Phylogenetic Construction And Comparative Mitogenomic Analysis Of Onchidiids

Onchidiidae belongs to Mollusca, gastropods, Pulmonata and Systellommatophora. Onchidiids are shell-less pulmonate gastropods which breath air.? It habitats beach and mangrove in the intertidal zone or the high tide zone, it has important scientific value and nutritional value. Onchidiids have a worldwide distribution, with the exception of the Arctic and Antarctic. They are the only group of euthyneura gastropods to be found in the sea, the fresh water and on land, it might help us understand better macroevolutionary patterns of transitions between aquatic and terrestrial habitats.?Therefore, Onchidiidae play a vital role?in?phylogenetic research and classification of mollusks.Via long and accurate polymerase chain reaction (LA-PCR), a complete mitochondrial genome sequence of Onchidium struma was determined and analyzed. The results indicated that the complete mitochondrial genome of Onchidium struma a circular molecule of 13957 nucleotides.The gene content includes 22 tRNAs, 2 rRNAs and 19 non-coding regions from 2 bp to 138 bp in size. Its overall composition is 27.27%A, 16.78 %C, 20.23%G, and 35.72%T;four protein genes and 8 tRNA genes are transcribed from L strand,and the other 11 protein genes and 18 tRNA genes are transcribed genes from H strand. All protein coding genes of the O. struma mitochondrial genome start with a typical ATN codon, except for ND2 with TTG. The usual TAA or TAG termination codons were found for eleven protein-coding genes. However, the COIII and Cytb genes have an incomplete termination codon (T). Leucine (Leu) was the highest amino acid usage in O. struma, The lowest frequency amino acids was Asp, the most commonly codon suage was UUA, The lowest frequency Codon was CGC.With respect to the secondary structures of 22 tRNA genes. The absence of DHU arm of tRNASer was observed as in Onchidium struma tRNASer and tRNAThr with 9 bases in anticodon loop was found, compared with typical 7 bases in others. The longest non-coding had secondary structures which similar to the tRNA.The complete mitochondrial genome of the Onchidiidae is important for general molecular and evolutionary studies in Gastropoda. A complete mitochondrial genome sequence of the Platevindex mortoni, Paraoncidium reevesii, Peronia verruculata and Onchidium struma were determined and analyzed using gene content, gene size, gene arrangement, base composition and structural characteristics. The results indicated that the size of the complete mitochondrial genomes is 13,842, 13,861, 13,957 and 13,991bp , and the base counts yield A+T composition are 63.64%, 64.90%, 64.67% and 62.99% for P. reevesii, P. verruculata, O. struma and P. mortoni, respectively. A relatively high level of conservation in the order of genes was observed in these genomes. All protein coding genes of the the four Onchidiids mitochondrial genome start with a typical ATN codon, except for GTG,TTG and CTA. The usual TAA or TAG termination codons were found for eleven protein-coding genes. However, the Cytb genes of the the four Onchidiids all have an incomplete termination codon (T). The transcripts of the latter genes would be modified to form a complete termination signal UAA by polyadenylation after cleavage of the polycistronic RNA, as demonstrated for other metazoan mtDNAs.Gastropoda is one of the largest group of mollusks and the species accounted for 80% of the mollusk. It can be found in the marine, freshwater and terrestrial.Gastropod mollusk mt genomes present high diversity of gene orders , and offer a suitable model system to study the rates and mechanisms of mt genome rearrangement,as well as the phylogenetic utility of arrangement comparisons. The facts of their convergent and divergent evolutions, very fast radiations, long evolutionary history, have made their phylogenetic relationships been controversial. The mitogenome were sequenced for 4 species from Onchidiids species, and the mitogenome sequences of another 9 species of Gastropoda were downloaded from GenBank. After the multiple sequence alignments, the sequence variation analyses were conducted and the molecular phylogenetic trees of Gastropoda were reconstructed withNJ,ME,MP and bayesian methods, using Katharina tunicata of Polyplacophora as an outgroup. The results of phylogenetic analyses showed: (1) the phylogenetic relationships of the Gastropoda taxa was basically in consistent with morphology-based ones. (2)New molecular data support Euthyneura as a natural group, rejecting Pulmonata and Opisthobranch as natural groups. (3)The Basommatophora S. pectinata is more closely related to opisthobranchs than to any other group of pulmonates considered in this study. (4)Phylogenetic analyses showed that Onchidium struma and Platevindex mortoni were inferred as sister group, and Paraoncidium reevesii was robustly inferred to be a sister species of Peronia verruculata, which may related to their living environment. In all of the main coding genes of Onchidiidae mitochondrial genomes, the variation in ND2 and ND6 genes were shown the maximum. Therefore, ND2 and ND6 genes can be used as molecular markers assisted with COI gene, providing a reference for the conservation of Onchidiidae biological diversity and use of the biological resources rationally.