Sequence comparison of the mitochondrial genomes of five brackish water species of the family Neritidae: Phylogenetic implications and divergence time estimation.

Neritids are ancient gastropod species which can live in marine, brackish water, and freshwater environments. In this study, we sequenced and annotated the mitochondrial genomes of five brackish water neritids (i.e., Clithon corona, Clithon lentiginosum, Clithon squarrosum, Neritina iris, and Septaria lineata). The mitogenomes ranged from 15,618 to 15,975 bp, and all contain 13 protein-coding genes (PCGs), 22 tRNA genes, and two rRNA genes, with a closed ring structure. We calculated the Ka/Ks values of all 13 PCGs of Neritidae species, all ratios are less than 1, under purification selection. Phylogenetic analysis of the 13 PCGs showed that Neritimorpha is a sister group with Vetigastropoda and Caenogastopoda, genus Clithon is a sister group with Neritina and Septaria. Estimation of divergence time for all species of Neritidae showed that the main differentiation of Neritidae occurred in Cenozoic period (65 Mya), C. corona and C. lentiginosum were differentiated in the Cenozoic Neogene, the other three species diverged in the Cenozoic Paleogene. These results will help to better understand the evolutionary position of Neritidae and provide reference for further phylogenetic research on Neritidae species.

Insights into the Deep Phylogeny and Novel Divergence Time Estimation of Patellogastropoda from Complete Mitogenomes.

To further understand the origin and evolution of Patellogastropoda, we determined the mitochondrial genome sequence of Cellana toreuma, and compared its mitogenome characteristics with the other four limpets of Nacellidae. The ratio of Ka and Ks indicated that these Nacellidae species were suffering a purifying selection, with exception of the atp6 gene. The gene sequence is basically consistent among families, while there are great differences among Lottidae species. According to the mitogenome sequences of selected gastropod species, we reconstructed a new phylogenetic tree with two methods. The data complement the mitogenome database of limpets and is a favorable research tool for the phylogenetic analysis of Gastropoda. It is found that there is a long-branch attraction (LBA) artefact in the family Lottiidae of Patellogastropoda. Therefore, the Patellogastropoda was separated by Heterobranchia, and Lottiidae is located at the root of the whole phylogenetic tree. Furthermore, we constructed the divergence time tree according to the Bayesian method and discussed the internal historical dynamics, and divergence differences among the main lineages of 12 Patellogastropoda under an uncorrelated relaxed molecular clock. In turn, we made a more comprehensive discussion on the divergence time of limpets at the molecular level.

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.

Novel gene rearrangement in the mitochondrial genome of Siliqua minima (Bivalvia, Adapedonta) and phylogenetic implications for Imparidentia.

Siliqua minima (Gmelin, 1791) is an important economic shellfish species belonging to the family Pharidae. To date, the complete mitochondrial genome of only one species in this family (Sinonovacula constricta) has been sequenced. Research on the Pharidae family is very limited; to improve the evolution of this bivalve family, we sequenced the complete mitochondrial genome of S. minima by next-generation sequencing. The genome is 17,064 bp in length, consisting of 12 protein-coding genes (PCGs), 22 transfer RNA genes (tRNA), and two ribosomal RNA genes (rRNA). From the rearrangement analysis of bivalves, we found that the gene sequences of bivalves greatly variable among species, and with closer genetic relationship, the more consistent of the gene arrangement is higher among the species. Moreover, according to the gene arrangement of seven species from Adapedonta, we found that gene rearrangement among families is particularly obvious, while the gene order within families is relatively conservative. The phylogenetic analysis between species of the superorder Imparidentia using 12 conserved PCGs. The S. minima mitogenome was provided and will improve the phylogenetic resolution of Pharidae species.

Characterization of the complete mitochondrial genomes of two species of Penaeidae (Decapoda: Dendrobranchiata) and the phylogenetic implications for Penaeoidea.

