A MOLECULAR SYSTEMATIC INVESTIGATION OF THE ACROCHAETIALES (FLORIDEOPHYCIDAE, RHODOPHYTA) AND RELATED TAXA BASED ON NUCLEAR SMALL-SUBUNIT RIBOSOMAL DNA-SEQUENCE DATA

Small-subunit ribosomal DNA (SSU) sequences for 14 members of the rhod ophyte order Acrochaetiales were used to generate phylogenetic trees t o determine whether the Acrochaetiales is monophyletic and to assess t he relationships of acrochaetioid algae to closely related taxa. Withi n the Acrochaetiales, two strongly supported groups are recognized: on e group that includes, in addition to three other species, the type sp ecies of the genera Arochaetium, Audouinella and Rhodochorton; and a s econd group that includes an unequivocal representative of Colaconema and nine additional species. The relationships of these groups with th e Batrachospermales, Palmariales and Nemaliales were not resolved and the possibility that the Acrochaetiales is polyphyletic was not elimin ated. Resolution of relationships for species within these groups was strong in only a few cases, with many of the phylogenetic issues at th is level remaining equivocal. Our results do not position Camontagnea in the Acrochaetiales, as has recently been suggested, but strongly al ly this genus to the family Rhodothamniellaceae of the Palmariales. Th e phylogenetic position of Rhododraparnaldia, an alga intermediate bet ween the Acrochaetiales and Batrachospermales, remains unresolved. The se molecular results provide a foundation for assessing the taxonomic significance of phenotypic characters (e.g. anatomy, life histories, m orphology, phycoerythrin type and plastid details) variously considere d taxonomically significant in the Acrochaetiales.