A MOLECULAR PHYLOGENY OF THE MARINE RED ALGAE (RHODOPHYTA) BASED ON THE NUCLEAR SMALL-SUBUNIT RIBOSOMAL-RNA GENE

A phylogeny of marine Rhodophyta has been inferred by a number of meth ods from nucleotide sequences of nuclear genes encoding small subunit rRNA from 39 species in 15 orders. Sequence divergences are relatively large, especially among bangiophytes and even among congeners in this group. Subclass Bangiophycidae appears polyphyletic, encompassing at least three lineages, with Porphyridiales distributed between two of t hese. Subclass Florideophycidae is monophyletic, with Hildenbrandiales , Corallinales, Ahnfeltiales, and a close association of Nemaliales, A crochaetiales, and Palmariales forming the four deepest branches. Cera miales may represent a convergence of vegetative and reproductive morp hologies, as family Ceramiaceae is at best weakly related to the rest of the order, and one of its members appears to be allied to Gelidiale s. Except for Gigartinales, for which more data are required, the othe r florideophyte orders appear distinct and taxonomically justified. A good correlation was observed with taxonomy based on pit-plug ultrastr ucture. Tests under maximum-likelihood and parsimony of alternative ph ylogenies based on structure and chemistry refuted suggestions that Ac rochaetiales is the most primitive florideophyte order and that Gelidi ales and Hildenbrandiales are sister groups.