The house mouse and its relatives: systematics and taxonomy

A quarter of a century before the advent of biochemical tools for taxonomic approaches, Schwarz and Schwarz (1943) published a pioneering survey of the systematics of the house mouse. All taxa were then considered as subspecies of the house mouse (Mus musculus). In other words, there was only one species within the subgenus Mus. It was a relatively large-scale study, but the taxonomic criteria were obviously based solely on external morphology and distribution of species. It is nowadays well established that the subgenusMus exhibits high interspecific morphological conservatism and that all species share an important part of their intra-specific variation. Most, if not all, classical external criteria such as coat colour, length of head, body, tail, ear, or foot, are poorly discriminating between species, and, except for very few (e.g. the zygomatic index forM. musculus; Orsini et al., 1983), none has a general and specific diagnostic value. When only one or two species occur sympatrically, some characters may be diagnostic, such as the length of the tail, which is useful to distinguishMus musculus domesticus, one of the longest-tailed mice, from all sympatric species. Thus, there is only a poor correspondence between the 15 subspecies of Mus musculus described by Schwarz and Schwarz (1943) and the species that are now diagnosed by taxonomists. Biochemical and molecular tools have considerably clarified our knowledge of the taxonomy within the subgenusMus. The revision began with the deciphering of the Mus musculus complex of species in Europe (Selander et al., 1969), though the two subspecies M. m. musculus and M. m. domesticus had already been recognized by their respective tail lengths. This was followed by the establishment of Mus spretus as a valid species by Britton et al. (1976) and the recognition of the other European species, M. macedonicus and M. spicilegus. By the late 1990s, 9 of the 14 taxa known today had been clearly identified, with criteria that left no doubt as to their species status: musculus, spretus, spicilegus, macedonicus, cervicolor, caroli, cookii,