title: The complete mitochondrial genomes of two freshwater snails provide new protein-coding gene rearrangement models and phylogenetic implications.
abstract: BACKGROUND
  Mitochondrial (mt) genome sequences are widely used for species identification and to study the phylogenetic relationships among Gastropoda. However, to date, limited data are available as taxon sampling is narrow. In this study we sequenced the complete mt genomes of the freshwater gastropods Radix swinhoei (Lymnaeidae) and Planorbarius corneus (Planorbidae). Based on these sequences, we investigated the gene rearrangement in these two species and the relationships with respect to the ancestral gene order and assessed their phylogenetic relationships.
  
  
  METHODS
  The complete mt genomes of R. swinhoei and P. corneus were sequenced using Illumina-based paired-end sequencing and annotated by comparing the sequence information with that of related gastropod species. Putative models of mitochondrial gene rearrangements were predicted for both R. swinhoei and P. corneus, using Reishia clavigera mtDNA structure as the ancestral gene order. The phylogenetic relationships were inferred using thirteen protein sequences based on Maximum likelihood and Bayesian inference analyses.
  
  
  RESULTS
  The complete circular mt genome sequences of R. swinhoei and P. corneus were 14,241 bp and 13,687 bp in length, respectively. Comparison of the gene order demonstrated complex rearrangement events in Gastropoda, both for tRNA genes and protein-coding genes. The phylogenetic analyses showed that the family Lymnaeidae was more closely related to the family Planorbidae, consistent with previous classification. Nevertheless, due to the position recovered for R. swinhoei, the family Lymnaeidae was not monophyletic.
  
  
  CONCLUSIONS
  This study provides the complete mt genomes of two freshwater snails, which will aid the development of useful molecular markers for epidemiological, ecological and phylogenetic studies. Additionally, the predicted models for mt gene rearrangement might provide novel insights into mt genome evolution in gastropods.
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Xidong
authors.initials: X
authors.lastname: Mu
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Yexin
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Du
authors.initials: D
authors.lastname: Luo
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Meng
authors.initials: M
authors.lastname: Xu
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Wei
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Dangen
authors.initials: D
authors.lastname: Gu
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China.
authors.forename: Hongmei
authors.initials: H
authors.lastname: Song
authors.affiliation: Key Laboratory of Tropical&Subtropical Fishery Resource Application & Cultivation, Ministry of Agriculture, Pearl River Fisheries Research Institute, Chinese Academy of Fishery Sciences, Xingyu Road1, Guangzhou, 510380, China. huyc22@163.com.
authors.forename: Yinchang
authors.initials: Y
authors.lastname: Hu
keywords: Gene order; Mitochondrial genome; Phylogeny; Pulmonate