Identification of three distinct Polytomella lineages based on mitochondrial DNA features

Polytomella is composed of colorless green algae closely related to Chlamydomonas reinhardtii. Species in the genus have been used in diverse fields of biological research, most recently to study mitochondrial function and mitochondrial genome evolution in the Chlorophyceae, but the phylogenetic relationship between the various available taxa has not yet been clarified and it is not known whether they also possess fragmented mitochondrial genomes, as reported for Polytomella parva. We therefore examined cox1 sequence from seven Polytomella taxa with the goal of establishing their phylogenetic relationships and relating this information to their mitochondrial DNA (mtDNA) fragmentation pattern. We found that the Polytomella isolates examined fall into three distinct lineages, two of which possess fragmented mitochondrial genomes. The third and earliest branching lineage, represented by Polytomella capuana, appears to possess an intact mtDNA. In addition, there is evidence for variation in both size and number of mtDNA fragments between various Polytomella isolates, even within the same lineage. The considerable amount of sequence divergence between lineages seems to correlate with the geographic origin of the strains, leading us to believe that greater amounts of sequence divergence could be uncovered by a broader sampling of Polytomella.