| Orthoptera is one of the oldest extant insect lineages with more than20thousands described species widely distributed throughout the world but mainly focused on the tropical areas. It is one of the largest and best researched of the hemimetabolous insect orders and consists of two suborders Caelifera and Ensifera. So far, complete47orthopteran mitogenomes were available in the GenBank, of which27for Acridoidea in Caelifera, and the number of complete mitogenomes sequenced were not balanced in the two suborders of Orthoptera. Currently, few studies on Orthoptera focus on using the comparative genomics to analyse the mitogenome sequence divergence and molecular evolution and using different data partitioning strategies in mitogenome to resolve the phylogenetic relationships at various taxonomic levels. Previous studies were limited to the phylogenetic relationships at higher taxonomic levels, and it is needed to pay more attention to the phylogenetic analyses at the middle and low taxonomic levels.based on the mitogenome sequence.Pamphagidae and Gomphoceridae belong to the Caelifera. The two genus Filchnerella and Pseudotmethis in Pamphagidae is so similar in the morphology and both are mainly distributed in the northwestern area of China. Additionally, the difference of G. sibiricus and G. licenti in Gomphoceridae are identified only according to whether the anterior and posterior cubitus connate or not, and the geographical ranges of the two species overlap in some northwestern areas of China. Do the mitogenomes of these taxa similar in the morphology and close in the distributed area have many common important characters?In this study, the four complete mitogenome sequence in Acridoidea were successfully sequenced, annotated and analysed. Additionally, a total of43orthopteran mitogenomes available in the GenBank were selected based on our different analyses. The evolutionary patterns of orthopteran mitogenomes were firstly investigated using the comparative genomics based on the bioinformatics. Additonally, the phylogeny of Orthoptera was analysed based on12datasets from a total of47orthopteran mitogenomes. Followings are the main results: 1. The complete mitogenome sequences of G. sibiricus, A. zacharjini, F. helanshanensis and P. rubimarginis are15,590bp,15,660bp,15,657bp and15,661bp in size, respectively. All four mitogenomes share the same37typical metazoan genes (13protein-coding genes,22transfer RNA genes and2ribosomal RNA genes), and they have identical gene arrangement and orientation with all previously determined caeliferan mitogenomes.2. The average values of P-distance between G. sibiricus and G Licenti are lower than those between other two species in Gomphoceridae based on the whole mtDNA,22tRNAs or2rRNAs, however, that between G. sibiricus and G tibetanus is the lowest based on the13mitogenome protein coding genes. The ratio of cox1, cox2, cox3and nad4L were lower either on the nucleotide and amino acid sequence heteromorphosis or KalKs, which may indicate the four PCGs are highly conserved and evolve slowly. The evolutionary rate of atp6, atp8and nad6is relatively higher, and the three PCGs may under lower selection pressure. Two bulged adenines insertion after the third couplet in the TΨC stem of trnThr are common in the four Gomphoceridae species. Whether it is a molecular synapomorphy to Gomphocerinae may require more data to verify. The rrnS is very consistent between G. licenti and G. sibiricus. However, variable regions in domain Ⅰ and domain Ⅱ are obvious in G. tibetanus.3. The average values of P-distance between F. helanshanensis and P. rubimarginis are lower than those between other two species in Pamphagidae based on the whole mtDNA,13PCGs,22tRNAs and2rRNAs. In the thirteen PCGs of Pamphagidae mitogenomes, the coxl, cox2, cytb and atp6are the slowest evolving genes, while the nad3and nad5have higher evolutionary rate.4. In the two families of Caelifera, Pamphagidae and Gomphoceridae, both nad6and atp8reveal much higher A+T content, Without exception, A+T contents in all three COX genes are lower than the other gene categories. The number of adenine is almost equal to thymine in all PCGs distributed in J-strand except in atp8which has a moderate A-skew value, while each of the other four PCGs (nad5, nad4, nad4L and nadl) coded by N-strand has an obvious T-skew value. The hairpin in A+T-rich region can be predicted in all four species respectively from Gomphoceridae and Pamphagidae. one T-stretch in the majority strand was only found in the A+T-rich region of the four Pamphagidae mitogenomes, however, they are not adjacent to the trnIle but inside the stem-loop sequence in the majority strand.5. We evaluate the nucleotide-compositional behavior of the insecta mitogenome by the three parameters:A+T content (AT%), AT-skew and GC-skew. The A+T contents of the mitogenomes from6orders of insecta range from75%to80%, the AT-skew values range from0to0.05and the GC-skew values ranges from-0.3to0. Among these species, the A+T content and AT-skew in orthopteran mitogenomes are dispersed, but both seems to be positive correlations. The disparity index value is lower among those orthopteran species near in A+T content and near the average A+T content value or among the closely related species. The average ID value are different in three codon positions of PCGs, the highest ID value was observed in the third codon positions, the second codon positions shows the lowest ID value.6. In the13PCGs of orthopteran mitogenomes, the A+T contents of coxl and cox3are the lowest in most species, while atp8, nad4L and nad6reveal higher A+T content than other PCGs. The four PCGs coded by N-strand in Orthoptera have obvious T-skew value. The9PCGs coded by J-strand in Ensifera also have obvious T-skew value, while these PCGs show A-skew in most caeliferan species. Each PCG in J-strand is C-skew, However, each PCG in N-strand is G-skew. We compared overall mean distance of13PCGs in two suborders of Orthoptera based on the JC and K2P model and found the cox2is the slowest evolutionay gene, the nad5is the fastest evolutionary gene in Caelifera, while the nadl is the fastest in Ensifera. We compared the sequence heteromorphosis of13PCGs at the DNA and amino acid level, the cytochrome oxidase subunits and cytochrome b show overall much slower rates of evolution, while the nad6is the fastest evolving protein-coding gene. Generally, the amino acid sequence should be more conserved than the nucleotide sequence. However, the amino acid of all13PCGs show higher divergence than the DNA sequence with the exception of the cox1gene, especially obvious in the nad6and atp8.7. The pattern of nucleotide conservation in tRNA genes was markedly majority strand-biased. Eleven tRNAs showed%INUC>50, only one of them was located on the minority strand. Indeed, trnLeuUUR, trnAsn and trnLys, which showed the highest levels of nucleotide conservation (%INUC≥70), were all located on the majority strand. In the three domains of rrnS in orthopteran mitogenomes, the domain Ⅰ (excluding the5’ half) is the most conserved region, while the domain Ⅱ has many fewer conserved nucleotide strings than other two domains. Similarly, in the rrnL of Orthoptera mitogenomes, Domains Ⅰ and Ⅱ, on average, are less conserved than domains Ⅳ and Ⅴ.8. In terms of the ability to resolve deeper level relationships in Orthoptera, the conserved datasets, PCG1, PCG2, COX, COX+cytb and rRNA(C) may be the best choice, while the genes or regions of intermediate rates might be of better phylogenetic utility for the orthopteran phylogenetic analyses at various taxonomic level, nevertheless, the analytical regimen is essential. Those regions including slightly more variable sites may be useful for the phylogenetic studies at the lower taxonomic level. However, the faster small evolutionary gene such as ATP genes seem to be usefulless in the phylogenetic studies. |
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