MOLECULAR SYSTEMATICS OF THE XANTHOPHYCEAE

Species belonging to the Xanthophyceae have traditionally been classified in six (or seven) orders corresponding to amoeboid, coccoid, filamentous, monadoid, palmelloid and siphonous lines of descent. We have determined plastid rbcL and nuclear 18S rRNA gene sequences for selected coccoid, filamentous and siphonous species. Phylogenetic trees inferred from these data indicate that the orders Mischococcales (coccoid) and Tribonematales (filamentous) are not monophyletic. Instead, our results indicate that coccoid and filamentous life forms have arisen independently in different xanthophyte lineages. Some coccoid and filamentous species are resolved as close allies. For example, in our analyses coccoid and filamentous species possessing bipartite cell walls (e.g., Bumilleria, Ophiocytium, Tribonema, Xanthonema) form a distinct and previously unrecognized lineage within the class. The phylogenetic positions of Asterosiphon, Botrydium and Vaucheria are not robustly resolved, but our data suggest that these siphonous xanthophytes probably do not form a monophyletic group. Our results imply that Vaucheria is only distantly related to other xanthophytes we have examined. Nucleotide sequence divergence values among Vaucheria species were found to be equal to, or greater than, those observed between other xanthophyte species that are classified in different families or orders. Furthermore, our trees do not always support the infrageneric system of classification for Vaucheria species, which is based primarily upon antheridial features.