9527884 Wallace The cactus family, Cactaceae, comprises some 1500 species in North and South America (with one introduction to West Africa) and includes morphologically diverse life forms ranging from columnar, tree-like to barrel to viny and epiphytic. In adapting to dry and often desert conditions, these "stem succulents" have likely in parallel converged on similar vegetative shapes and forms, which renders evolutionary analysis designed to distinguish the lineages within the family difficult and confusing. In order to obtain robust and independent comparative data for taxonomic and phylogenetic analyses, Dr. Robert Wallace of Iowa State University is studying DNA variation in genes of the chloroplast and nucleus, from samples of cacti from the two large subfamilies Opuntioideae and Cactoideae. DNA nucleotide sequences are being gathered from several genes, and these will be aligned for comparison and assessed for mutational differences between species. Phylogenetic trees are then constructed based upon these measures of mutation difference. With the phylogenies (or taxon genealogies) as guide to lineage relationships among genera and species, one can study the pattern of adaptive radiation into desert habitats throughout the Americas, and the morphological changes that accompany or enable evolution into various regions and habitats. The phylogenetic analysis also will inform a more stable classification of cacti, and direct the choice of key characters for purposes of species identification.
