The mitochondrial genome of <i>Palythoa tuberculosa</i> (Esper, 1805) contains a novel ORF with unknown function

Recent advances in sequencing technology have enabled rapid and cost-effective recovery of mitochondrial genomes (mitogenomes), which are used in phylogenetic and evolutionary studies. Among cnidarians, the order Zoantharia possesses one of the largest mitogenomes on average 20,976 bp. <i>Palythoa tuberculosa</i> (Hexacorallia: Zoantharia: Sphenopidae) has the largest reported mitogenome in this order (23,044 bp), approximately 2,000 bp larger than the zoantharian average. In this study, to determine the basis of this expansion, we generated and annotated the complete mitogenome of <i>P. tuberculosa</i>. Synteny analysis revealed that this increased genome size is attributed to the insertion of a novel open reading frame (ORF) located between <i>trnW</i> and <i>cox2</i>, encoding 665 amino acids, initiated with the codon ATG and terminated with TAA. This ORF was also present in the closely related species <i>P. caribaeorum</i>, suggesting its origin in their common ancestor. This represents the first case of a novel ORF insertion in zoantharians and demonstrates that mitogenome expansion in zoantharians can result from lineage-specific protein-coding insertions, revealing greater structural dynamism in anthozoan mitogenomes than previously recognized.