tu and Rb(10.13)42Tu represent new arm combinations not found previously in any wild mouse population. the remaining translocations have previously been found in different Mediterranean countries, in Scotland and in southern Germany. The findings suggest that each translocation arose only once and that different translocations have come together in different populations to generate a unique karyotype characterizing this population.