XVI International Botanical Congess
The cholla cacti includes ca. 35 species of trees and shrubs primarily distributed in the deserts of northern Mexico and southwestern U.S. There are few morphological phylogenetically informative characters in the group as a result of great morphological plasticity, asexual reproduction, and hybridization between close and distantly related taxa. Cladistic analysis of sequence data from the psbA-trnH intergenic spacer provides low resolution of closely related species, yet is very valuable as a maternal marker for hybrid detection. One surprising result is that the triploid and mostly asexual O.bigelovii was the maternal parent in several hybrization events.