In the present study, mitogenomes of the species Trachypenaeus curvirostris and Parapenaeus fissuroides (Decapoda: Dendrobranchiata: Penaeidae) were sequenced. The total lengths of the two species were 15,956 bp and 15,937 bp in length with A + T biases of 67.08% and 67.69%, respectively. Both two species showed positive AT skews (0.016, 0.058) and negative GC skews (-0.254, -0.310). Both mitogenomes contained 13 protein-coding genes, 22 transfer RNA genes, and 2 ribosomal RNA genes. Results of phylogenetic analyses support close relationships among Aristeidae, Benthesicymidae and Solenoceridae. The family Sicyoniidae was observed to be deeply nested within Penaeidae. Within Penaeidae, T. curvirostris and P. fissuroides were most closely related to the genus Parapenaeopsis and Metapenaeopsis, respectively, indicated that these two species belong to Penaeidae. These results will help to better understand the evolutionary position of Penaeidae and provide reference for further phylogenetic research on Penaeoidea species.

The complete mitochondrial genome and phylogenetic analysis of Batillaria zonalis (Gastropoda: Batillariidae).

We determined the complete mitochondrial genome of Batillaria zonalis. The B. zonalis mitochondrial genome is 15748 bp in length, comprising 13 protein-coding genes, 22 transfer RNA genes, and two ribosomal RNA genes. The nucleotide composition for B. zonalis is 17.71% of C, 16.74% of G, 34.99% of T, and 30.3% of A. In 13 protein-coding genes, all genes start with ATG. For the stop codon, the cox2 gene stops with TTC, the cytb, nad1, nad2 genes stop with TAG, the other nine genes are with TAA. Of these 37 genes identified, nine protein-coding genes and six transfer RNA genes are encoded on the heavy strand and the other genes on the light strand. The phylogenetic tree was constructed based on 13 protein-coding genes of the B. zonalis and other 19 Gastropoda species, Sepia latimanus as outgroup using the Neighbour-joining method. The result showed that B. zonalis is most closely related to the Tylomelania sarasinorum in Cerithioidea. We believe that this result will be helpful for the study of population genetic and phylogenetic analysis of the family Batillariidae.

The complete mitochondrial genome and phylogenetic analysis of Littorina brevicula (Gastropoda, Littorinidea).

The complete mitochondrial genome of the Littorina brevicula was determined in this study. The complete mitogenome (mtDNA) is 16,356 base pairs (bp) in length and contains 13 protein-coding genes, two rRNA genes, and 22 tRNA genes. The control region was divided into two parts. The overall base composition of the genome in descending order was 35.33%-T, 28.41%-A, 20.43%-C, and 15.81%-G. In 13 protein-coding genes, 12 genes start with ATG, except nad5 starts with ATT. For the stop codon, seven genes end with TAA, atp6, nad41, and nad3 end with TAG. Phylogenetic analysis indicated that L. brevicula is close to Naticidea family. This study first determined the complete mitochondrial genome of L. brevicula. It would be a supplement for the genetic analysis of L. brevicula and promote the phylogenetic of Littorinidea.

The complete mitochondrial genome and phylogenetic analysis of Omphalius rusticus (Gastropoda, Tegulidae).

The complete mitochondrial genome of the Omphalius rusticus has been determined. The complete genome is 18,067 bp and contained 13 protein-coding genes, two rRNA genes and 22 tRNA genes. The overall base composition is 33.48% (A), 33.52% (T), 15.58% (G) and 17.42% (C). The all start codon for 13 protein-coding genes is ATG and the most common termination codon is TAA. The phylogenetic tree showed that O. rusticus is most closely related to the Tectus pyramis. We suggest that this result will further supplement the genome information in mitochondria of the family Tegulidae and facilitate the study on population genetics.

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.

Sequence comparison of the mitochondrial genomes in two species of the genus Nerita (Gastropoda: Neritimorpha: Neritidae): phylogenetic implications and divergence time estimation for Neritimorpha.

