Sequencing And Analysis Of The Complete Mitochondrial Genomes Of Two Lepidoptera Species

The mitochondrion is a semi-autonomous organelle, which has its own genome. The mitochondial DNA has been widely used as an informative genetic marker in the study of insect taxonomy, population genetics, phylogeography, and comparative and evolutionary genomics, since it has simple molecular mass, simple structure, homologous single-copy, conservative gene composition and sequence, materilineal inheritance with a rapid evolutionary rate and rarely genetic recombination, has no introns and a large number of repeat sequences.Lepidoptera is the second-most diverse order in insecta, mainly including agricultural and forestry insect pests and economic insect. The research of lepidopteran insect has an important significance on the management of agricultural and forestry insect pests, and the using of economic insect. In this study, we have finished the sequencing and analysis of Bombyx mori strain Baiyun and the common cutworm Spodoptera litura, the conclutions of this study are as follows:(1) The mitochondrial genomes of Bombyx mori strain Baiyun and the common cutworm Spodoptera litura are 15629 bp and 15383 bp, each of them has 37 genes (13 protein-coding genes,22 tRNA and two rRNA genes) and a non-coding A+T-rich region like other animal.(2) The genome composition of the major strand of Bombyx mori strain Baiyun and the common cutworm Spodoptera litura showed highly A+T biased, for 81.32% and 81.08%. The A+T-rich regions exhibite the highest A+T content:95.56% and 93.87%. There were positive AT-skew(0.0585) and negative GC-skew(-0.215) in the mitochondrial genome of Bombyx mori strain Baiyun.(3) All protein-coding genes(PCGs) of Bombyx mori strain Baiyun were initiated by ATN codons (4 PCGS were ATT,5 PCGs were ATG and 3 PCGs were ATT) except for the COI gene, which began with CGA codon.11 PCGs ended with termination codon TAA, whereas the COI and COII stopped with a single T. In the common cutworm Spodoptera litura 12 PCGs started with ATN start codon (2 PCGs were ATA,6 PCGs were ATG and 4 PCGs were ATT), just COI begined with CGA. 8PCGs stoped with complete termination codons (TAA or TAG), whereas 5 PCGs used an imcomplete stop codon T.(4) The mitochondrial genomes of Bombyx mori strain Baiyun and the common cutworm Spodoptera litura all contain 22 tRNA genes, ranging in size from 64-71bp and 65-71b, and all the tRNA genes had a typical cloverleaf secondary structure. The two rRNA genes of Bombyx mori strain Baiyun:rrnL and rrnS, were 1378bp and 783bp in length, with A+T contents of 84.40% and 85.57%. In the common cutworm Spodoptera litura, the rrnL was 1402bp in length with an A+T content of 84.95%, while the rrnS’xs 814bp long with an A+T content of 85.14%.(5) The A+T-rich region of Bombyx mori strain Baiyun located between rrnS and the tRNA cluster tRNAMel-tRNAIle-tRNAGln, was 496bp in length, exhibiting an A+T content of 95.56%. Additionally, it contented the motif ATAGA followed by an 18bp poly-T stretch and two microsatellite-like(TA)8 and (AT)9 elements preceded by the ATTTA motif.In the common cutworm Spodoptera litura the A+T-rich region located between rrnS and trnM, was 326bp long with an A+T content of 93.87%, contented three tandem repeat elements, which is four 19bp tandem repeats, two 32bp tandem repeats and two 36bp tandeats. Additionally, the were two conserved motifs: ATAGA and ploy-T-stretch.(6) For Bombyx mori strain Baiyun, contented 20 intergenic spacer sequence, ranging in size from 1 bp to 59 bp, the longest was 59 bp, located between tRNIle and tRNAGln, the shortest was only 1 bp, located between tRNAAla and tRNArg, totalling 344 bp. Whereas, there were just 4 overlapping sequences, the largest and smallest were 1 bp and 7 bp, located between tRNASer(AGN) and tRNAGlu, tRNAHis and ND4.There were 12 intergenic spacer sequence in the mitochondrial genomen of the common cutworm Spodoptera litura totalling 187 bp, ranged in size from 1 bp to 48bp, the intergenic spacer sequence between trnI and trnQ was the largest with 48 bp, while the lbp intergenic spacer sequence between trnW and trnC was the shortest. Additionlly, there were 6 overlapping sequences, ranging in size from 1 bp to 8 bp, among them, the longest sequence was between nad2 ande trnW Wtih 8 bp-base, and there was only one base between atp8 and atp6, nad3 and trnA, trnN and trnS2.(7) 14 mitochondrial genomes have been sequenced in Noctuoidea. Using the PCGs of Bombyx mori, Timelaea maculata and this 14 sequences to construct the phylogenetic tree of Spodoptera litura. The Spodoptera litura was in Noctuidae of Noctuoidae, and Noctuoidae had a closer affinity with Bombycodiea than with papilionoidea by analysising the phylogenetic tree.