The position of rodents among mammalian species

Mice are rodents. They belong to the most abundant (around 40%) and diversified order of living placental mammals, with slightly over 2000 species grouped in 28 families (Huchon et al., 2002). Because of their great diversity, the phylogenetic relationships between the different species of this order has been a matter of controversy for many years, especially when morphological markers were the only criteria available for the establishment of phylogeny. Nowadays, with the use of various molecular (mostly DNA) markers and possible references to the complete genomic sequence of numerous orthologous genes, the situation is much clarified and Figure 1.1 represents the most likely phylogenetic tree for a sample of 40 different eutherian mammals. Based on comparisons at the level of nuclear DNA sequences, the divergence between man and murid rodents (Mus or Rattus genus) has been set somewhere between 65 and 75 Myrs ago (Waterston et al., 2002).

Mice among rodents

The rodent family of Muridae encompasses at least 1326 species grouped in 281 genera (Musser and Carleton, 1993). The establishment of the evolutionary systematics in this group has also been disputed but, this time, it was because many mammals in this family are very similar in size and shape. Here again studies making use of DNA sequences of various types (Michaux et al., 2001; Lundrigan et al., 2002) have greatly contributed to clarify the situation and Figure 1.2 represents the evolutionary relationships among a sample of 21 rodent species anchored into the broader phylogeny of eutherian mammals. The divergence between the Mus and Rattus genus has been estimated at around 10–15 Myrs ago (Jaeger et al., 1986; Murphy et al., 2001), while the divergence of these two genera with Peromyscus maniculatus, the deer mouse (subfamily Sigmodontinae), occurred at around 25 Myrs ago. This is to be remembered because deer mice, which are abundantly used as laboratory models, are often considered close relatives of the laboratory mice while, in fact, they are no more related to them than hamsters.

Systematics in the genus Mus

Figure 1.3 (Guénet and Bonhomme, 2003 and references therein) summarizes the phylogenetic relationships within the genus Mus (subfamily Murinae). The individualization of the subgenus Mus sensu stricto occurred around 5 Myrs ago with the split of three other different subgenera, the African Nannomys and the Asian Coelomys and Pyromys.

The subgenus Mus comprises several species that are extremely similar in size and shape but never hybridize in the wild. Among the Asian species are Mus cervicolor, Mus cookii, and Mus caroli as well as the group of Indian pigmy mice related to Mus dunni. Mus famulus from India should also be cited as well as the recently discovered species Mus fragilicauda (Auffray et al., 2003) from Thailand.

Mus spicilegus and Mus macedonicus are short tailed mice that are found in central Europe and the eastern Mediterranean, respectively, while mice belonging to the species Mus spretus are common in the western Mediterranean regions (south east France, Spain, Portugal and North Africa).

Mice of the Mus musculus complex are closely related. They have their evolutionary origins in the Indian subcontinent (Bonhomme et al., 1994) but are now spread over the five continents. The best known representatives of the complex are the three Mus musculus subspecies: Mus m. domesticus, common in western Europe, Africa, the near-East, and transported by man to the Americas and Australia; Mus m. musculus, whose habitat spans from eastern Europe to Japan, across Russia, and northern China, and Mus m. castaneus, which is found from Sri Lanka to south east Asia including the Indo-Malayan archipelago. Various molecular criteria discriminate easily between these different species (Figure 1.4; Boursot et al., 1993; Moriwaki et al., 1994)