WON, HYOSIG* and SUSANNE S. RENNER. Department of Biology, University of Missouri-St. Louis, St. Louis, MO 63121-4499. - Towards a phylogeny of Gnetum: first results from rbcL, Leafy, and ITS sequences.
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