The Common Shrew Gene Map.

Among mammals, gene mapping efforts have concentrated mainly on humans and house mice. However, although studies of these 2 species can add much to our understanding of eutherian genome evolution (for example, Ehrlich and others 1997), it is important to expand the range of species to be examined for a more complete picture of the genomic changes that have occurred during the eutherian radiation (O'Brien and others 1997). Gene mapping studies much more modest than those conducted on humans and mice can add enormously to our knowledge of eutherian genome evolution, especially if (1) species from a wide range of different eutherian orders are analyzed, and (2) the same set of comparative anchor loci are examined (O'Brien and others 1993, 1997). Because the relationship of many of the orders of eutherian mammals is difficult to infer (Novacek 1992), gene mapping studies on species from as many eutherian orders as possible are needed both to help clarify the relationships between the orders and to properly deduce the structure of the ancestral eutherian genome. The order Insectivora includes shrews, hedgehogs, moles, and tenrecs. It is the third most speciose eutherian order (after Rodentia and Chiroptera), of which the shrew family (Soricidae) is particularly large. Morphologically, insectivores appear to be closer to the ancestral eutherian condition than other orders (Nowak 1991), and mitochondrial DNA sequences also suggest that insectivores are more basal within the eutherian phylogenetic tree than cetaceans, artiodactyls, primates, and rodents (Krettek and others 1995). Insectivores date back to the late Cretaceous in the fossil record (Stahl 1974). Given their status as a large group of "primitive" eutherians, insectivores are clearly an important model for gene mapping. Among the Insectivora, the common shrew (Sorex araneus L. 1758) has been subjected to particularly detailed population genetic analysis, both with respect to chromosomes (see below) and to other genetic markers (Heikkila 1989; Taberlet and others 1994). (For recent studies see also Fredga and Searle 1996; Wojcik and Wolsan 1998.) This