New mitochondrial gene order arrangements and evolutionary implications in the subclass Octocorallia

The complete mitochondrial genomes of octocorals typically range from 18.5 kb to 20.5 kb in length, and include 14 protein coding genes (PCGs), two ribosomal RNA genes and one tRNA. To date seven different gene orders (A-G) have been described, yet comprehensive investigations of the actual number of arrangements, as well as comparative analyses and evolutionary reconstructions of mitochondrial genome evolution within the whole subclass Octocorallia have been often overlooked. Here we considered the complete mitochondrial genomes available for octocorals and explored their structure and gene order variability. Our results updated the actual number of mitochondrial gene order arrangements so far known for octocorals from seven to twelve, and allowed us to explore and preliminarily discuss the role of some of the structural and functional factors in the mitogenomes. We performed comparative mitogenomic analyses on the existing and novel octocoral gene orders, considering different mitogenomic structural features such as genome size, GC percentage, AT- and GC-skewness. The mitochondrial gene order history mapped on a recently published nuclear loci phylogeny showed that the most common rearrangement events in octocorals are inversions, and that the mitochondrial genome evolution in the subclass is discontinuous, with rearranged gene orders restricted only to some regions of the tree. We believe that different rearrangement events arose independently and most likely that new gene orders, instead of being derived from other rearranged orders, came from the ancestral and most common gene order. Finally, our data demonstrate how the study of mitochondrial gene orders can be used to explore the evolution of octocorals and in some cases can be used to assess the phylogenetic placement of certain taxa.