Mitogenomes are useful in analyzing phylogenetic relationships and also appear to influence energy metabolism, thermoregulation and osmoregulation. Much evidence has accumulated for positive selection acting on mitochondrial genes associated with environmental adaptation. Hence, the mitogenome is a likely target for environmental selection. The family Bufonidae (true toads) has only nine complete and four partial mitogenomes published compared to the 610 known species of this family. More mitogenomes are needed in order to obtain a clearer understanding of the phylogenetic relationships within Bufonidae that are currently controversial. To date, no mitogenomes have been reported from the genera Anaxyrus and Bufotes. Anaxyrus americanus can live in low temperature environments and Bufotes pewzowi can live in high salinity environments. We sequenced the mitogenomes of these two species to discuss the phylogenetic relationships within Bufonidae and the selection pressures experienced by specimens living in low temperature or saline environments. Like other toads, the circular mitogenomes of both species contained the typical 37 genes. Anaxyrus americanus had the highest A+T content of the complete mitogenome among the Bufonidae. In addition, A. americanus showed a negative AT-skew in the control region, whereas Bufotes pewzowi showed a positive AT-skew. Additionally, both toad species had unique molecular features in common: an ND1 gene that uses TTG as the start codon, an extra unpaired adenine (A) in the anticodon arm of trnS (AGY), and the loss of the DHU loop in trnC. The monophyly of Bufonidae was corroborated by both BI and ML trees. An analysis of selective pressure based on the 13 protein coding genes was conducted using the EasyCodeML program. In the branch model analysis, we found two branches of A. americanus and Bufotes pewzowi that were under negative selection. Additionally, we found two positively selected sites (at positions 115 and 119, BEB value > 0.90) in the ND6 protein in the site model analysis. The residue D (119) was located only in A. americanus and may be related to adaptive evolution in low temperature environments. However, there was no evidence of a positively selected site in Bufotes pewzowi in this study.
The mitochondrial genome of Fejervarya kawamurai is a circular molecule of 17,650 bp in length, containing 13 protein-coding genes, two rRNA genes, 23 tRNA genes (including an extra tRNA-Met), and the control region. The AT content of the whole genome is 56.9%. In Bayesian inference (BI) and Maximum likelihood (ML) analyses, we found that F. kawamurai is a sister clade to F. multistriata and F. limnocharis. The monophyly of Fejervarya, Quasipaa, Nanorana was well supported (1.00 in BI and 100% in ML).
The complete mitochondrial genome sequence of the hybrid of
Hoplobatrachus chinensis
(♀) ×
H. rugulosus
(♂) was obtained in this study. The circular mitochondrial genome was 20,282 bp in length (including extra ND5 genes). Compared with the complete mitogenome of the parents, the results indicated that the mitochondria of the hybrid tiger frog was consistent with a maternal inheritance. Phylogenetic analyses using concatenated nucleotide sequences of the 11 protein-coding genes with two different methods (maximum likelihood and MrBayes analysis) both highly supported a close relationship of the hybrid frogs with the Chinese tiger frog (=
H. chinensis
).
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