Linear Mitochondrial genome in Anthozoa (Cnidaria): A case study in Ceriantharia

Sequences and structural attributes of mitochondrial genomes have played a key role in the clarification of relationships among Cnidaria, a key phylum of early-diverging animals. Among the major lineages of Cnidaria, Ceriantharia ("tube anemones") remains one of the most enigmatic groups in terms of its phylogenetic position. We sequenced the mitochondrial genomes of two ceriantharians to see whether the complete organellar genome would provide more support for the phylogenetic placement of Ceriantharia. For both ceriantharian species studied, the mitochondrial gene sequences could not be assembled into a circular genome. Instead, our analyses suggest both species have fragmented mitochondrial genomes consisting of multiple linear fragments. Linear mitogenomes are characteristic of members of Medusozoa, one of the major lineages of Cnidaria, but are unreported for Anthozoa, which includes the Ceriantharia. The number of fragments and the variation in gene order between species is much greater in Ceriantharia than among Medusozoa. The novelty of the mitogenomic structure in Ceriantharia highlights the distinctiveness of this lineage but, because it appears to be both unique to and diverse within Ceriantharia, it is uninformative about the phylogenetic position of Ceriantharia relative to other anthozoan groups.