The complex species Mus musculus

The polytypic species Mus musculus finds its origins in Asia, like many murid rodents. It can be resolved into several subspecies as described below:

Peripheral species

Mus musculus domesticus (Rutti, 1772) Mice of this species are found in western Europe, around the Mediterranean basin, in Africa, in Arabia and in the Middle East. This subspecies includes geographical races sometimes improperly designated with other Latin trinomens, such as Mus m. brevirostris, Mus m. poschiavinus, Mus m. praetextus etc., but these races are biochemically and genetically identical with Mus m. domesticus.

Although most of the animals belonging to Mus m. domesticus exhibit a karyotype with 40 arocentric chromosomes, several subpopulations have been reported with varying numbers of Robertsonian translocations in different combinations.

Mus musculus musculus (Linnaeus, 1758) Individuals of this species occupy the region from eastern Europe to Japan across Russia and northern China. Like the former subspecies, individuals of this group are either commensal or wild whereas other groups are almost exclusively wild.

Mus musculus castaneus (Waterhouse, 1843) This species is found from Sri Lanka to South-East Asia including the Indo-Malayan archipelago.

The central populations

Mus musculus bactrianus (Blyth, 1846) is found from Iran to Pakistan. This trinomen has long been attributed to populations ranging from Iran to Pakistan. According to recent work by Din and colleagues, this is a rather heterogeneous ensemble, and the bactrianus species sensu stricto are probably limited to the Afghan valleys. An accurate taxonomical description of these central populations is still needed.

Origins of the species

None of the above-mentioned subspecies are completely genetically isolated from the other three and, in those locations where they meet, there is evidence of exchanges ranging from limited introgression to complete blending. Studying the genctic relationships between the above-mentioned subspecies, Din and colleagues have proposed that the northern part of the Indian subcontinent was the cradle of the species Mus musculus. According to the pattern of genetic-divergence observed, they proposed that the extant peripheral subspecies resulted from a radiation proceeding from the central populations in several directions outwards (Figure 1). This would have taken place between 0.5 and 1 million years ago, according to the analysis of mitochondrial DNA by Boursot and coworkers, and would account for most of the genetic variability presently available in the species.

These taxa would have secondarily and much more recently colonized the rest of the world because

Figure 1 Map showing the proposed radiation of subspecies from a central population of Mus musculus.

of their capacity to interact with humans (commensalism). These exchanges justify the use of subspecific names (Latin trinomens) for their designation. The best documented cases of such gene exchanges are those occurring between Mus m. mus-culus and Mus m. domesticus in Europe, and between Mus m. musculus and Mus m. castaneus in Japan. In this archipelago, the two subspecies have hybridized extensively, giving rise to a unique population often referred to as Mus m. molossinus.

For a more exhaustive review of the genetical and ecological interactions between the various species, see Boursot et al (1993) and Sage et al (1993).

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