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To most people, all small rodents are virtually indistinguishable from each other, and as such, they are lumped together and considered to be mice of one kind or another. In Webster's Third New International Dictionary, one finds the following definition for a mouse: "any of numerous small rodents typically resembling diminutive rats with pointed snout, rather small ears, elongated body and slender hairless or sparsely haired tail, including all the small members of the genus Mus and many members of other rodent genera and families having little more in common than their relatively small size". 6 In fact, the order Rodentia (in the kingdom Animalia, phylum Chordata, and subphylum Vertebrata) is very old and highly differentiated with 28 separate families, numerous genera, and over 1,500 well-defined species accounting for 40% of all mammalian species known to be in existence today (Corbet and Hill, 1991). All families, subfamilies and genera in this order that contain animals commonly referred to as mice are listed in Table 2.1. The family Muridae encompasses over 1,000 species by itself including mice, rats, voles, gerbils, and hamsters. Within this family is the subfamily Murinae, which contains over 300 species of Old World mice and rats, and within this subfamily is the genus Mus. The Mus genus has been divided into four subgenera, of which one is also called Mus. This subgenus contains all of the "true Old World mice" including the house mouse M. musculus, the main focus of this book. A humorous view of mouse evolution is reproduced in Figure 2.1, and a more serious phylogenetic tree with all extant members of the Mus subgenus is presented in Figure 2.2.

There is still a great deal of confusion in the field of rodent systematics, and the proper classification of species into and among genera is now undergoing serious revision with the results of new molecular analyses. Just recently, it was suggested that the guinea pig is not a rodent at all, contrary to long-held beliefs (Graur et al., 1991). And in other studies (based on DNA-DNA hybridization and quantitative immunological cross-reactivity), a series of African species known as "spiny mice" were found to be more closely related to gerbils than to true Old World mice (Wilson et al., 1987; Chevret et al., 1993).

The major reason for the confusion is that classical systematics has always been dependent on taxonomy, and taxonomy has always been dependent on the demonstration of distinct morphological differences — measurable on a macroscopic scale — that can be used to distinguish different species. Unfortunately, many small rodent species have developed gross morphological characteristics that are convergent with those present in other relatively distant species. Thus, traditional taxonomy can fail to provide an accurate systematic description of mice. (An illustration of the close similarity of Mus species can be seen in Figure 3.3). Fortunately, the tools of molecular phylogenetics — and in particular, DNA sequence comparisons — have proven highly effective at sorting out the evolutionary relationships that exist among different mouse groups. With continued molecular analysis, it may be possible to clear up all of the confusion that now exists in the field.

Excellent sources of information concerning the systematics and phylogeny of Mus and related species are the proceedings from two conferences, The Wild Mouse in Immunology (Potter et al., 1986) and The Fifth International Theriological Congress (Berry and Corti, 1990) as well as a review by the Montpellier group (Boursot et al., 1993). A concise description of Mus systematics is provided by Bonhomme and Guénet (Bonhomme and Guénet, 1989).

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