THE PHYLOGENETIC AFFINITIES OF MYAGROPSIS-MYAGROIDES (FUCALES, PHAEOPHYCEAE) AS DETERMINED FROM 18S RDNA SEQUENCES

Phylogenetic affinities of the fucalean brown alga, Myagropsis myagroi des (Mertens ex Turner) Fensholt (Fucales, Phaeophyceae), were determi ned from 18S rDNA sequences. We have sequenced 10 species of fucalean algae collected from various parts of Japan: Fucus distichus ssp. evan escens (C. Agardh) Powell, Pelvetia babingtonii (Harvey) De Toni (Fuca ceae), Cystoseira hakodatensis (Yendo) Fensholt, Coccophora langsdorfi i (Turner) Greville, Myagropsis myagroides (Cys toseiraceae), Sargassu m horneri (Turner) C. Agardh, S. confusum C. Agardh, S. macrocarpum C. Agardh, Sargassum sp., and Hizikia fusiformis (Harvey) Okamura (Sarga ssaceae). Phylogenetic trees constructed using maximum likelihood, max imum parsimony, and distance matrix methods resulted in similar topolo gy. Although currently Myagropsis is regarded as a member of the Cysto seiraceae, our results revealed that Myagropsis is more closely relate d 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 t hese two families was also demonstrated by the smaller number of nucle otide differences compared to those between other families. This resul t is consistent with previous molecular work that used a different set of species mainly collected in Australian waters.