Gnetum has many unique features, such as bisexual strobili, net-veined leaves, lignin composition typical of angiosperms, and a climbing habit, that make it unusual among extant gymnosperms. Recent molecular phylogenies strongly suggest the inclusion of Ephedra (Gnetum + Welwitschia) in conifers as earlier suggested by morphologists. Gnetum has ca. 30 species of which 7 occur in the Neotropics, 2 in Africa, and ca. 21 in tropical Asia. All but two are canopy lianas. Previous classifications were based mostly on geography and variation in strobilus morphology. To assess the evolution of different strobilus types, growth forms, pollen morphology, and wood anatomy (studied in detail by S. Carlquist), we are constructing a molecular phylogeny for the genus. So far, we have sequenced the chloroplast gene rbcL, the second intron of the nuclear floral homeotic gene Leafy, and nuclear ribosomal ITS from 11 species (more will have been sequenced by the time this poster is presented). The species sampled represent the major groups recognized in the recentmost classification and the group's geographic distribution. The rbcL sequences show up to 2.6% sequence divergence, Leafy up to 5.8% divergence, and ITS shows such wide variation (with lengths varying from 410 to 986 bp) that it has not so far been alignable. However, the length variation in ITS is congruent with topologies resulting from parsimony analysis of Leafy and with geography. Combined rbcL and Leafy sequences yield highly resolved most parsimonious trees with bootstrap values between 83-100%. The topology suggests that, contrary to previous hypotheses, South American and Asian species are more closely related to each other than either is to the African species. Also, arborescent growth and prominent sterile ovules on male inflorescences appear to have evolved recently within Gnetum.

Key words: biogeography, bisexual strobilus evolution, Gnetum, ITS, Leafy, rbcL