MOLECULAR PHYLOGENY OF EXTANT GYMNOSPERMS AND SEED PLANT EVOLUTION - ANALYSIS OF NUCLEAR 18S RIBOSOMAL-RNA SEQUENCES

To study the evolutionary relationships among the four living gymnospe rm orders and the interfamilial relationships in each order, a set of 65 nuclear 18S rRNA sequences from ferns, gymnosperms, and angiosperms was analyzed using the neighbor-joining and maximum-parsimony methods . With Selaginella as the outgroup, the analysis strong ly indicates t hat the seed plants form a monophyletic group with the ferns as a sist er group. Within the seed plants the angiosperms are clearly a monophy letic group. Although the bootstrap support for the monophyly of the g ymnosperm clade is moderate, the monophyly is further supported by its lack of angiosperm-specific indels. Within the gymnosperms there appe ar to be three monophyletic clades: Cycadales-Ginkgoales, Gnetales, an d Coniferales. The cycad-ginkgo clade is the earliest gymnosperm linea ge. Given the strong support for the sister group relationship between Gnetales and Coniferales, it is unlikely that Gnetales is a sister gr oup of the angiosperms, contrary to the view of many plant taxonomists . Within Coniferales, Pinaceae is monophyletic and basal to the remain ing conifer families, among which there are three monophyletic clades: Phyllocladaceae-Podocarpaceae, Araucariaceae, and Sciadopityaceae-Tax aceae-Cephalotaxaceae Within the latter clade, Sciadopityaceae may be an outgroup to the other four families. Among the angiosperms, no sign ificant cluster at the level of subclass was found, but there was evid ence that Nymphaeaceae branched off first. Within the remaining angios perms, the monocots included in this study are nested and form a monop hyletic group. This study attests to the utility of nuclear 18S rRNA s equences in addressing relationships among living gymnosperms. Conside rable variation in substitution rates was observed among the ferns and seed plants.