WALLACE, ROBERT S. Department of Botany, Iowa State University, Ames, Iowa 50011. - Phylogenetic relationships among North American columnar cacti revisited.
The evolutionary diversification of columnar cacti in North America
has resulted in a range of morphologies from large trees, shrubs, and
epiphytes, to geophytes, and single-stemmed dwarf forms. This group of
cacti include members of the tribes Pachycereeae, Leptocereeae, which
are predominantly large to moderately-sized trees and shrubs, and the
Hylocereeae which are primarily epiphytes. The South American genus
Corryocactus (formerly placed in tribe Notocacteae) also has
been allied to this clade. Comparative sequencing evidence from
variation in plastid markers supports an origin for these North
American cacti from within a sub-clade of the presently circumscribed
tribe Browningieae of the central Andean region. The present study
examined the primary patterns of phylogenetic divergence in this North
American clade and reviews the systematic relationships between these
tribes. Additionally, the phylogeny within each of the represented
tribes was examined in the context of understanding the evolution of
systematically useful morphological characters (vegetative and
floral), and to evaluate present geographic distribution as related to
phylogenetic divergence. Sequences of plastid non-coding regions of
the psbA-trnH intergenic spacer and the rpl16 intron
were used in concert with maximum parsimony techniques to develop a
phylogenetic assessment of the various groups. It was discovered that
the two subtribes of the Pachycereeae, Pachycereinae and
Stenocereinae, were strongly supported, the latter containing the
genus Echinocereus. The relationship between the tribes
Leptocereeae and Hylocereeae was found to be closer than previously
hypothesized. Bat pollination is prevalent throughout these clades,
however multiple shifts to entomophily were also suggested.
Key words: Cactaceae, evolution, Hylocereeae, Leptocereeae, Pachycereeae, phylogeny