Many Nerita species live in warm-water environments, and they are some of the few organisms from the intertidal zone that can live in both freshwater and seawater. Previous comparative studies of the mitogenomes of Nerita species suggest that the genome rearrangements are very conservative. Generally, the species possess a set of similar mitochondrial gene arrangements, but nucleotide sequences can be used to elucidate phylogenetic relationships at various levels of divergence. Here, the mitogenomes of Nerita undata and Nerita balteata were sequenced and found to be 15,583 bp and 15,571 bp, respectively. The mitogenomes of both species contain 13 protein-coding genes, 22 tRNA genes, and two rRNA genes. The nucleotides of the two mitogenomes are highly similar, with the same gene composition and genomic organization as those present in other Nerita species. The tRNA secondary structures were different from those of other gastropods: trnS2 is not folded into typical secondary structures, and the dihydrouridine (DHU) arm simply forms a loop. The phylogenetic analysis showed that Neritimorpha is a sister group of Vetigastropoda and Caenogastropoda. Nerita balteata is a sister group of Nerita versicolor and Nerita undata, and all three species belong to Neritimorpha. This study contributes towards the comparative mitogenomic analysis of Neritidae and phylogenetic considerations among Neritimorpha species. The estimation of divergence time revealed that the two Nerita species were differentiated in the late Paleogene of the Cenozoic Era, and their evolution may be related to environmental changes.

Strong genetic differentiation of the clam Meretrix lamarckii in the China Sea revealed by mitochondrial DNA marker.

The hard clam Meretrix lamarckii is ecologically and economically important in the coastal regions of China. In this study, we evaluated the genetic diversity and population structure of six M. lamarckii populations in the East China Sea and the South China Sea using mitochondrial cytochrome c oxidase subunit 1 (COI) and cytochrome b (Cytb) genes. We obtained 582 bp of partly sequences comprising 28 novel haplotypes of COI gene from 138 specimens and 1168 bp of partly sequences comprising 22 novel haplotypes of Cytb gene from 125 specimens. The haplotype diversity of COI and Cytb genes ranged from 0.606 to 0.862 and 0.562 to 0.863, respectively. The nucleotide diversity ranged from 0.0015 to 0.0038 in COI gene and ranged from 0.0007 to 0.0032 in Cytb gene. Thus, there is moderate-level genetic diversity in M. lamarckii in the China Sea. The F-statistics showed that the Zhoushan (ZS) and Xiangshan (XS) populations were significantly (P < 0.01) differed from the populations of Wenzhou (WZ), Zhangpu (ZP), Shantou (ST), and Zhanjiang (ZJ) in both COI and Cytb genes. Both haplotypes network and plot of STRUCTURE analysis suggested obviously genetic divergence between East China Sea and South China Sea regions. Knowledge on genetic variation and population structure of M. lamarckii populations along the Southeast China Sea obtained from this study will support the aquaculture management and conservation of M. lamarckii in China.

The complete mitochondrial genome and phylogenetic analysis of Batillaria cumingi (Gastropoda: Batillariidae).

We determined the complete mitochondrial genome of Batillaria cumingi. The B. cumingi mitochondrial genome is 16,100 bp in length, comprising 13 protein-coding genes, 22 transfer RNA genes, and two ribosomal RNA genes. The nucleotide composition for B. cumingi is 17.5% of C, 16.88% of G, 35.3% of T, and 30.31% of A. In 13 protein-coding genes, all genes start with ATG. For the stop codon, the cox2 gene stops with TTC, the cytb, nad1, and nad2 genes stop with TAG, and the other nine genes are with TAA. Of these 37 genes identified, nine protein-coding genes and six transfer RNA genes are encoded on the heavy strand and the other genes on the light strand. The phylogenetic tree was constructed based on 13 protein-coding genes of the B. cumingi and other 19 Gastropoda species, Sepia latimanus as outgroup using the Neighbour-joining method. The tree showed that the B. cumingi is closely related to the Semisulcospira coreana in Cerithioidea. We believe that this result will be helpful for the study of population genetic and phylogenetic analysis of the family Batillariidae.