We sequenced the complete mitochondrial genomes of Lycosa shansia, and Lycosa singoriensis by combining Sanger and next-generation sequencing methods and analyzed the sequenced genomes in order to explore the phylogenetic placement and the mitogenome composition and evolution of these species. The mitochondrial genome of L. shansia was 14,638 bp, whereas that of L. singoriensis was 13,686 bp. The type of genes and direction of the coding strand present in the mitogenomes were the same as those in other species of Lycosoidea, including two ribosomal RNA genes (rRNAs), 22 transfer RNA genes (tRNAs), and 13 protein-coding genes (PCGs). The mitogenomes of the two species exhibited negative AT and positive GC skews. This indicated that the nucleotide compositions of the mitogenomes of L. singoriensis and L. shansia tended to be T and G. Both the mean and median values of Ka/Ks of ATP8 were the highest among the 13 protein-coding genes, indicating that it might have evolved more rapidly than the other protein-coding genes in both species. ATP8 may have undergone more relaxed selection constraints and accumulated more mutations. In addition, many tRNAs lacked T and D stem loops; a few had no acceptor stems. The assessed species were recovered nested within Lycosidae with high support. The present findings will be useful for future studies on the mitogenome evolution of spiders.

In this study, mitochondrial genomes (mitogenomes) of seven cichlid species (Lamprologus kungweensis, L. meleagris, L. ornatipinnis, Neolamprologus brevis, N. caudopunctatus, N. leleupi, and N. similis) are characterized for the first time. The newly sequenced mitogenomes contained 37 typical genes [13 protein-coding genes (PCGs), two ribosomal RNA genes (rRNAs) and 22 transfer RNA genes (tRNAs)]. The mitogenomes were 16,562 ~ 16,587 bp in length with an A + T composition of 52.1~58.8%. The cichlid mitogenomes had a comparable nucleotide composition, A + T content was higher than the G + C content. The AT-skews of most mitogenomes were inconspicuously positive and the GC-skews were negative, indicating higher occurrences of C than G. Most PCGs started with the conventional start codon, ATN. There was no essential difference in the codon usage patterns of these seven species. Using Ka/Ks, we found the fastest-evolving gene were atp8. But the results of p-distance indicated that the fastest-evolving gene was nad6. Phylogenetic analysis revealed that L. meleagris did not cluster with Lamprologus species, but with species from the genus Neolamprologus. The novel information obtained about these mitogenomes will contribute to elucidating the complex relationships among cichlid species.

We determined the complete mitochondrial genome of Thereuopoda clunifera (Chilopoda: Scutigeridae) to clarify the phylogenetic status within Chilopoda. Using Sanger sequencing and next-generation sequencing technologies, the entire mitogenome of T. clunifera was assembled and annotated, with 14,898 bp in length and 37 genes (13 protein-coding genes, 22 transfer RNA genes, and two ribosomal RNA genes), which are usually found in arthropod mitogenomes. Only one D-loop contained no repeat element. The base composition of T. clunifera was found to be A + T content of 69.55% and G + C content of 30.45%. The AT-skew of T. clunifera was positive, while the GC-skew was negative. The gene order of T. clunifera was identical to that of Scutigera coleoptrata that has been unique among those so far determined for the Arthropoda. We also performed phylogenetic analyses of 25 Myriapoda species to further explore the taxonomic and evolutionary relationships within Myriapoda and Chilopoda. Phylogenetic analyses supported that the division of Chilopoda into subclasses Notostigmophora and Pleurostigmophora.

Channidae (snakehead fish) is a family of medium-to-large freshwater carnivorous fish and contain the genus, Channa. Here, the complete mitogenomes of two Channa fish were determined and comparatively analyzed with the mitogenomes of 16 other Channidae fish species. The two newly sequenced complete mitogenomes were circular DNA molecules with sizes of 16,953 bp (Channa burmanica; OP954106) and 16,897 bp (Channa aurantimaculata; OQ134162). The mitogenomes were composed of 37 genes and one D-loop region. Positive AT skews and negative GC skews were found in the mitogenomes. Most protein-coding genes (PCGs) started with the conventional start codon, ATG; however, the sequence of the stop codon was variable. There was no obvious difference in relative synonymous codon usage among the two mitogenomes, and the two species shared a similar number of codon usage of mitogenomic PCGs, which was also similar to the mean values for the other 15 species of Channa. All Ka/Ks values were <1; cox1 had the lowest value, and atp8 had the highest. All of the tRNAs were typical clover structures, except trnS1. Phylogenetic analysis showed that C. burmanica and C. aurantimaculata shared a close relationship and that they were also closely related to C. gachua. These findings enrich the gene database of Channidae species, clarify the mitochondrial genome structure of the two species, and provide basic data for invasive biological surveillance in the future.

Soil macrofauna, such as Spirobolus bungii, are an important component of ecosystems. However, systematic studies of the genetic diversity, population genetic structure, and the potential factors affecting the genetic differentiation of S. bungii are lacking. We performed a population genetic study of 166 individuals from the mountains to the south of the Yangtze River, north of the Yangtze River in Nanjing city, and near Tianjin city, in order to investigate the correlations between geographical distance and genetic diversity. A total of 1182 bp of COX2 and Cytb gene sequences of mitochondrial DNA, and 700 bp of the 18S rRNA gene sequence were analyzed. There were two haplotypes and one variable site in the 18S rRNA gene, and 28 haplotypes and 78 variable sites in the COX2 and Cytb genes. In this study, the 18S rRNA gene was used for species identification, and mtDNA (concatenated sequences with Cytb and COX2) was used for population genetic analysis. Structure cluster analysis indicated that the genetic structures of the different populations of S. bungii tended to be consistent at small geographical scales. Phylogenetic trees revealed that the haplotypes were clearly divided into three branches: the area south of the Yangtze River, the area to the north of the Yangtze River in Nanjing, and the area in Tianjin. Large geographical barriers and long geographical distance significantly blocked gene flow between populations of S. bungii. Our results provide a basic theoretical basis for subsequent studies of millipede taxonomy and population genetic evolution.

Millipedes (Diplopoda) are crucial decomposers in soil ecosystems, as they play a vital role in organic matter degradation while also holding potential as bioindicators of environmental health. This study deciphered the complete mitogenomes of four millipede species (Diplopoda: Spirostreptida and Spirobolida) using next-generation sequencing technology, thus revealing evolutionary relationships among diplopod taxa and characterizing mitochondrial genomic features. The full mitochondrial sequences of Agaricogonopus acrotrifoliolatus, Bilingulus sinicus, Paraspirobolus lucifugus, and Trigoniulus corallinus, ranged in size from 14,906 to 15,879 bp, with each containing 37 typical genes and one D-loop region. Notably, the D-loop regions of A. acrotrifoliolatus and B. sinicus were positioned atypically, thus indicating structural rearrangements. A nucleotide composition analysis revealed pronounced AT-skews, with tRNA sequences exhibiting the highest A+T content. Ka/Ks ratios demonstrated that the ND5 gene experienced the weakest purifying selection pressure, thus suggesting its potential role in adaptive evolution. The results of the phylogenetic analysis showed genetic relationships between the three orders of ((Julida, Spirostreptida), Spirobolida), which was inconsistent with the previous conclusion regarding the three orders, obtained through morphological studies: ((Julida, Spirobolida), Spirostreptida). These findings highlight the role of the mitochondrial genome in resolving phylogenetic conflicts and provide important insights for further studies on millipedes.