Interspecific introgressive origin of genomic diversity in the house mouse

We report on a genome-wide scan for introgression between the house mouse (Mus musculus domesticus) and the Algerian mouse (Mus spretus), using samples from the ranges of sympatry and allopatry in Africa and Europe. Our analysis reveals wide variability in introgression signatures along the genomes, as well as across the samples. We find that fewer than half of the autosomes in each genome harbor all detectable introgression, whereas the X chromosome has none. Further, European mice carry more M. spretus alleles than the sympatric African ones. Using the length distribution and sharing patterns of introgressed genomic tracts across the samples, we infer, first, that at least three distinct hybridization events involving M. spretus have occurred, one of which is ancient, and the other two are recent (one presumably due to warfarin rodenticide selection). Second, several of the inferred introgressed tracts contain genes that are likely to confer adaptive advantage. Third, introgressed tracts might contain driver genes that determine the evolutionary fate of those tracts. Further, functional analysis revealed introgressed genes that are essential to fitness, including the Vkorc1 gene, which is implicated in rodenticide resistance, and olfactory receptor genes. Our findings highlight the extent and role of introgression in nature and call for careful analysis and interpretation of house mouse data in evolutionary and genetic studies.

The genetic architecture of chemosensory cues involved in species recognition: a behavioral approach in the house mouse

The genetics of chemical signals is poorly understood. We addressed this issue in two subspecies of mice, Mus musculus musculus and M. m. domesticus, comparing their odor phenotypes with that of their hybrids. Earlier studies indicated that these subspecies could be discriminated on the basis of their urinary odor. We assessed male odor phenotypes from perception of musculus mice acting as olfactometers. Our results point to a complex genetic determinism. Reciprocal F1 hybrids produced a distinct odor phenotype, with shared characteristics distinguishing them from their parents, and stronger similarity to domesticus than to musculus. These results are consistent with implications of genes with partial dominance and a parent of origin effect. Further, similarities between reciprocal F2 allowed us to reject a direct role of the Y-chromosome in shaping the odor phenotype. However we show that the X-chromosome could be involved in explaining domesticus phenotype, while epistasis between genes on the sex chromosomes and the autosomes might influence musculus phenotype.

Why is the house mouse karyotype so variable?

rates of robertsonian chromosomal evolution in the Western European house mouse are about two orders of magnitude greater than for most other mammals. This has resulted in a remarkable diversity of karyotypic races in a very short period of time. Recent studies are beginning to shed light on the relative contributions of mutation, drift, selection and meiotic drive in producing this pattern.

Genetic variation and phylogeography of central Asian and other house mice, including a major new mitochondrial lineage in Yemen

The mitochondrial DNA (mtDNA) control region and flanking tRNAs were sequenced from 76 mice collected at 60 localities extending from Egypt through Turkey, Yemen, Iran, Afghanistan, Pakistan, and Nepal to eastern Asia. Segments of the Y chromosome and of a processed p53 pseudogene (Psip53) were amplified from many of these mice and from others collected elsewhere in Eurasia and North Africa. The 251 mtDNA types, including 54 new ones reported here, now identified from commensal house mice (Mus musculus group) by sequencing this segment can be organized into four major lineages-domesticus, musculus, castaneus, and a new lineage found in Yemen. Evolutionary tree analysis suggested the domesticus mtDNAs as the sister group to the other three commensal mtDNA lineages and the Yemeni mtDNAs as the next oldest lineage. Using this tree and the phylogeographic approach, we derived a new model for the origin and radiation of commensal house mice whose main features are an origin in west-central Asia (within the present-day range of M. domesticus) and the sequential spreading of mice first to the southern Arabian Peninsula, thence eastward and northward into south-central Asia, and later from south-central Asia to north-central Asia (and thence into most of northern Eurasia) and to southeastern Asia. Y chromosomes with and without an 18-bp deletion in the Zfy-2 gene were detected among mice from Iran and Afghanistan, while only undeleted Ys were found in Turkey, Yemen, Pakistan, and Nepal. Polymorphism for the presence of a Psip53 was observed in Georgia, Iran, Turkmenistan, Afghanistan, and Pakistan. Sequencing of a 128-bp Psip53 segment from 79 commensal mice revealed 12 variable sites and implicated >/=14 alleles. The allele that appeared to be phylogenetically ancestral was widespread, and the greatest diversity was observed in Turkey, Afghanistan, Pakistan, and Nepal. Two mice provided evidence for a second Psip53 locus in some commensal populations.

Genomic incompatibilities in the hybrid zone between house mice in Denmark: evidence from steep and non-coincident chromosomal clines for Robertsonian fusions

The pattern of chromosomal variation is investigated in house mice from the Danish hybrid zone between the translocation-prone Mus musculus domesticus and the chromosomally conservative M. m. musculus. The cytogenetic analysis confirmed the non-introgression of three pairs of Robertsonian (Rb) fusions from M. m. domesticus into the M. m. musculus genome. The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal clines which increased in steepness the closer they were to the centre of the hybrid zone as defined by allozymes. Analysis of alternate hypotheses suggests that chromosomal differentiation of the Danish domesticus occurred after contact was established with musculus. The staggering of the clines would reflect the order of arrival of the Rb fusions into the hybrid zone. Several models with different processes of underdominance of the chromosomal heterozygotes are discussed to account for the difference in width between clines. A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome provides the best fit for the observed pattern. Selection against Rb fusions with little effect on the recombination of linked allozyme markers supports the view that no reduction in gene flow due to chromosomal heterozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musculus genome.

Fluctuating asymmetry in the Mus musculus hybrid zone: a heterotic effect in disrupted co-adapted genomes

Developmental stability reflects the organism's ability to buffer minor developmental accidents and is often estimated by measuring the fluctuating asymmetry. Either implicitly or explicitly, numerous authors have assumed that developmental stability is correlated with overall fitness. If this is the case, changes in morphological asymmetry across a hybrid zone could be used as a measure of the selection on hybrid genomes. Developmental stability in hybrid populations is theoretically related to the genetic distance between hybridizing taxa, and results from a balance between the stabilizing effect due to increased heterozygosity and the disruptive effect caused by breakdown of genomic co-adaptation. Here we have compared the amount of fluctuating asymmetry across a transect of the hybrid zone between the two European subspecies of the house mouse (Mus musculus domesticus and M. m. musculus) in Denmark. For the first time in any natural hybrid zone we found an increased developmental stability in the populations with mixed genomes. Moreover, the apparently beneficial effect of hybridization on the developmental stability of the hybrid mice contrasts with the results of both genetic and parasitological studies which show that hybrid dysgenesis occurs in this zone. Our results suggest that the barrier to gene flow in the Mus musculus hybrid zone may result from the disruption of relatively few gene systems. They also lead us to reassess the relation between developmental stability expressed as fluctuating asymmetry, co-adaptation and overall fitness.