Characterization of four mitochondrial genomes of family Neritidae (Gastropoda: Neritimorpha) and insight into its phylogenetic relationships.

Neritidae is one of the most diverse families of Neritimorpha and possesses euryhaline properties. Members of this family usually live on tropical and subtropical coasts and are mainly gregarious. The phylogenetic relationships between several subclasses of Gastropoda have been controversial for many years. With an increase in the number of described species of Neritidae, the knowledge of the evolutionary relationships in this family has improved. In the present study, we sequenced four complete mitochondrial genomes from two genera (Clithon and Nerita) and compared them with available complete mitochondrial genomes of Neritidae. Gene order exhibited a highly conserved pattern among three genera in the Neritidae family. Our results improved the phylogenetic resolution within Neritidae, and more comprehensive taxonomic sampling of subclass Neritimorpha was proposed. Furthermore, we reconstructed the divergence among the main lineages of 19 Neritimorpha taxa under an uncorrelated relaxed molecular clock.

Comparative analysis of the complete mitochondrial genomes in two limpets from Lottiidae (Gastropoda: Patellogastropoda): rare irregular gene rearrangement within Gastropoda.

To improve the systematics and taxonomy of Patellogastropoda within the evolution of gastropods, we determined the complete mitochondrial genome sequences of Lottia goshimai and Nipponacmea fuscoviridis in the family Lottiidae, which presented sizes of 18,192 bp and 18,720 bp, respectively. In addition to 37 common genes among metazoa, we observed duplication of the trnM gene in L. goshimai and the trnM and trnW genes in N. fuscoviridis. The highest A + T contents of the two species were found within protein-coding genes (59.95% and 54.55%), followed by rRNAs (56.50% and 52.44%) and tRNAs (56.42% and 52.41%). trnS1 and trnS2 could not form the canonical cloverleaf secondary structure due to the lack of a dihydrouracil arm in both species. The gene arrangements in all Patellogastropoda compared with those of ancestral gastropods showed different levels of gene rearrangement, including the shuffling, translocation and inversion of single genes or gene fragments. This kind of irregular rearrangement is particularly obvious in the Lottiidae family. The results of phylogenetic and gene rearrangement analyses showed that L. goshimai and Lottia digitalis clustered into one group, which in turn clustered with N. fuscoviridis in Patellogastropoda. This study demonstrates the significance of complete mitogenomes for phylogenetic analysis and enhances our understanding of the evolution of Patellogastropoda.

Genetic Population Structure of Macridiscus multifarius (Mollusca: Bivalvia) on the Basis of Mitochondrial Markers: Strong Population Structure in a Species with a Short Planktonic Larval Stage.

The clam Macridiscus multifarius with a planktonic larval stage of about 10 days is an ecologically and economically important species in the coastal regions of China. In this study, 3 mt-DNA markers (COI, 12S rRNA, and ND1) were used to investigate the population structure and demography of wild M. multifarius populations in 3 coastal localities of the East China Sea (ZS and ZP populations) and Beibu Gulf in the South China Sea (BH population). Sequences of 685 bp in COI, 350 bp in 12S rRNA, and 496 bp in ND1 were determined. High level and significant FST values were obtained among the different localities on the basis of either COI (FST = 0.100-0.444, p < 0.05) or 12S rRNA (FST = 0.199-0.742, p < 0.05) gene, indicating a high degree of genetic differentiation among the populations. FST values were significant but weak for the ND1 gene because it is highly conservative. The median-joining network suggested an obvious genetic differentiation between ZS and BH populations, and the finding is consistent with the results of our demographic analyses using the unweighted pair group method with arithmetic mean. Our study unraveled the extant population genetic structure of M. multifarius and explained the strong population structure of a species with a short planktonic larval stage species; this information could be useful for fishery management measures, including artificial breeding and conservation.