Phylogenetic relationships of species of uncertain taxonomic position within the Acrochaetiales-Palmariales complex (Rhodophyta): Inferences from phenotypic and 18S rDNA sequence data

Nucleotide sequences of the small-subunit ribosomal RNA gene (18S rDNA) were newly determined for four members of the related rhodophyte orders Palmariales and Acrohaetiales and were compared by neighbor-joining and parsimony analyses with four previously published sequences from these orders. A published rDNA sequence from Nemalion helminthoides (Velley in Withering) Batters (Nemaliales) and a gene sequence determined herein for Nothocladus nodosus Skuja (Batrachospermales) were used as outgroups. Sequences were very similar within the Palmariales with a maximum difference of 13 nucleotides out of 1772 between members of the families Palmariaceae and Rhodophysemataceae. Despite similarities in life history, the acrochaetioid algae Rhodochorton purpureum (Lightfoot) Rosenvinge and Rhodothamniella floridula (Dillwyn) J. Feldmann in Christensen were phylogenetically separated with R. purpureum affiliated weakly with Acrochaetiales, and R. floridula forming a sister branch unequivocally to palmarialean algae of the families Rhodophysemataceae and Palmariaceae. Rhodothamniella floridula is postulated on both molecular and phenotypic grounds (encompassing biochemical and anatomical characters, and life history features) to represent an early lineage, Rhodothamniellaceae fam. nov., in the Palmariales. The family Rhodophysemataceae appeared paraphyletic in the sequence analyses, but a monophyletic Rhodophysemataceae could not be rejected by Templeton-Felsenstein or Kishino-Hasegawa tests, and the family was clearly monophyletic in analysis of the phenotypic data. The earlier tentative placement of Rhodophysemataceae in the Palmariales rather than Acrochaetiales was confirmed.