Parsimony analysis of plastid rpl16 sequences from 62 members of Tribe Cacteae, and four outgroup taxa yielded 1296 equally parsimonious trees of length 666. Strict consensus evaluation of these trees established a highly pectinate topology, which delimited clades within the tribe that correspond to several previously considered generic groups. Aztekium and Geohintonia, which manifest ribs in their stem morphology were shown to represent an early divergence in the tribe, forming a sister group to remaining members of the tribe. Clades containing other genera having ribbed stems also are basal to those that develop tubercles. The most derived clade forms a distinct group of typically small stemmed species with tubercular stem morphology. Within Mammillaria, species formerly placed in the genus Cochemiea and members of the Series Ancistracanthae formed a well-supported, sister clade to the remaining members of Mammillaria. Length variation of the intron in two members of Mammillaria series Stylothelae was also observed.

Communicating Editor: Thomas G. Lammers