Complete mitochondrial genome of Anadara vellicata (Bivalvia: Arcidae): A unique gene order and large atypical non-coding region

Mitogenomics reveals phylogenetic relationships of Arcoida (Mollusca, Bivalvia) and multiple independent expansions and contractions in mitochondrial genome size

Arcoida, comprising about 570 species of blood cockles, is an ecologically and economically important lineage of bivalve molluscs. Current classification of arcoids is largely based on morphology, which shows widespread homoplasy. Despite two recent studies employing multi-locus analyses with broad sampling of Arcoida, evolutionary relationships among major lineages remain controversial. Interestingly, mitochondrial genomes of several ark shell species are 2-3 times larger than those found in most bilaterians, and are among the largest bilaterian mitochondrial genomes reported to date. These results highlight the need of detailed phylogenetic study to explore evolutionary relationships within Arcoida so that the evolution of mitochondrial genome size can be understood. To this end, we sequenced 17 mitochondrial genomes and compared them with publicly available data, including those from other lineages of Arcoida with emphasis on the subclade Arcoidea species. Our phylogenetic analyses indicate that Noetiidae, Cucullaeidae and Glycymerididae are nested within a polyphyletic Arcidae. Moreover, we find multiple independent expansions and potential contractions of mitochondrial genome size, suggesting that the large mitochondrial genome is not a shared ancestral feature in Arcoida. We also examined tandem repeats and inverted repeats in non-coding regions and investigated the presence of such repeats with relation to genome size variation. Our results suggest that tandem repeats might facilitate intraspecific mitochondrial genome size variation, and that inverted repeats, which could be derived from transposons, might be responsible for mitochondrial genome expansions and contractions. We show that mitochondrial genome size in Arcoida is more dynamic than previously understood and provide insights into evolution of mitochondrial genome size variation in metazoans.

Complete mitochondrial genome of Trachycardium flavum (Linnaeus, 1758)

The complete mitochondrial genome of Trachycardium flavum from South China Sea was first determined in this study, which is 16,596 bp in length. The base composition of the mitogenome is a less biased (A, G, T, and C was 21.5%, 26.3%, 37.0%, and 15.2%, respectively) with A + T contents of 58%. It consists of 24 tRNA genes, two rRNA genes, and 12 protein-coding genes (PCGs). No typical control region was found in non-coding sequence. Interestingly, ATP8 genes was absence same as most shellfish. Nine PCGs use a special start codon TTG, and the other three genes use a normal initiation codon ATN. Except ND6 use a single base G as the stop codon, the others all end with the normal stop codon TAG. The phylogenetic tree showed that T. flavum was first clustered with Fulvia mutica, Cerastoderma edule, and Acanthocardia tuberculate, which all belong to Cardiidae.

Complete mitochondrial genome of the cockle Anadara antiquata (Linnaeus, 1758)

The complete mitochondrial genome of Anadara antiquata was first determined. With a length of 45,454 bp, it consists of 29 tRNA, 2 rRNA, and 17 protein-coding genes (PCGs). The non-coding region was large and atypical around the genome with total 24,162 bp long. The nucleotide composition is significantly biased with AT contents of 62.2%. PCGs have five types of start codon, and terminate with a complete stop codon TAA or TAG. 19 microsatellites (SSRs) were identified in mitogenome sequences. Phylogenetic analysis demonstrated that A. antiquata was first clustered with Anadara vellicate, then together with Tegillarca granosa.

Two new mitogenomes of Pellorneidae (Aves : Passeriformes) and a phylogeny of the superfamily Sylvioidea

The superfamily Sylvioidea contains the most diversified species within the Passerida. The grey-cheeked fulvetta (Alcippe morrisonia) and the eyebrowed wren-babbler (Napothera epilepidota) are birds with a weak flight that live in lightly wooded or scrubland environments. In the present study, two new mitogenomes of A. morrisonia (KX376475) and N. epilepidota (KX831093) within the superfamily Sylvioidea were sequenced and their total lengths were 17788bp and 17913bp, respectively. Both mitogenomes comprised 13 protein-coding genes, 22 tRNAs, 2 rRNAs and two control regions (CR and CCR). Similar to most metazoans, both mitogenomes and their protein-coding genes encoded on the H-strand displayed typical positive AT skews and negative GC skews. Bayesian inference and maximum-likelihood phylogenetic analyses were conducted on the basis of partitioned data of mitogenomes and two identical topologies were observed. The family-level phylogenetic relationships ((((Pellorneidae, Leiothrichidae) Timaliidae) Zosteropidae) Sylviidae) among the superfamily Sylvioidea were strongly supported. Within the family Pellorneidae, A. morrisonia clustered with N. epilepidota. Within Leiothrichidae, we further demonstrated that Babax lanceolatus is sister to Garrulax perspicillatus, and Spizixos semitorques was nested within the genus Pycnonotus according to the mitogenomic data and we propose that the generic placement of Spizixos should be reconsidered.

Complete mitochondrial genomes of Trisidos kiyoni and Potiarca pilula: Varied mitochondrial genome size and highly rearranged gene order in Arcidae

We present the complete mitochondrial genomes (mitogenomes) of Trisidos kiyoni and Potiarca pilula, both important species from the family Arcidae (Arcoida: Arcacea). Typical bivalve mtDNA features were described, such as the relatively conserved gene number (36 and 37), a high A + T content (62.73% and 61.16%), the preference for A + T-rich codons, and the evidence of non-optimal codon usage. The mitogenomes of Arcidae species are exceptional for their extraordinarily large and variable sizes and substantial gene rearrangements. The mitogenome of T. kiyoni (19,614 bp) and P. pilula (28,470 bp) are the two smallest Arcidae mitogenomes. The compact mitogenomes are weakly associated with gene number and primarily reflect shrinkage of the non-coding regions. The varied size in Arcidae mitogenomes reflect a dynamic history of expansion. A significant positive correlation is observed between mitogenome size and the combined length of cox1-3, the lengths of Cytb, and the combined length of rRNAs (rrnS and rrnL) (P < 0.001). Both protein coding genes (PCGs) and tRNA rearrangements is observed in P. pilula and T. kiyoni mitogenomes. This analysis imply that the complicated gene rearrangement in mitochondrial genome could be considered as one of key characters in inferring higher-level phylogenetic relationship of Arcidae.