Molecular phylogeny and evolution of the order Tribonematales (Heterokonta, Xanthophyceae) based on analysis of plastidial genes rbcL and psaA

Tribonematales is an order of filamentous algae in the class Xanthophyceae (Heterokonta). Few molecular studies, all with a limited taxon sampling, have previously investigated its evolutionary history and phylogenetic relationships. We sequenced the chloroplast-encoded rbcL and psaA genes of several tribonematalean species and of several coccoid and siphonous forms that previous studies revealed to be strictly related to Tribonematales. Multiple alignments included mostly new sequences obtained from 42 taxa. Phylogenetic reconstructions were performed using the maximum likelihood method. The rbcL and psaA data sets were analyzed independently and combined in a single multiple alignment. Neither rbcL nor psaA genes showed intraspecific sequence variation. The former proved to be a better diagnostic marker than the latter for characterization of species. We explored effects produced on phylogenetic outcomes by selected genes. Congruent results were obtained from analyses performed on single gene multiple alignments as well as on the combined data set. There is strong statistical support for trees that show several currently recognized taxonomic groups to be polyphyletic. The siphonous orders Botrydiales and Vaucheriales do not form a clade. Botrydiales and Tribonematales are polyphyletic as are the families Botrydiaceae, Centritractaceae and Tribonemataceae and the genera Xanthonema and Bumilleriopsis. We tentatively define new boundaries of the Tribonematales to include both coccoid and filamentous species having a bipartite cell wall and also the siphonous members of the genus Botrydium. Also, our results support morphological convergence at all taxonomic ranks in the evolution of the Xanthophyceae.