In the absence of reliable taxonomic characters, species relationships in Gelidium (Gelidiales, Rhodophyta) are poorly understood. In order to produce an independent data set for developing phyloge...

A phylogeny for the Rhodophyta has been inferred by parsimony analysis of plastid rbcL sequences representing 81 species, 68 genera, 38 families, and 17 orders of red algae; rbcL encodes the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. Levels of sequence divergence among species, genera, and families are high in red algae, typically much greater than those reported for flowering plants. The Rhodophyta traditionally consists of one class, Rhodophyceae, and two subclasses, Bangiophycidae and Florideophycidae. The Bangiophycidae with three orders (Porphyridiales, Compsopogonales, and Bangiales) appears to be polyphyletic, and the Florideophycidae with 17 orders is monophyletic in this study. The current classification of the Florideophycidae based on ultrastructure of pit connections is supported. With the exception of the Rhodogorgonales, which appears to be misplaced, orders with one or two pit-plug cap layers (Hildenbrandiales, Corallinales, Acrochaetiales, Palmanales, Batrachospermales, and Nemaliales) terminate long branches of basal position within Florideophycidae in the most parsimonious rbcL tree. Orders that lack typical cap layers but possess a cap membrane are resolved as a monophyletic clade sister to the Ahnfeltiales. The large order Gigartinales, which is distributed among five rbcL clades, is polyphyletic. Families that possess typical carrageenan in their cell walls are resolved as a terminal clade containing two family complexes centered around the Solieriaceae and Gigartinaceae.