Mitochondrial genome structure, phylogenetic analyses and substitution rate estimation of the Oedogoniales

Abstract

The order Oedogoniales comprises three genera, Oedogonium, Oedocladium and Bulbochaete, which were classified based on traditional morphological criteria, and includes more than 600 described species. This group is economically important in astaxanthin production and the energy sector. However, only one mitochondrial genome (mitogenome) has been reported so far. This study determined the mitochondrial genomes of seven Oedogoniales species, including six Oedogonium species and Oedocladium prescottii. Comparative analyses between the newly determined mitogenomes and the previously reported Bulbochaete rectangularis var. hiloensis mitogenome showed that all eight mitogenomes comprised 12 protein-coding genes (PCGs) and two rRNAs; however, the mitogenomes differed in their genome sizes, GC content, tRNAs, non-coding regions and introns. Synteny analysis of the eight mitogenomes revealed a high degree of syntenic conservation in general, with some rearrangements and inversions. The average nucleotide identity (ANI) analysis of the eight mitogenomes indicated Oedogonium dentireticulatum showed high similarity with Oedogonium sp3 (ANI of 96.32%). Most of the PCGs of the eight mitogenomes presented the conventional start codon ATG and stop codon TAR (TAA/TAG/TGA), and the synonymous codon preferences were conserved. Phylogenetic results indicated that Oedogonium was polyphyletic, and species of Oedocladium clustered with Oedogonium, while the position of B. rectangularis var. hiloensis was uncertain for the incongruent phylogenetic results. Statistical analyses of substitution rates demonstrated no significant differences among the three genera, and the dN/dS ratios based on branch model showed that cob, cox1 and nad4 of Oedocladium prescottii and B. rectangularis var. hiloensis were putative fast-evolving genes. These findings suggested that the traditional taxonomy of Oedogoniales did not define natural groups, and that species of Oedocladium and Bulbochaete may have undergone rapid evolution.