Phylogenetic analysis of diatom coxI genes and implications of a fluctuating GC content on mitochondrial genetic code evolution.

In order to address the relationships among diatom groups and to investigate possible changes in their mitochondrial (mt) genetic codes, we have analyzed a 1.1-kb region of the cytochrome c oxidase subunit I (coxI) gene from eight diverse diatom species. A phylogenetic analysis of these coxI sequences including representative species of the Phaeophyta, Xanthophyta, Eustigmatophyta and Haptophyta showed that the diatoms (Bacillariophyta) formed a well-supported monophyletic group. Of the eight species investigated, four have been classified together as radial centric diatoms based on morphology. However, in our coxI tree, the two radial centrics belonging to the order Thlassiosirales (Skeletonema costatum and Thalassiosira nordenskioldii) were placed as the sister group to the multipolar centric diatoms, while the other two radial centrics (Melosira ambigua and Rhizosolenia setigera) were in another clade. Also, in two species of the Tharassiosirales we found UGA codons that occur at conserved tryptophan (Trp) sites in the coxI sequences, strongly indicating that UGA codes for Trp in these diatoms. No evidence of a deviant genetic code was detected in the other analyzed diatom species. There was no apparent relationship between the nucleotide third-position GC content of mtDNA (based on the sequenced coxI region) and the presence of a deviant genetic code.