ORIGINS AND AFFINITIES OF THE FILAMENTOUS GREEN ALGAL ORDERS CHAETOPHORALES AND OEDOGONIALES BASED ON 18S RIBOSOMAL-RNA GENE-SEQUENCES

The order Chaetophorales includes filamentous green algae whose taxono mic relationships to other chlorophycean orders is uncertain. Chaetoph oralean taxa include filamentous species which are both branched and u nbranched. Ultrastructural studies of zoospores have revealed similar flagellar apparatuses in a number of genera, including Uronema, Stigeo clonium, and Fritschiella, suggesting a chose phylogenetic relationshi p among these taxa. The order Oedogoniales represents a second group o f branched and unbranched filamentous green algae whose relationships to other chlorophycean orders also has been unclear. A possible close relationship between the Chaetophorales and Oedogoniales has been sugg ested. Using DNA sequences from the small-subunit ribosomal RNA gene ( SSU rRNA) of several members of each order, we have examined the monop hyly of the Chaetophorales and Oedogoniales, as well as the nature of their relationship to other chlorophycean orders. Our results show tha t chaetophoralean and oedogonialean taxa form separate monophyletic gr oups. Results also suggest that the two orders are not closely related to each other.