XVI International Botanical Congess
Plastid structure has been used to a limited extent in brown algal systematics to define higher taxa. I have studied species with a stellate arrangement of plastids and a single or several grouped pyrenoids. Small and large subunit nrDNA and ITS sequences of Asterocladon lobatum, Asteronema spp., Bachelotia antillarum, Scytothamnus spp., Stereocladon rugulosus and Splachnidium rugosum, suggest that Scytothamnus, Splachnidium, Stereocladon and Asteronema ferruginea form a clade. Scytothamnales ord. nov. was proposed for these taxa occurring in southern temperate to polar regions. The other taxa occur in warm-temperate to tropical regions and do not belong to the Scytothamnales, yet their former placement in Ectocarpaceae is not supported by molecular data either. Sequences suggest that cytological characters in brown algae are phylogenetically more informative than morphology or life histories.