PHYLOGENY OF THE GENUS PYRAMIMONAS (PRASINOPHYCEAE, CHLOROPHYTA) INFERRED FROM THE RBCL GENE

A 1089-basepair fragment (approx. 75%) of the large subunit of the chl oroplast-encoded gene, ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL), was sequenced from 16 species of the genus Pyramimonas Schmar da. Electron microscopic and biochemical studies of Pyramimonas, one o f the most morphologically diverse genera within the potential sister groups to the chlorophyll a- and b-containing plants, suggest that thi s genus consists of at least four separate subgenera. Using the homolo gous sequence of rbcL from Cymbomonas tetramitiformis Schiller (Halosp haeraceae) as an outgroup and applying the maximum likelihood method, we show that the inferred topology is congruent with traditional delim itations of the taxa based on observations of periplast, internal ultr astructure, and biochemical features. A bootstrap analysis also suppor ts division at the subgeneric level; however, the low bootstrap suppor t associated with the deep nodes precludes resolution of these branche s. A maximum likelihood relative rate test revealed that the rbcL gene in these single-celled green flagellates has a heterogeneous rate of substitution. The rbcL gene in species of the subgenus Pyramimonas has evolved at an accelerated rate relative to that of congenerics.