PHYLOGENETIC RELATIONSHIPS OF SPECIES OF UNCERTAIN TAXONOMIC POSITION WITHIN THE ACROCHAETIALES‐PALMARIALES COMPLEX (RHODOPHYTA): INFERENCES FROM PHENOTYPIC AND 18S rDNA SEQUENCE DATA1

Nucleotide sequences of the small‐subunit ribosomal RNA gene(18S rDNA)were newly determined for four members of the related rhodophyte orders Palmariales and Acrochaetiales and were compared by neighbor‐joining and parsimony analyses with four previously published sequences from these orders. A published rDNA sequence fromNemalion helminthoides (VelleyinWithering)Batters(Nemaliales)and a gene sequence determined herein forNothocladus nodosusSkuja(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 algaeRhodo‐chorton purpureum (Lightfoot) Rosenvinge and Rhodothamniella floridula (Dillwyn)J. FeldmanninChristensen were phylogenetically separated withR. purpureum affiliated weakly with Acrochaetiales, and R. floridulaforming a sister branch unequivocally to palmarialean algae of the families Rhodophysemataceae and Palmariaceae. Rhodothamniella floridulais 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‐Husegawa 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.