PHYLOGENY OF CHLAMYDOMONAS (CHLOROPHYTA) BASED ON CLADISTIC-ANALYSIS OF NUCLEAR 18S RIBOSOMAL-RNA SEQUENCE DATA

The genus Chlamydomonas Ehrenberg may contain as many as 450 described species. Morphological, physiological and molecular data show that variation among some Chlamydomonas species can be great, leading to speculation that multiple, generic-level lineages exist within this genus. The most recent systematic studies of Chlamydomonas have led to proposals of nine distinct morphological and 15 distinct sporangial autolysin groups. Partial sequences from the nuclear small subunit rNAs from 14 Chlamydomonas species representing 12 autolysin and four morphological groups, and from three flagellates thought to be related to Chlamydomonas were determined in a phylogenetic study of relationships among these algae. Sequence comparisons among some Chlamydomonas species revealed differences comparable to the sequence divergence between soybeans and cycads. Cladistic analysis of the sequence data suggests that multiple lineages exist among species of Chlamydomonas. Some of these lineages represent alliances of both Chlamydomonas and non-Chlamydomonas taxa; thus, the current taxonomy does not reflect natural, or monophyletic, groups. Collectively, these lineages may represent distinct families or even orders.