Applicability of Mitochondrial Genome Data to Annelid Phylogeny and the Evolution of Group II Introns

Abstract

Annelida is a very diverse group of segmented worms with over 16,500 described species. They play an important role in both terrestrial and aquatic environments. Despite this, their phylogenetic and evolutionary relationships of many groups within annelids are still poorly understood. This study focused on Terebelliformia annelids, a group of tubedwelling worms used for investigation, comprises five recognized families with ambiguous phylogeny and a myzostomid worm, whose annelid affinity has been debated in recent years. In view of the conserved composition across bilaterians and the hypothesis of conserved gene order pattern across annelids, the mitochondrial genomic data have been becoming increasingly useful for applications to resolve this issue. The main aim of this research is to characterize the mtDNA genomes in above annelids and resolve certain relationships within annelids with mitochondrial genome to evaluate their applicability to annelid phylogeny. Additionally, two group II introns that unexpectedly discovered in the myzostomid mtDNA drew our attentions to explore the evolution of such introns as they are rarely found in bilaterian genomes. This study showed that the mitochondrial gene arrangement pattern is evolutionarily conserved as previously hypothesized, especially for protein-coding genes. Phylogenetic analyses based on the mitochondrial genome data indicated a well-resolved phylogeny within Terebelliformia group: Pectinariidae was placed as a basal clade to all other Terebelliformia families; Ampharetidae and Alvinellidae were sister to each other; Trichobranchidae and Terebellidae were sister clade with strong support. This suggests the great potential applicability of mitochondrial genomes which could likely be applied to the phylogenetic reconstruction of other annelid clades. Two group II introns (divergent Mintron1 and degenerated Mintron2) which are characterized here in a partial mitochondrial genome of Endomyzostoma sp. (Myzostomida), is the first report of multiple introns in bilaterian genomes. The study implicated that both introns belong to the mitochondrial class and they could have independent origins given the dissimilarity between their RNA structures. It offers an important basis for the future studies in regard to the evolution and function of bilaterian group II introns. Overall, the study implies an increasingly potential applicability to explore the mitochondrial genomes of annelids in terms of the phylogenetic and evolutionary examinations.