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 investigat e possible changes in their mitochondrial (mt) genetic codes, we have analy zed a 1.1-kb region of the cytochrome c oxidase subunit I (coxI) gene from eight diverse diatom species. A phylogenetic analysis of these co,vr sequen ces including representative species of the Phaeophyta, Xanthophyta, Eustig matophyta 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 morpholog y. However, in our coxI tree, the two radial centrics belonging to the orde r Thlassiosirales (Skeletonema costatum and Thalassiosira nordenskioldii) w ere placed as the sister group to the multipolar centric diatoms, while the other two radial centrics (Melosira ambigua and Rhizosolenia setigera) wer e in another clade. Also, in two species of the Tharassiosirales we found U GA codons that occur at conserved tryptophan (Trp) sites in the coxI sequen ces, strongly indicating that UGA codes for Trp in these diatoms. No eviden ce of a deviant genetic code was detected in the other analyzed diatom spec ies. There was no apparent relationship between the nucleotide third-positi on GC content of mtDNA (based on the sequenced coxI region) and the presenc e of a deviant genetic code.