MusDivision. Extant species of Mus are contained in subgenera Coelomys, Mus, Nannomys, and Pyromys, each diagnosed by a suite of discrete morphological traits (see J. T. Marshall, Jr., 1977b, 1986, for diagnoses of Coelomys, Mus, and Pyromys; morphological characters distinguishing each subgenus are listed by Chevret et al., 2003), morphometric features (Macholán, 2001), and biochemical characteristics (Bonhomme, 1986; She et al., 1990). Nannomys, Pyromys, and Coelomys alternatively have been treated as genera (Bonhomme, 1986; She et al., 1990). The DNA-DNA hybridization and morphological study by Catzeflis and Denys (1992) using selected species of subgenera Nannomys, Pyromys, Coelomys, and Mus indicated the two African species sampled are more closely allied to Mus than to other African and European non Mus genera sampled, and at least as close to subgenus Mus as the subgenera Coelomys and Pyromys: Nannomys "appears to be an offshoot of the Asian mice radiation, and this split could have occurred without important morphological changes at the beginning of the Pliocene" (p. 228). Their study reinforced the inclusion of the suite of endemic African species in Mus. Results of phylogenetic analysis using mitochondrial 12S rDNA gene sequences, which focused on the relationships of Sumatran Mus crociduroides, also demonstrated the monophyly of Mus compared with other murine genera (Sourrouille et al., 1995), as did albumin immunology (Watts and Baverstock, 1995b), and analyses of sequences from six genes "representing paternally, maternally, and biparentally inherited regions of the genome" (Lundrigan et al., 2002:410). Recent analyses of combined morphological traits, DNA/DNA hybridization, and mitochondrial 12S rRNA sequences indicate subgenera Mus, Nannomys, Pyromys, and Coelomys represent four distinctive monophyletic groups (Chevret et al., 2003; also see review by Guénet and Bonhomme, 2003). Results of Lundrigan et al.’s (2002) analyses also recognized the same four clades, but at the same time indicated all four to be in a larger monophyletic cluster relative to Hylomyscus, Mastomys, and Rattus, which were used to root the phylogenies. Lindrigan et al. (2002:424) noted that a denser sampling of Mus species and murine genera are needed to rigorously test the monophyly of Mus, ". . . But for now there is no phylogenetic justification for excluding Pyromys, Coelomys, or Nannomys from the genus Mus." Relevance of metrical, chromosomal, and allozymic variation to systematics of Mus, particulary the impulse to elevate subgenera to genera, was discussed by Corbet (1990). Enamel microstructure of incisors and molars in Mus and its significance documented by Patnaik (2002).
Reviews of variable depth and content are available for Asian (J. T. Marshall, Jr., 1977b) and European species (J. T. Marshall, Jr., 1981, 1986, 1998; J. T. Marshall, Jr. and Sage, 1981; Bonhomme et al., 1984; Gerasimov et al., 1990; She et al., 1990), and "wild mice" as a popular mammalian model (Guénet and Bonhomme, 2003). Appraisal of fossil evidence and morphological traits for European species summarized by Thaler (1986). A key to European house mice (M. musculus, M. spretus, M. macedonicus, and M. spicilegus) based mostly on cranial and dental traits provided by Macholán (1996a), along with summaries of diagnostic characters, habitats, and geographic ranges. Morphometric analysis of 12 population samples representing those four species were also documented by Macholán (1996b), and he provided results of multivariate analysis of morphometric variation in Asian species of subgenus Mus and two species of Nannomys and contrasted it with the phylogenies reconstructed from allozymic analyses (Macholán, 2001). Most African species require careful systematic review (see accounts of species in subgenus Nannomys); Ansell (in Meester et al., 1986:280) believed the African segment of the genus to be "over-split," but our study of specimens suggests a current underestimate of actual species-diversity.
Reconstructing evolutionary relationships among species is a prime research goal of Mus systematists and they are using phylogenetic analyses of an increasing variety of molecular data (sequences from a range of mitochondrial and nuclear genes) along with sophisticated chromosomal information to test the current phylogenetic picture based upon morphological traits. Much of the molecular data is concordant with morphology in supporting current estimate of species affinities, but not all. Nishioka et al. (1993) surveyed a repetitive Y chomosomal sequence (1.1 kb long and designated 142.4) among species of Mus in subgenera Mus, Pyromys, Coelomys, and Nannomys and found a restriction fragment length polymorphism that was different in M. m. musculus and M. m. domesticus. Among the other species sampled, however, the accumulation patterns of this repetitive sequence did not correlate with the phylogenetic relationships of species as determined by morphological and other molecular traits, and Nishioka et al. concluded that the 142.4 related sequences are evolutionarily unstable.
