Systematic Relationships in the Artiodactyl Tribe Bovini (Family Bovidae), as Determined from Mitochondrial DNA Sequences

Abstract

-The systematic relationships of genera in the artiodactyl tribe Bovini were evaluated using two sets of mitochondrial DNA (mtDNA) sequences available for Bison, Bos, and Bubalus. One of these sets included 2,726 aligned positions from the two relatively conserved ribosomal RNA genes and three flanking transfer RNA genes; the other consisted of 247 base pairs from the rapidly evolving D-loop region. Most-parsimonious cladograms derived from the two data sets indicated that Bison and Bos are more closely related to each other than either is to Bubalus, as suggested from morphological, paleontological, and reproductive data. However, the traditional arrangement of the genus Bos is seriously challenged, as Bos grunniens groups first with Bison bison rather than with its congener Bos taurus. These results, when coupled with reproductive and other molecular data, strongly suggest that Bison should be treated as a congener of Bos rather than as a distinct genus in the tribe Bovini. [DNA sequences; mitochondrial DNA; molecular systematics; Bovini.] The mitochondrial genome of animals provides an excellent system to infer phylogenetic relationships at lower taxonomic levels (Wilson et al., 1985). Its rapid rate of nucleotide change ensures that a large number of reliable sequence differences accumulate for the resolution of evolutionary events within the last 20 million years or so (Miyamoto and Boyle, 1989). At the lower hierarchical levels, mitochondrial DNA (mtDNA) divergence exhibits little (Moritz et al., 1987), because insufficient time is available to accumulate parallel and back mutations (Miyamoto and Boyle, 1989). As a strong transition to transversion bias exists (Brown et al., 1982), transitions in the absence of saturation are responsible for most of the observed base differences at these levels. In spite of their obvious economic importance, the systematic relationships and taxonomic status of Recent genera in the artiodactyl tribe Bovini (subfamily Bovinae, family Bovidae) remain unclear, as reflected by the different classifications available for this group. Typically, four extant genera are recognized (Bison H. Smith [bison], Bos Linnaeus [cattle], Bubalus H. Smith [Asian water buffalo], and Syncerus Hodgson [African buffalo]) (McDonald, 1981; Kingdon, 1982; Walker et al., 1983; Simpson, 1984), but other classifications exist. Haltenorth (1963) and Lawlor (1979) recognize only one genus of buffalo (Bubalus) rather than two. Van Gelder (1977) and Gentry (1978) acknowledge only a single genus (Bos) for bison and cattle. Given its outstanding attributes for studying recent evolutionary events, the mtDNA system is therefore of potential importance to the resolution of systematic and taxonomic problems in the tribe Bovini (Miyamoto and Boyle, 1989). The relatively recent origin of the tribe (five to six million years ago in the upper Miocene to lower Pliocene [Romer, 1966; Sinclair, 1977; Kingdon, 1982] falls well within the time range in which mtDNA variation exhibits its greatest utility and resolving power (Moritz et al., 1987). We have employed mtDNA sequences to evaluate the molecular systematics and current taxonomic status of the two closely related genera Bison and Bos (Tanhauser, 1985). This study is restricted to a discussion of the systematic and taxonomic implications of the sequence data. Other aspects of our ongoing work with artiodactyl