Utility of the mitochondrial nad4L gene for algal and protistan phylogenetic analysis

Mitochondrial gene sequences, in contrast to their plastid counterparts, have been rarely exploited for use in determining algal phylogenetic relationships. Their utilization would prove advantageous particularly for the assessment of evolutionary relationships among protistan lineages that include photosynthetic species and their colorless heterotrophic descendents that contain only a reduced plastid genome as well as ancestral nonphotosynthetic relatives. Recent studies that have examined mitochondrial protein (cox1, cox2, cob, and nad5) rather than ribosomal RNA genes confirm that these algal sequences are sufficiently conserved to be used routinely in algal systematics. In this investigation, we sequenced the mitochondrial nad4L gene, encoding a small (ca. 99-amino acid) polypeptide subunit of the NADH dehydrogenase complex, from two chromophytic algae and an oomycete to determine its suitability for use in determining algal and protistan evolutionary relationships. Our analysis demonstrated that nad4L could clearly resolve these three organisms as a distinct clade, the stramenopiles, to the exclusion of terrestrial plants/chlorophytes and eu-fungi. However, the position of deeply rooted species within the stramenopiles proved tenuous and would best be examined in conjunction with the analysis of longer mitochondrial protein gene sequences.