Studies On The Comparative Mitochondrial Genomics Of Thysanoptera (Insecta)

The insect order Thysanoptera,commonly known as thrips,are distributed worldwide.Some herbivorous species can directly damage crops or serve as vectors of plant viruses,e.g.the quarantine pest,western flower thrips(Frankliniella occidentalis),which poses a serious threat towards the agriculture.Thysanoptera is divided into two suborders and eight families with about 700 genera and nearly 6,000 species.Prior to this study,whole mitochondrial(mt)genomes of only five species in Thripidae are available in GenBank.The mt genomes of these thrips have high evolution rate and drastic gene rearrangement,which are second only to the mt genomes of parasitic lice(Phthiraptera).Furthermore,fragmentation of mt genome has been found in the yellow tea thrips,Scirtothrips dorsalis SA1.To explore further into the diversity and the evolution of mt genome structure of thrips,we sequenced the whole mt genomes of four more thrips species from three families using PCR and high-throughput sequencing:Anaphothrips obscurus,Echinothrips americanus,Desmothrips sp.,and Idolothrips spectrum.This study presents a comparative and phylogenetic analysis of mitochondrial genomes among nine species of thrips from both suborders and three families of Thysanoptera.The main results are as follows:(1)The size of mt genomes of thrips is relatively stable at 14.6 kb-15.7 kb.The variation in the genome size is due to the number and size of non-coding regions(control region,CR).The mt genomes of thrips generally exhibit a high AT content,74.8-92.5%.(2)Fragmentation of mt genome is not common in Thysanoptera and was found so far only in Scirtothrips dorsalis SA1.The mt genomes of other thrips have one single circular chromosome with the 37 genes typical of animals.(3)Within Thysanoptera,the mt gene order is highly variable.Each of the species sequenced in this study has a unique gene arrangement pattern;the gene order even varies in the same genus.Compared with the ancestral gene arrangement of the arthropods,only two mt gene clusters are conserved in the thrips:ATP8-ATP6 and ND5-tRNAHis-ND4-ND4L.The breakpoint distance between the mt gene orders of a thrips and the ancestor of arthropods is 27-33,indicating extensive rearrangements in Thysanoptera.(4)There is a correspondence between the degree of variation in mt gene order and the phylogenetic relationship between two thrips.The breakpoint is 29-34 between species of the two suborders,22-29 between species from different families,9-25 between species from different genera,and 5 between the two species within the genus Frankliniella.In summary,this study sequenced the complete mt genomes of four species of thrips;conducted comparative analyses of the mt genomes of nine species from 2 suborders and three families;reconstructed the phylogeney of Thrips with mt genome sequences,and provided novel insights into the evolution of mt genomes in Thysanoptera.