Molecular phylogeny and phenotypic variation in the heterotrophic green algal genus Prototheca (Trebouxiophyceae, Chlorophyta)

Species of the heterotrophic green microalgal genus Prototheca and related taxa were phylogenetically analyzed based on the nuclear small subunit (SSU) and the 5' end of the large subunit (LSU) rRNA gene (rDNA) sequences. We propose restricting the genus Prototheca to the four species: P. moriformis Kruger, P. stagnora (Cooke) Pore, P. ulmea Pore, and P. zopfii Kruger. The main diagnostic feature of these taxa is the absence of growth on trehalose.Of these, it was suggested that P. moriformis should be merged into P. zopfii; P. moriformis and three varieties of P. zopfii constituted a paraphyletic assemblage with estimated short evolutionary distances. The trehalose-assimilating strains (Prototheca wickerhamii Tubaki et Soneda strains and Auxenochlorella protothecoides (Kruger) Kalina et Puncocharova SAG 211-7a), together with an invertebrate pathogen Helicosporidium sp., diverged before the radiation of the four species of Prototheca in the SSU rDNA and composite (SSU rDNA plus LSU rDNA) analyses. Comparison between the results from physiological data in this work (fermentative pattern) and those described earlier (growth requirements) lead us to propose a hypothesis that the phenotypic variation, which did not represent diagnostic characters for species delimitation, may reflect the history of genetic diversification within the genus Prototheca as inferred from rDNA sequence characters.