Comparative Studies On Mitochondrial Genomes Of Anoplan Nemerteans (Nemertea:Anopla)

Mitochondrial genome data have been widely used in resolving phylogenetic relationships of various animal groups. Until now, only 14 mitochondrial genomes of the phylum Nemertea, a phylum containing about 1280 species, were submitted to GenBank. Here I report 9 new mitogenomes of Anopla nemerteans. In combination with the mitogenome data from GenBank, phylogenetic analyses are conducted for the phylum.Seven of the nine new mitochondrial genomes belong to Heternemertea, they are Baseodiscus hemprichii (15205 bp). Gogonorhynchus sp. (15334 bp). Micrura bella (16847 bp), Ramphogordius sanguineus(15322 bp), Paralineopsiscf.taki(15134 bp) Notospermus geniculatus (15053 bp) and Iwatanemertes piperata (16382 bp). All these heteronemertean mitogenomes consist of the 37 genes typical for metazoan mitogenomes, including 13 protein-coding genes,22 transfer RNA genes and two ribosome RNA genes. Six of them share the same gene order with four previously sequenced heternemertean mitogenomes, whereas in Gogonorhynchus sp., the position of two leucine tRNA genes is exchanged.The studied two palaeonemertean (Tubulanus punctutus and Hubrechtella ijimai mitogenomes are 15224 bp and 15307 bp in length, respectively. They show different pattern of gene arrangements in comparison with other known nemertean mitogenomes, and an additional lysine tRNA gene (with different anticodon loop sequences) is found in Hubrechtella ijimai.The average length of nemertean mitogenomes is 15354 bp, with hoplonemerteans presenting the smallest genomes (14558 bp to 15365 bp; mean 14793 bp), the palaeonemerteans bearing the largest ones (15224 bp to 16296; mean 15657 bp). The Heteronemertea mitogenomes hold the lowest AT content (55.68%-66.03%), and the Palaeonemertea ones hold the highest AT content (63.04%-75.78%).All the 13 protein-coding genes reveal purifying selection effect (Ka/Ks<)), which is the strongest for coxl gene and the weakest for atp8 gene. Most transfer RNA genes can be folded into cloverleaf-like structure, while some serine trns lose their DHU-arm. The anticodons of trnS(AGN) gene in hoplo-and palaeonemerteans are GCT, and it is TCT in hheteronemerteans except Baseodiscus hemprichii (GCT).As consistent with previous studies, present phylogenetic analyses support the monophyly of Heteronemertea and Hoplonemertea, and the paraphyly of Palaeonemertea. Heteronemertea and Hubrechtella form a clade (Pilidiophora) sister to Hoplonemertea.