Floral Pigments and Phylogeny in Echinocereus (Cactaceae)

Abstract

A phylogenetic hypothesis is presented based upon floral flavonoid glycoside char- acters from 87 geographically separated populations representing 14 species of Echinocereus including E. chloranthus, E. engelmannii, E. enneacanthus, E. fendleri, E. pectinatus, E. polyacanthus, E. reichenbachii, E. rigidissimus, E. scopulorum, E. stoloniferus, E. stramineus, E. subinermis, E. triglochidiatus, and E. viridiflorus. A biosynthetic scheme involving flavanone-, dihydroflavonol-, flavone-, and flavonol-7-0-glyco- sides is put forth to clarify critical assumptions of character polarity. The unifying model proposes that intensification of ultraviolet solar radiation accompanied climatic change and increasing aridity of post-pluvial natural habitats. It is assumed that climatic change was the driving force behind the modification of floral flavonoids which act in cactus flowers as possible ultraviolet screens. Taxa which share the derived micromolecular chemosyndrome (presence of flavanone- and dihydrofla- vonol-7-0-glycosides) are considered more advanced than those species with 3-0-mono- and dig- lycosides of the common flavonols, because these flavonoids presumably better shield the cactus flowers from potentially damaging radiation in the UV-B. Flavone- and flavonol-7-0-glycosides are also considered to be apomorphic since they are immediate biosynthetic derivations of the UV-B flavonoid screening agents. Camin-Sokal Parsimony with known ancestral states (determined from the unifying model) allowed the construction of hypothetical phylogenies for 14 species.