PHYLOGENETIC-RELATIONSHIPS AMONG THE GENERA OF TAXODIACEAE AND CUPRESSACEAE - EVIDENCE FROM RBCL SEQUENCES

DNA sequences from the plastid gene rbcL were used to assess relations hips among genera of the conifer families Taxodiaceae and Cupressaceae s. str. Phylogenetic analyses produced four most parsimonious trees t hat differ only in the positions of three genera (Athrotaxis, Taxodium , and Taiwania) and the degree of resolution of a trichotomy within Cu pressaceae s. str. Taxodiaceae and Cupressaceae form a monophyletic gr oup. The major lineages of Taxodiaceae diverged first, and a monophyle tic Cupressaceae s. str. are derived from within Taxodiaceae. These re sults are consistent with recent suggestions that the two families be treated as a single family, Cupressaceae s.l. Sciadopitys, often class ified in Taxodiaceae, is not closely related to Cupressaceae s.l. and should be excluded from the family. Sequence evolution of rbcL is extr emely slow in this group of long-lived trees. The rate of silent nucle otide substitutions is 2.5 x 10(-11) per site per year, approximately 13 times slower than rates estimated in short-lived monocots.