Genome-wide Characterization And Expression Profiling Of ATP-binding Cassette Transporter Genes In The Diamondback Moth,Plutella Xylostella(L.)

The diamondback moth,Plutella xylostella(L.),is a worldwide insect pest specifically attacking cruciferous crops.It is known to have high genetic plasticity and is thus difficult to control.This insect pest has developed resistance to almost all chemical insecticides and toxins from biological insecticide Bt.ATP-binding cassette(ABC)transporters are one of the major transmembrane protein families,which transport a broad range of substrates across lipid membranes.An increasing number of studies have demonstrated that ABC transporters are a novel family of detoxification-involved proteins following cytochrome P450 monooxygenases,glutathione S-transferases and carboxyl esterases.In this study,82 ABC transporter genes have been identified in P.xylostella genome,grouping into 8 subfamilies(ABCA-H)by phylogenetic analysis.We found 15 ABC transporters belonging to the subfamily ABC A,14 to ABCB,21 to ABCC,3 to ABCD,1 to ABCE,3 to ABCF,19 to ABCG,and 6 to ABCH.An additional 19 fragments lacked conserved nucleotide binding domains(NBDs)and showed homology to ABC transporter genes.The ABC transporter genes were dispersed on 59 scaffolds,42 genes were located on 19 scaffolds each containing more than 2 genes,while the rest were found being individually located on a single scaffold.The full lengths of most ABC transporters ranged from 124 to 2714 aa.Two genes were considered very long containing 3796 and 4008 aa and both belonging to ABCA subfamily.The numbers of exons ranged from 2 to 73 showing high complexity of the genomic structure.Conserved motif analysis indicated that the P.xylostella ABC transporters had six conserved motifs:Walker A,Walker B,Signature C,Q-loop,D-loop and H-loop.Subfamilies ABCA,ABCC and ABCG showed extensive gene expansion in P.xylostella.In this study,a phylogenetic analysis were performed among D.melanogaster,P.xylostella and Bombyx mori,the Tetranychus urticae,and Homo sapiens to further understand the evolution of each subfamily.Many ABC transporter genes in P.xylostella were orthologous to extensively studied ABC transporter genes in other species.Further studies would be required to determine whether they have identical functions.Moreover,the expression patterns of 82 ABC genes have been studied in different developmental stages and various tissues of P.xylostella and the differential expression among different susceptible and insecticide-resistant strains.Gene expression profiling based on the RNA-seq data showed thatalthough ABCD,ABCE and ABCF subfamilies have few members,they have higher expression levels than the others,indicating a potential house-keeping role.Three ABCC transporter genes including CCG002415.1,CCG002416.1 and CCG008256.1 were preferentially expressed in midgut of the 4th-instar larvae of a susceptible strain(Fuzhou-S),suggesting a potential involvement in detoxification of host defense chemicals.Compared to the susceptible strain,we identified a number of candidate genes differentially expressed in the fipronil-and chlorpyrifos-resistant strains.Five ABCC genes including CCG002418.1,CCG008999.2,CCG009835.1,CCG002416.1(ABCC2),and CCG002419.1 were significantly upregulated in 3rd instar larvae of the two insecticide resistant strains.One of the ABCH genes(CCG014955.1)was differentially up-regulated in two resistant strains,while scarcely expressed in insecticide-susceptible strain.Results of qRT-PCR validation for the expression pattern of 18 selected genes were consistent with the RPKM data.In this study,a genome-wide study was carried out to identify and analyze the ABC transporter gene family of P.xylostella,with focus on the gene number,genomic structure,evolutionary relationship,and expression patterns.We proposed a possible role of ABC transporters in the detoxification of chemical insecticides.Our work provides a solid foundation for future functional studies of the P.xylostella ABC transporter genes to further validate their regulation pathways and mechanisms in insecticide resistance.