To gain further insight into the evolution of mitochondrial genomes (mtDNAs) in Phaeophyceae, the first recorded characterization of an Ishigeophycidae mtDNA from Ishige okamurae (Yendo), and only the second recorded characterization of a Dictyotophycidae mtDNA from Dictyopteris divaricata (Okamura) Okamura are presented in this study. The 35,485 bp I. okamurae mtDNA contained 36 protein-coding genes (PCGs), 22 tRNAs, three rRNAs, and four open reading frames (orfs), and the 32,021 bp D. divaricata mtDNA harbored 35 PCGs, 25 tRNAs, three rRNAs, and three orfs. The A + T content in D. divaricata (61.69%) was the lowest recorded in sequenced brown algal mtDNAs. The I. okamurae mtDNA displayed unique genome features including an elevated start-codon usage bias for GTG, while the organization of D. divaricata mtDNA was identical to that of Dictyota dichotoma. Phylogenetic analysis based on the amino acid sequence dataset of 35 PCGs indicated that I. okamurae (Ishigeophycidae) diverged early from the Fucophycidae-Dictyotophycidae complex, which was confirmed by the comparative analysis of the mitogenome structure. The novel mitogenome data made available by this study have improved our understanding of the evolution, phylogenetics, and genomics of brown algae.
Keywords: Brown algae; Dictyotales; Evolution; Ishigeales; Mitochondrial genome; Phaeophyceae.