Assessing the phylogenetic position of the Prasiolales (Chlorophyta) using rbcL and 18S rRNA gene sequence data

Partial sequences of the genes coding for the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) and the nuclear ribosomal small subunit (18S rRNA) were used to determine the phylogenetic position of the order Prasiolales among other members of the Chlorophyta. Sequences were obtained for seven representatives of the Prasiolales (Prasiola fluviatilis, P. furfuracea, P. linearis, two collections of P. mexicana, P. meridionalis and Rosenvingiella sp.) together with two Ulvophyceae, for which sequences were not previously available. Sequences of the representative Prasiolales were aligned with those of 17 green algal representatives for the rbcL gene and 20 for the 18S rRNA gene. Parsimony and distance analyses of the rbcL and 18S rRNA sequences indicate that the Prasiolales is a well-defined order. Parsimony and distance analyses of the rbcL gene give moderate support to an alliance with the Ulvophyceae, but support for the remaining nodes of the trees is very low to negligible. Both parsimony and distance analysis of the 18S rRNA gene provide moderate support for a relationship with the Trebouxiophyceae, which has been postulated in past studies based on cellular and ultrastructural characters. Although a large polytomy was determined by parsimony analysis containing many of the lineages within the Chlorophyta, the analyses of the 18S rRNA gene are more resolved than the rbcL gene analyses. Sequence divergence values within the Prasiolales for the rbcL gene are low (0-6.1%) compared to values among the other green algal sequences used in our analyses, confirming the cohesive nature of the order. Divergence values among the Prasiolales for the 18S rRNA gene are also low (0.4-3.8%).