EVOLUTION OF NUCLEAR RDNA ITS SEQUENCES IN THE CLADOPHORA ALBIDA SERICEA CLADE (CHLOROPHYTA)/

Ribosomal DNA ITS sequences were compared among 13 different species a nd biogeographic isolates from the monophyletic ''abbida/sericea clade '' in the green algal genus Cladophora. Six distinct ITS sequence type s were found, characterized by multiple insertions and deletions and h igh levels of nucleotide substitution. Conserved domains within the IT S regions indicate the presence of ITS secondary structure. Low transi tion/transversion ratios among the six types and nearly symmetrical tr ee-length frequency distributions indicate some saturation, and low ph ylogenetic signal. Although branching order among five of the six ITS sequence types could not be resolved, estimates of ITS sequence diverg ence as compared with 18S divergence in a subset of the taxa suggests that the origin of the different ITS types is probably in the mid-Mioc ene (12 Ma ago) but that biogeographic isolates within a single ITS ty pe (including both Pacific and Atlantic representatives) have probably dispersed on a time scale of thousands rather than millions of years.