Distribution of the mitochondrial deviant genetic code AUA for methionine in heterokont algae

The DNA sequence of the cytochrome oxidase subunit I (COXI) gene (1059 bp), was determined in a number of heterokont algae, including five species of the Phaeophyceae [Chorda filum (Linnaeus) Stackhouse, Colpomenia bullosa (Saunders) Yamada, Ectocarpus sp., Pseudochorda nagaii (Tokida) Inagaki, Undaria pinnatifida (Harvey) Suringar], and a member of the Raphidophyceae [Chattonella antiqua (Hada) Ono]. The distribution of a deviant mitochondrial code, the AUA codon for methionine (AUA/Met), which was previously reported in the Xanthophyceae, was inferred from these COXI sequences. Comparative analyses of these sequences revealed that all the algae described above bear the universal genetic code, including the assignment for the AUA codon. A phylogenetic tree was constructed using the obtained sequences along with already-published COXI sequences of various heterokont algae. The clusters of the Xanthophyceae and the Phaeophyceae were resolved as sister groups with high bootstrap support, excluding a bacillariophycean species, a raphidophycean species, and three species of the Eustigratophyceae. Taking the distribution of the deviant code and the COXI phylogenetic tree together, the genetic code change most probably occurred in an ancestor of the Xanthophyceae after it had branched off from the Phaeophyceae.