The complete mitochondrial genome of Calicogorgia granulosa (Anthozoa: Octocorallia): potential gene novelty in unidentified ORFs formed by repeat expansion and segmental duplication.

Mitochondrial genomes of many nonbilaterian animals show high diversity of genome size and gene content, revealing many intergenic regions (IGRs), diverse repeats and additional genes. Here we present a new complete mitogenome of the cnidarian sea fan species, Calicogorgia granulosa (Anthozoa: Octocorallia) and its novel genomic features. The 20,246 bp of the complete mitogenome, which is the largest among the nine octocorals sequenced to date, contains 13 protein coding genes, 2 rRNAs and a tRNA within its circular form of mitochondrial DNA. We found an identical segmental duplication (S1 and S2, 913 bp) composed of an ORF (672 bp) coding for a hypothetical protein within which Direct Variant Repeat (DVR) expansions reside in-frame to the coding sequence. Additionally, the duplicated segmental DNA showed no variation in nucleotide sequences both between S1 and S2 and across multiple individual samples. Upon these observations, we discuss plausible causes for the intramitochondrial segmental duplication and the absence of sequence variation, and a need for further investigation of the novel ORF as well. In conclusion the present mitogenome of C. granulosa adds more information to our understanding of the diversity and evolution of mitogenomes of nonbilaterian animals.