What did the first cactus look like? An attempt to reconcile the morphological and molecular evidence

Cacti have fascinated students of natural history for many millennia. Evidence exists for use of cacti as food, medicine, and ornamental plants by peoples of the New World for at least 9,000 years (Anderson, 2001), and cacti have captivated the attention of Old World plant scholars, agriculturists, and hobbyists since their introduction in the late 15th century. The striking visual aspect of the succulent, spiny, often leafless plants has received much attention, and botanists have been pondering their evolutionary relationships for over a century. Three subfamilial groups have been traditionally recognized within Cactaceae. The easiest group to circumscribe morphologically is Opuntioideae, with minute, barbed, deciduous spines (termed glochids), and small, often ephemeral leaves. Additionally, Opuntioideae have a bony seed aril (Vaupel, 1925; Anderson, 2001). This subfamily includes the familiar prickly pears of the genus Opuntia Mill. (Fig. lA), plus many other morphotypes. Another large group of cacti are subfamily Cactoideae, which include leafless, spiny stem succulents such as the genus Trichocereus Riccob. (Fig. 1B). Cactoideae are very diverse in habit and morphology, and the vast majority of described cactus species fall into this group. The remaining subfamily Pereskioideae includes leafy, spiny, often non-succulent trees and shrubs of the genus Pereskia Mill. (Fig. 1C). In addition to these three longand widely-recognized subfamilies, recent molecular evidence supports the recognition of a fourth subfamily of cacti, the monogeneric Maihuenioideae (Wallace, 1995a, b; Nyffeler, 2002). Maihuenia Phil. contains two little-studied species of smaller, succulent, cushion-forming cacti that are restricted to western and southern South America