A REASSESSMENT OF SPECIES BOUNDARIES IN CYSTOSEIRA AND HALIDRYS (PHAEOPHYCEAE,FUCALES) ALONG THE NORTH AMERICAN WEST COAST1

Application of phylogenetic species recognition to morphologically recognized species in the genera Cystoseira Agardh and Halidrys Lyngbye on North American west coasts revealed little genetic variation despite a remarkable degree of morphological variation currently used to recognize and delineate species. Whereas morphological characteristics allow recognition of two genera, four morphological species and three informal forms, maximum genetic variation among them was similar to that characteristic of the intraspecific level in European congeners and other Fucales. Among morphological species and forms, nucleotide variation in a combined 26S (large subunit (LSU)) and internal transcribed spacer (ITS) ribosomal DNA analysis was below 3% while it was 1% or less for the RUBISCO spacer of the chloroplast DNA. Comparison of the LSU data to available data for European congeners showed that the genera Cystoseira and Halidrys are not monophyletic and that the previously recognized Cystoseiraceae should be included within the family Sargassaceae. These observations suggest that the current taxonomy for the Sargassaceae fails to reflect evolutionary history because Atlantic and Pacific Cystoseira and Halidrys appear to have arrived at similar morphologies independently. Our results indicate a comparatively recent establishment on the west coast of North America of a sargassacean progenitor whose descendant taxa have experienced limited genetic divergence and are characterized by a high capacity for phenotypic variation despite their overall genetic similarity.