Background Hoplobatrachus rugulosus (Anura: Dicroglossidae) is distributed in China and Thailand and the former can survive substantially lower temperatures than the latter. The mitochondrial genomes of the two subspecies also differ: Chinese tiger frogs (CT frogs) display two identical ND5 genes whereas Thai tiger frogs (TT frogs) have two different ND5 genes. Metabolism of ectotherms is very sensitive to temperature change and different organs have different demands on energy metabolism at low temperatures. Therefore, we conducted studies to understand: (1) the differences in mitochondrial gene expression of tiger frogs from China (CT frogs) versus Thailand (TT frogs); (2) the differences in mitochondrial gene expression of tiger frogs (CT and TT frogs) under short term 24 h hypothermia exposure at 25 °C and 8 °C; (3) the differences in mitochondrial gene expression in three organs (brain, liver and kidney) of CT and TT frogs. Results Utilizing RT-qPCR and comparing control groups at 25 °C with low temperature groups at 8 °C, we came to the following results. (1) At the same temperature, mitochondrial gene expression was significantly different in two subspecies. The transcript levels of two identical ND5 of CT frogs were observed to decrease significantly at low temperatures (P < 0.05) whereas the two different copies of ND5 in TT frogs were not. (2) Under low temperature stress, most of the genes in the brain, liver and kidney were down-regulated (except for COI and ATP6 measured in brain and COI measured in liver of CT frogs). (3) For both CT and TT frogs, the changes in overall pattern of mitochondrial gene expression in different organs under low temperature and normal temperature was brain > liver > kidney. Conclusions We mainly drew the following conclusions: (1) The differences in the structure and expression of the ND5 gene between CT and TT frogs could result in the different tolerances to low temperature stress. (2) At low temperatures, the transcript levels of most of mitochondrial protein-encoding genes were down-regulated, which could have a significant effect in reducing metabolic rate and supporting long term survival at low temperatures. (3) The expression pattern of mitochondrial genes in different organs was related to mitochondrial activity and mtDNA replication in different organs.
Based on morphological characteristics to make species identification, the cryptic species of the Scutigeromorpha can be greatly underestimated. The mitochondrial genome provides a desirable tool for the biological identifications and the discovery of the cryptic species. The capacity to acquire mitochondrial genome sequences has substantially improved in recent years using next-generation sequencing (NGS) technology. On the basis of the next-generation sequencing, we obtained four complete mitochondrial genomes of Thereuonema tuberculata (Wood, 1862) from Nanyang, Henan Province (NY), Nanchang, Jiangxi Province (NC), Jinan, Shandong Province (JN), and Dali, Yunnan Province (DL) in China with GenBank numbers OK513221, OL449685, ON058988 and ON058989, respectively. The lengths of the four mitochondrial genomes ranged from 14,903 to 14,909 bp. The composition and order of genes of the four mitochondrial genomes were identical to the published mitochondrial genome of Scutigera coleoptrata (Linnaeus, 1758) (Scutigeromorpha: Scutigerdae). It was the first time that the tandem repeats in the control region were detected in Scutigeromorpha. We also calculated the corrected pairwise genetic distance of four complete mitochondrial genomes of T. tuberculata, ranging from 7.7 to 15.2%. The results showed that the T.tuberculata NC belonged to the typical sample of T. tuberculata, and T. tuberculata DL was hypothesized as a cryptic species of T. tuberculata. Meanwhile, T. tuberculata NY and T. tuberculata JN were hypothesized as potential cryptic species of T. tuberculata in this study. In both BI and ML trees, the monophyly of Scutigeromorpha, Scolopendromorpha, Geophilomorpha, and Lithobiomorpha was forcefully advocated. Moreover, Scutigeromorpha was recovered as the sister clade of (Scolopendromorpha + (Lithobiomorpha + Geophilomorpha)). Four specimens of T. tuberculata were clustered into one clade, which was the sister to the clade of S. coleoptrata.
The complete mitochondrial (mt) genome of Leptomantella tonkinae (Hebard, 1920) was 15,527 bp in length and contained 13 protein-coding genes, 22 transfer RNAs, two ribosomal RNAs, and one control region. The gene arrangement of mt genome of L. tonkinae was identical to the primitive mantis. The overall AT content of the mt genome was 74%. In ML and BI phylogenetic analyses, the monophyly of Leptomantellidae was robustly supported and the clade of Leptomantellidae is a sister clade to the group of (Gonypetidae+(Leptomantellidae+(Amorphoscelidae+Nanomantidae))).
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