The complete mitochondrial genome of Conus striatus (Neogastropoda: Conidae)

Mitogenome Characterization of Four Conus Species and Comparative Analysis

Cone snails, as a type of marine organism, have rich species diversity. Traditionally, classifications of cone snails were based mostly on radula, shell, and anatomical characters. Because of these phenotypic features' high population variability and propensity for local adaptation and convergence, identifying species can be difficult and occasionally inaccurate. In addition, mitochondrial genomes contain high phylogenetic information, so complete mitogenomes have been increasingly employed for inferring molecular phylogeny. To enrich the mitogenomic database of cone snails (Caenogastropoda: Conidae), mitogenomes of four Conus species, i.e., C. imperialis (15,505 bp), C. literatus (15,569 bp), C. virgo (15,594 bp), and C. marmoreus (15,579 bp), were characterized and compared. All 4 of these mitogenomes included 13 protein-coding genes, 2 ribosomal RNA genes, 22 tRNA genes, and non-coding regions. All the Protein Codon Genes (PCGs) of both newly sequenced mitogenomes used TAA or TAG as a terminal codon. Most PCGs used conventional start codon ATG, but an alternative initiation codon GTG was detected in a gene (NADH dehydrogenase subunit 4 (nad4)) of C. imperialis. In addition, the phylogenetic relationships were reconstructed among 20 Conus species on the basis of PCGs, COX1, and the complete mitogenome using both Bayesian Inference (BI) and Maximum Likelihood (ML). The phylogenetic results supported that C. litteratus, C. quercinus, and C. virgo were clustered together as a sister group (PP = 1, BS = 99), but they did not support the phylogenetic relation of C. imperialis and C. tribblei (PP = 0.79, BS = 50). In addition, our study established that PCGs and complete mitogenome are the two useful markers for phylogenetic inference of Conus species. These results enriched the data of the cone snail's mitochondrion in the South China Sea and provided a reliable basis for the interpretation of the phylogenetic relationship of the cone snail based on the mitochondrial genome.

Complete mitochondrial genome of Conus lischkeanus Weinkauff, 1875 (Neogastropoda, Conidae) and phylogenetic implications of the evolutionary diversification of dietary types of Conus species

The family Conidae, commonly known as cone snails, is one of the most intriguing gastropod groups owing to their diverse array of feeding behaviors (diets) and toxin peptides (conotoxins). Conuslischkeanus Weinkauff, 1875 is a worm-hunting species widely distributed from Africa to the Northwest Pacific. In this study, we report the mitochondrial genome sequence of C.lischkeanus and inferred its phylogenetic relationship with other Conus species. Its mitochondrial genome is a circular DNA molecule (16,120 bp in size) composed of 37 genes: 13 protein-coding genes (PCGs), 22 transfer RNA genes, and two ribosomal RNA genes. Phylogenetic analyses of concatenated nucleotide sequences of 13 PCGs and two ribosomal RNA genes showed that C.lischkeanus belongs to the subgenus Lividoconus group, which is grouped with species of the subgenus Virgiconus, and a member of the largest assemblage of worm-hunting (vermivorous) species at the most basal position in this group. Mitochondrial genome phylogeny supports the previous hypothesis that the ancestral diet of cone snails was worm-hunting, and that other dietary types (molluscivous or piscivorous) have secondarily evolved multiple times from different origins. This new, complete mitochondrial genome information provides valuable insights into the mitochondrial genome diversity and molecular phylogeny of Conus species.

Mitochondrial genome sequencing of a vermivorous cone snail Conus quercinus supports the correlative analysis between phylogenetic relationships and dietary types of Conus species

Complete mitochondrial genome (mitogenome) sequence of a worm-hunting cone snail, Conus quercinus, was reported in this study. Its mitogenome, the longest one (16,460 bp) among reported Conus specie, is composed of 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, two ribosomal RNA (rRNA) genes and one D-loop region. The mitochondrial gene arrangement is highly-conserved and identical to other reported. However, the D-loop region of C. quercinus is the longest (943 bp) with the higher A+T content (71.3%) and a long AT tandem repeat stretch (68 bp). Subsequent phylogenetic analysis demonstrated that three different dietary types (vermivorous, molluscivorous and piscivorous) of cone snails are clustered separately, suggesting that the phylogenetics of cone snails is related to their dietary types. In conclusion, our current work improves our understanding of the mitogenomic structure and evolutionary status of the vermivorous C. quercinus, which support the putative hypothesis that the Conus ancestor was vermivorous.

Sequencing of the complete mitochondrial genomes of eight freshwater snail species exposes pervasive paraphyly within the Viviparidae family (Caenogastropoda)

Phylogenetic relationships among snails (Caenogastropoda) are still unresolved, and many taxonomic categories remain non-monophyletic. Paraphyly has been reported within a large family of freshwater snails, Viviparidae, where the taxonomic status of several species remains questionable. As many endemic Chinese viviparid species have become endangered during the last few decades, this presents a major obstacle for conservation efforts. Mitochondrial genomes (mitogenomes) carry a large amount of data, so they can often provide a much higher resolution for phylogenetic analyses in comparison to the traditionally used molecular markers. To help resolve their phylogenetic relationships, the complete mitogenomes of eight Chinese viviparid snails, Viviparus chui, Cipangopaludina chinensis, C. ussuriensis, C. dianchiensis (endangered), Margarya melanioides (endangered), M. monodi (critically endangered), Bellamya quadrata and B. aeruginosa, were sequenced and compared to almost all of the available caenogastropod mitogenomes. Viviparidae possess the largest mitogenomes (16 392 to 18 544 bp), exhibit the highest A+T bias (72.5% on average), and some exhibit unique gene orders (a rearrangement of the standard MYCWQGE box), among the Caenogastropoda. Apart from the Vermetidae family and Cerithioidea superfamily, which possessed unique gene orders, the remaining studied caenogastropod mitogenomes exhibited highly conserved gene order, with minimal variations. Maximum likelihood and Bayesian inference analyses, used to reconstruct the phylogenetic relationships among 49 almost complete (all 37 genes) caenogastropod mitogenomes, produced almost identical tree topologies. Viviparidae were divided into three clades: a) Margarya and Cipangopaludina (except C. ussuriensis), b) Bellamya and C. ussuriensis, c) Viviparus chui. Our results present evidence that some Cipangopaludina species (dianchiensis and cathayensis) should be renamed into the senior genus Margarya. The phylogenetic resolution obtained in this study is insufficient to fully resolve the relationships within the 'b' clade, but if C. chinensis proves to be a valid representative of the genus, C. ussuriensis may have to be reassigned a different genus (possibly Bellamya, or even a new genus). Non-monophyly also remains pervasive among the higher (above the family-level) Caenogastropod taxonomic classes. Gene order distance matrix produced a different phylogenetic signal from the nucleotide sequences, which indicates a limited usability of this approach for inferring caenogastropod phylogenies. As phenotypic homoplasy appears to be widespread among some viviparid genera, in order to effectively protect the rapidly diminishing endemic Viviparid populations in China, further detailed molecular phylogenetic studies are urgently needed to resolve the taxonomic status of several species.