BUTTERWORTH, CHARLES A.* and ROBERT S. WALLACE. Department of Botany, Iowa State University, Ames, Iowa 50011. - What is the sister-group of the tribe Cacteae? (Cactaceae: Cactoideae).
Of the various tribes described for members of the cactus subfamily
Cactoideae, the tribe Cacteae is perhaps one of the most
morphologically distinguishable. The members of this tribe include the
barrel cacti of North America, containing such familiar genera as
Ferocactus, Astrophytum, Lophophora, and
Mammillaria. One of the major phylogenetic questions that
remains for this group is the assessment of the sister-group
relationship, if any, to other tribes of the subfamily. Based on
general stem morphology, the barrel cacti of South America, tribe
Notocacteae (e.g. Parodia, Frailea, Blossfeldia,
Copiapoa, and allied genera), have been implicated as the
possible sister group to the Cacteae. Comparative sequence studies of
representative members of both of these tribes, in addition to
representatives from all other tribes of the subfamily Cactoideae were
used to assess intertribal relationships, and to examine the
phylogenetic affinities of tribe Cacteae. Parsimony analysis of
sequence data from the plastid intron rpl16 supports the
monophyly of the tribe Cacteae, places it as a distinct clade within
the subfamily, and does not infer any direct relationship to members
of the tribe Notocacteae. The rpl16 phylogeny suggests that the
Cacteae represent an early discrete divergence from the cactoid
ancestor, with no apparent directly-related sister clades.
Furthermore, the data support the distinction of the Atacama endemic
genus Copiapoa as a discrete lineage from the ‘core’ members of
tribe Notocacteae. Additional sequence data are being accrued to
evaluate phylogenetic information from other molecular markers and to
examine the robustness of the relationships supported by the
rpl16 intron phylogeny.
Key words: Cactaceae, Cacteae, classification, molecular systematics, phylogeny