Evolutionary implications of analyses of complete mitochondrial genomes across order Zoantharia (Cnidaria: Hexacorallia)

Cnidarians are early-diverging metazoans, but evolutionary aspects of some taxa are still poorly understood, as in the order Zoantharia (Anthozoa: Hexacorallia). Zoantharians have been divided into two suborders based on the arrangement of the fifth septae as complete (Macrocnemina) or incomplete (Brachycnemina). Previous molecular phylogenetic analyses have indicated the need for re-evaluation as Macrocnemina has been found to be paraphyletic. Despite many phylogenetic studies, the recovery of complete mitochondrial genomes (mt-genomes) for systematic and evolutionary studies of zoantharians has been limited. The present study represents the first to sequence the complete mt-genomes of members of eight of nine zoantharian families. Although all examined mt-genomes had the same gene order arrangement, there were variations among mt-genomes' sizes, nucleotide substitution rates, and introns. Only two species did not have thecox1intron, which harbors a gene coding a homing endonuclease of the LAGLIDADG type. Our mitogenomic analyses also showed relatively high nucleotide diversity in mt-DNA regions other than the standard regions traditionally considered for DNA barcoding of this group. Phylogenetic analyses using 13 mt-genome protein-coding genes recovered a fully resolved tree with clear separation between macrocnemic representatives. Ancestral state reconstruction analyses revealed three main transitions in arrangement of the marginal musculature through the evolutionary history of the order. An early transition from reticulate mesogleal to a cteniform endodermal arrangement was followed by transitions that occurred in the common ancestor of the Brachycnemina and family Hydrozoanthidae. Our results indicate the need for clarification of higher-level phylogeny and taxonomy of Zoantharia.