The phylogenetic affinities of Myagropsis myagroides (Fucales, Phaeophyceae) as determined from 18S rDNA sequences

Phylogenetic affinities of the fucalean brown alga, Myagropsis myagroides (Mertens ex Turner) Fensholt (Fucales, Phaeophyceae), were determined from 18S rDNA sequences. We have sequenced 10 species of fucalean algae collected from various parts of Japan: Fucus distichus ssp. evanescens (C. Agardh) Powell, Pelvetia babingtonii (Harvey) De Toni (Fucaceae), Cystoseira hakodatensis (Yendo) Fensholt, Coccophora langsdorfii (Turner) Greville, Myagropsis myagroides (Cys toseiraceae), Sargassum horneri (Turner) C. Agardh, S. confusum C. Agardh, S. macrocarpum C. Agardh, Sargassum sp., and Hizikia fusiformis (Harvey) Okamura (Sargassaceae). Phylogenetic trees constructed using maximum likelihood, maximum parsimony, and distance matrix methods resulted in similar topology. Although currently Myagropsis is regarded as a member of the Cystoseiraceae, our results revealed that Myagropsis is more closely related to members of the Sargassaceae. This implies that the morphological character (mode of branching) used to discriminate the Cystoseiraceae and the Sargassaceae is not phylogenetically meaningful. Monophyly of the Cystoseiraceae and the Sargassaceae was strongly supported by both bootstrap analyses and likelihood ratio test. The close affinity of these two families was also demonstrated by the smaller number of nucleotide differences compared to those between other families. This result is consistent with previous molecular work that used a different set of species mainly collected in Australian waters.