Phylogenetic relationships of Mus are unclear. Study of molar traits led Misonne (1969) to place the genus in his Rattus Division, and to suggest an affinity with the deomyines Acomys and Uranomys. This grouping is uncorroborated when tested with molecular data, which isolates the latter two genera as a separate group not closely related to any murines (see account of Deomyinae). Cladistic analyses of complete mtDNA cytochrome b sequences (Martin et al., 2000) and sequences from nuclear IRBP gene and the cytochrome b and 12S rRNA mitochondrial markers (Michaux et al., 2002a) indicate a tighter link between Mus and Apodemus and its relatives than between Mus and Rattus, results corroborated by other molecular studies (see references in Michaux et al., 2002a). Generic sampling is not dense in these analyses, and the suggested Mus-Apodemus alliance must be tested by wider taxon and geographic sampling. Albumin immunology, for example, pointed to Mus as "a distinct monogeneric clade arising from the central murine stem . . . At about the same time as Apodemus and the African group [Rhabdomys, Grammomys, Arvicanthis, Thallomys, Praomys, Hybomys, Stochomys, Hylomyscus, Pelomys, Aethomys, Lemniscomys, and Otomys]" (Watts and Baverstock, 1995b:112), which may be a more realistic estimate of evolutionary origin. A species resembling the M1 occlusal pattern of Mus comes from the same C European beds containing the oldest European murines (early Vallesian, 11.1-9.7 million years ago) and is regarded as related to Mus; it, Apodemus, Progonomys, and two other groups comprise the earliest European murine lineages (Freudenthal and Martín Suárez, 1999; Mein et al., 1993). The fate of the Mus like form in Europe is unknown because true Mus is absent from the later European Miocene and Pliocene sediments and appears only in the Pleistocene (Auffray et al., 1990c; Kowalski, 2001), but Mein et al. (1993) noted the very close similarity between the Vallesian "Mus" and Siwalik Progonomys, from which Jacobs (1978) and others (Jacobs and Downs, 1994) would derive the earliest Mus. This phyletic ambiguity surrounding Mus prompts us to isolate it in its own group. Most systematists have interpreted, as all the molecular work has reinforced, the phylogenetic relationship between Mus and Rattus to be distant, and perhaps instead of focusing interminably and tediously on the imaginary Mus-Rattus split (dredged up in most paleontological and molecular works focusing on murine systematics; see also Adkins et al., 2003), researchers can instead look for data signals identifying the closest allies of Mus and time of its split from the ancestral murine stock.
Mus auctor, from late Miocene sediments in the Siwaliks of Pakistan, is claimed to be the earliest record of the genus (5.7 million years ago; Jacobs, 1978; Jacobs and Downs, 1994; Patnaik, 1997; Patnaik et al., 1996). But at that time the Mus dental specializations are already apparent and indicate an earlier evolutionary origin (see discussion in Chaimanee, 1998), which may be found in molar samples from early Vallesian (11-10 million years ago) of Europe considered closely related to Mus in morphology, if not a species of Mus (Mein et al., 1993; Suárez and Mein, 1998; those latter authors even noted that Mus, along with Apodemus, have the longest time range of any murine, referring to the Vallesian form). Jacobs and Downs (1994) derive Mus from Asian Progonomys debruijni or a species similar to it (but Mein et al., 1993, excluded debruijni from Progonomys). During Pliocene times Mus samples come from C Asia and Africa (see review in Auffray et al., 1990), and by the Plio-Pleistocene, subgenus Mus is represented by a cluster of species from N India that are dentally similar to the living M. booduga and M. terricolor from the Indian subcontinent (Gupta and Prasad, 2001; Kotlia, 1992, 1995; Patnaik, 1997, 2001; Patnaik et al., 1993, 1996). Recently, a 2-million year old skull (late Pliocene) has been found representing a species (M. linnaeus) that might be ancestral to M. musculus, which in the eyes of Patnaik et al. (1996) support the out-of-India origin of house mice. Boursot et al. (1996:406), e.g., speculated that "This species appears to have originated in the N Indian subcontinent, from where it colonised the Middle East," which is the hypothesis also favored by Din et al. (1996), by not by Prager et al. (1998). Elsewhere in Asia, Zheng (1993) recorded Mus sp. From late Pliocene cave sediments of the Sichuan-Guizhou region in S China, and Zheng et al. (1997) documented M. musculus from Pleistocene fissure deposits in the Shandong region. Chaimanee (1998:214) documented four extant Thai species based upon molars recovered from late Pliocene and Pleistocene cave sediments. In her opinion, "Southeast Asia is the center of evolution of [Mus] with four sympatric species occurring [in Thailand] since the Pleistocene." Examples of species in subgenus Mus have also been recovered from early Pleistocene sediments on Java (Van der Meulen and Musser, 1999).
The extinct Mus minotaurus is represented by only teeth and mandibular fragments "from a cave deposit in a small limestone ridge between Canea and Suda" on Crete Isl in the Mediterranean (Bate, 1942a:46); age range probably late Pleistocene to Holocene (found in Neolithic levels; Mayhew, 1977). This is one of the large-bodied species in the genus, approximating the size of the extant Indomalayan Mus shortridgei (Bate, 1942a). Kuss and Misonne (1968) suggested a close relationship between M. minotaurus and the Indomalayan M. pahari, M. mayori and M. shortridgei as well as the African M. bufo. Mayhew (1977, 1996) had described M. bateae, having molars smaller than M. minotaurus but larger than M. musculus, from probable middle Pleistocene sediments on Crete and postulated that it and M. minotaurus are part of a single lineage derived from an ancestral population resembling M. musculus. Both M. minotaurus and M. bateae are found in the higher biostratigraphic depositional levels on Crete, always in the zone above those containing the extinct endemic murine Kritimys (Dermitzakis and de Vos, 1987). Mus minotaurus was still present in late cave levels along with traces of human activities and probably became extinct after the arrival of humans (about 8000 years before present) and the subsequent introduction of Rattus and Mus musculus, which ultimately provided devastating competition (Dermitzakis and de Vos, 1987).
Earliest records of Mus from Subsaharan Africa come from late Pliocene sediments, and samples of the genus have also been obtained from Pleistocene and Holocene strata (Avery, 1998, 2000; Denys, 1999; Jaeger, 1976; Jaeger and Wesselman, 1976; Senut et al., 1992). Geraads (1998) described a Miocene-Pliocene species from Morocco that may be the earliest known Mus recorded from Africa. Mein and Pickford (1992) discussed species recovered from Pleistocene beds in Tunisia.