The complete mitochondrial genome of Galba pervia (Gastropoda: Mollusca), an intermediate host snail of Fasciola spp

Abstract

Complete mitochondrial (mt) genomes and the gene rearrangements are increasingly used as molecular markers for investigating phylogenetic relationships. Contributing to the complete mt genomes of Gastropoda, especially Pulmonata, we determined the mt genome of the freshwater snail Galba pervia, which is an important intermediate host for Fasciola spp. in China. The complete mt genome of G. pervia is 13,768 bp in length. Its genome is circular, and consists of 37 genes, including 13 genes for proteins, 2 genes for rRNA, 22 genes for tRNA. The mt gene order of G. pervia showed novel arrangement (tRNA-His, tRNA-Gly and tRNA-Tyr change positions and directions) when compared with mt genomes of Pulmonata species sequenced to date, indicating divergence among different species within the Pulmonata. A total of 3655 amino acids were deduced to encode 13 protein genes. The most frequently used amino acid is Leu (15.05%), followed by Phe (11.24%), Ser (10.76%) and IIe (8.346%). Phylogenetic analyses using the concatenated amino acid sequences of the 13 protein-coding genes, with three different computational algorithms (maximum parsimony, maximum likelihood and bayesian analysis), all revealed that the families Lymnaeidae and Planorbidae are closely related two snail families, consistent with previous classifications based on morphological and molecular studies. The complete mt genome sequence of G. pervia showed a novel gene arrangement and it represents the first sequenced high quality mt genome of the family Lymnaeidae. These novel mtDNA data provide additional genetic markers for studying the epidemiology, population genetics and phylogeographics of freshwater snails, as well as for understanding interplay between the intermediate snail hosts and the intra-mollusca stages of Fasciola spp..

Figure 1. The mitochondrial genome of Galba pervia.

Gene scaling is only approximate. All genes have standard nomenclature including the 22 tRNA genes, which are designated by the one-letter code for the corresponding amino acid, with numerals differentiating each of the two leucine- and serine-specifying tRNA (L1 and L2 for codon families CUN and UUR, respectively; S1 and S2 for codon families AGN and UCN, respectively).

Figure 2. Comparison of the mitochondrial gene arrangement between Galba pervia and Radix balthica.

All genes have standard nomenclature including the 22 tRNA genes, which are designated by the one-letter code for the corresponding amino acid, with numerals differentiating each of the two leucine- and serine-specifying tRNA (L1 and L2 for codon families CUN and UUR, respectively; S1 and S2 for codon families AGN and UCN, respectively). Underlined genes are coded on the light strand.

Figure 3. Inferred phylogenetic relationship among Pulmonata species based on mitochondrial DNA sequences.

The concatenated amino acid sequences of 13 protein-coding genes were analyzed utilizing maximum parsimony (MP), maximum likelihood (ML) and Bayesian analysis (Bayes), using Aplysia californica and A. dactylomela as outgroup. The numbers along branches indicate bootstrap probability (BP) and posterior probability (PP) resulting from different analyses in the order: MP/ML/Bayes. Alternative topologies were correspondingly shown on the downside of the tree. The colourred species indicated that the phylogenetic status of the family Ellobiidae remains problematic.