Organellar Genome Analysis of the Red Alga Acrosorium flabellatum (Rhodophyta, Ceramiales, Delesseriaceae): Evidence of Gene Duplication and Group II Intron in cox1 Gene

Abstract

Mitochondrial genomes (mitogenome) offer important information about evolutionary history, genomic diversity, and structural dynamics of Rhodophytes. The complete mitochondrial genome of Acrosorium flabellatum Yamada, 1930 (Delesseriaceae, Ceramiales), is reported in this study. The circular mitogenome spans 31,818 bp and has a biased A+T (78.8%) composition. The mitogenome contains 61 genes, including two rRNAs, 33 tRNAs, 24 protein-coding genes (PCGs), and two open reading frames (ORFs). Notably, the mitogenome contains several duplicated genes (nad6, sdh3, sdh2) and a Group II intron within the cox1 gene, suggesting continued intron mobility and structural expansion within Delesseriaceae. Codon usage indicated a strong preference for AT-rich codons, and intergenic regions were compact. A majority of PCGs showed positive GC skewness and negative AT skewness, reflecting conserved nucleotide bias across Ceramiales. Comparative analysis within the Ceramiales revealed lineage-specific differences in genome size, gene arrangement, gene content, with A. flabellatum revealing clear features of duplication and gene loss (sdh4, rps3). Several structural rearrangements, including gene order, presence of the Group II intron and ORFs, further support dynamic mitogenome evolution within Delesseriaceae. Phylogenetic analysis based on 23 conserved PCGs strongly supported the monophyly of Ceramiales and placed A. flabellatum within the Delesseriaceae clade with high bootstrap support. Overall, this study enriches the genomic resources of red algae, and sheds light on the comparative patterns of mitogenome structure within the Ceramiales. The mitogenome of A. flabellatum shows both conserved and dynamic features within this diverse red algal lineage.