Phylogenetic Analyses Of Complete Mitochondrial Genome Of Urechis Unicinctus (Echiura) Support That Echiurans Are Derived Annelids

Due to its many advantages over other molecular makers, mitochondrial genome has become a powerful tool for metazoan evolutionary history research. Traditionally, echiurans (spoon worms and innkeepers) form a minor phylum, characterized as unsegmented, coelomate, and bilaterally symmetrical invertebrates, and comprise about 150 species. Relationship between Echiura and Annelida is a question on hot debate. Many evidence, including morphological and molecular data, show that Echiura is likely derived from Annelida. Here we present the complete mitochondrial genome of an innkeeper worm, Urechisi unicinctus. This circular mtDNA is 15,761 bp in length and 61.9% in A+T composition, containing as the same typical 13 typical protein coding genes (PCGs), 22 tRNA genes, and 2 ribosomal RNA genes as found in other metazoan animals. All 37 genes are encoded on the same strand, and Echiura shared three gene arrangement blocks with the Annelida animals that has been studied. Phylogenetic analyses based on 12concatenated amino acid data strongly support the inclusion of Echiura within Annelida. Thus, many characteristics formerly hypothesized to separate echiurans from annelids, especially the segmentation characteristic, should be reevaluated.