Reassessment of Phylogenetic Relationships among Pitviper Genera Based on Mitochondrial Cytochrome b Gene Sequences

Abstract

Crotalines are biomedically important, venomous snakes that are widely distributed across Asia and the Western Hemisphere. Phylogenetic relationships and contingent taxonomy of these pitvipers, however, remain problematic. This has been particularly true for snakes of the moccasin complex of the tribe Agkistrodontini (Gloyd and Conant, 1990). Species of this group were assigned to the genus Agkistrodon (senso lato), first described by de Beauvois (1799) but more recently have been divided into three Asian genera (Calloselasma, Deinagkistrodon, and Hypnale) and a more narrowly redefined Agkistrodon (Gloyd, 1979). This general division is supported by immunological evidence (Cadle, 1992; Minton, 1990). The redefined genus Agkistrodon remains controversial. Morphological evidence led Hoge and Romano-Hoge (1981) to group Old World Agkistrodon species in a separate genus, which they named Gloydius. Cranial musculoskeletal data were interpreted by Goombridge (1986) as favoring recognition of Gloydius, whereas Kardong (1990) viewed the same data as compatible with retention of Agkistrodon for Old World species. Using osteological characters, Brattstrom (1964) supported a monophyletic Agkistrodon and suggested that the bushmaster, Lachesis muta, and the rattlesnakes (genera Crotalus and Sistrurus) formed a monophyletic group. Finally, molecular data (Knight et al., 1992) indicate that the genus Agkistrodon, as defined by Gloyd (1979), is paraphyletic; restriction fragment length polymorphism (RFLP) of mitochondrial DNA and 16S ribosomal RNA sequences group New World species of Agkistrodon with the New World rattlesnake, Crotalus molossus, and the tropical New World pit viper, Cerrophidion godmani, rather than with Old World species of Agkistrodon. In this study, we used mitochondrial cytochrome b gene sequences to test two phylogenetic hypotheses: (1) the genus Agkistrodon as presently defined is monophyletic; and (2) the bushmaster, Lachesis muta, and the rattlesnakes, represented by Crotalus viridis, constitute a monophyletic clade.