PHYLOGENY OF THE GENUS PYRAMIMONAS (PRASINOPHYCEAE,CHLOROPHYTA) INFERRED FROM THE rbcL GENE1

A 1089-basepair fragment (approx. 75%) of the large subunit of the chloroplast-encoded gene, ribulose-1,5-bis-phosphate carboxylase/oxygenase (rbcL), was sequenced from 16 species of the genus Pyramimonas Schmarda. Electron microscopic and biochemical studies of Pyramimonas, one of the most morphologically diverse genera within the potential sister groups to the chlorophyll a- and b-containing plants, suggest that this genus consists of at least four separate subgenera. Using the homologous sequence of rbcL from Cymbomonas tetramitiformis Schiller (Halosphaeraceae) as an outgroup and applying the maximum likelihood method, we show that the inferred topology is congruent with traditional delimitations of the taxa based on observations of periplast, internal ultrastructure, and biochemical features. A bootstrap analysis also supports division at the subgeneric level; however, the low bootstrap support associated with the deep nodes precludes resolution of these branches. 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.