Leeches (Clitellata: Hirudinida) are hermaphrodite worms and abundant predators or ecto-parasites in various freshwater habitats. Many leeches have a significant adverse impact on freshwater culture and animal health. Erpobdella octoculata Linnaeus, 1758 (Hirudinea: Arhynchobdellida: Erpobdelliformes) is one of the most common leech species in freshwaters. Its assemblage typically dominates the lowlands in lake, streams and rivers as well as in urban park ponds. Despite widely distributed, the current genomic knowledge of leeches is scant and taxon diversity is poorly constructed.
Animal mitochondrial DNA (mtDNA) is a single circular duplex molecule typically coding for 13 protein-coding genes, 2 rRNA genes and 22 tRNA genes. The mitochondrial genome has been extensively used to study the phylogenetic relationships at several taxonomic levels. Compared to other metazoan animals, only five complete mt genome sequences of leech species for the suborder Hirudiniformes have been sequenced and deposited in GenBank to date, and no mt genome for the suborder Erpobdelliformes has been reported. In this study, we sequenced the complete mitochondrial (mt) genomes of E. octoculata, analysized its mitochondrial gene arrangements, gene compositions, translation and initiation codons and codon usage as well as the structure of rRNA and tRNA genes. In addition, we used the currently available nucleotide sequences to reconstruct the phylogenetic relationship among the included annelids. The new mt genome sequence may provide novel and useful mtDNA markers for investigating genetic composition of suborder Erpobdelliformes, as it is the first reported complete mtDNA sequence in this suborder.
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