Comprehensive molecular phylogenetic analysis of a heterokont alga (NIES 548) using genes from all three cellular compartments

Abstract

We have determined a partial DNA sequence (approximately 1.1 kb) encoding the mitochondrial cytochrome oxidase subunit I (cox1) gene from the alga NIES 548, a strain which has been maintained as Tribonema marinum J. Feldmann (Xanthophyceae) at the National Institute of Environmental Study (NIES, Japan). Unexpectedly, phylogenetic analysis of cox1 sequences showed that NIES 548 does not group with Xanthophyceae, but groups strongly with the Phaeophyceae. Furthermore, the cox1 sequence from NIES 548 does not use the codon AUA for methionine (AUA/Met), a genetic marker characteristic for the Xanthophyceae. Given the cox1 results, the molecular phylogenetic position of NIES 548 was thus examined with DNA sequences from genes encoded in the two other subcellular compartments. In the plastid, we analyzed elongation factor tu (tufA) and in the nucleus, the small subunit ribosomal RNA (rrnS). These analyses clearly indicated that NIES 548 is not a member of the Xanthophyceae, but a member of the Phaeophyceae. This result is robust as it was supported by all three genes analyzed, each of which reside in different genomes. The phylogenetic resolutions of these trees were almost the same, proving the usefulness of mitochondrial cox1 gene as a tool for phylogenetic reconstruction in the phycological arena. Our light microscopic observations indicated that NIES 548 is a uniseriate filamentous alga with branches, a clear contradiction of its original descriptions. We conclude that NIES 548 is a phaeophycean alga and is not the same clone of the strain observed by Sartoni and his co-authors.