Identification, mRNA Expression, and Functional Analysis of Chitin Synthase 1 Gene and Its Two Alternative Splicing Variants in Oriental Fruit Fly, Bactrocera dorsalis

Two alternative splicing variants of chitin synthase 1 gene (BdCHS1) were cloned and characterized from the oriental fruit fly, Bactrocera dorsalis (Hendel). The cDNA of both variants (BdCHS1a and BdCHS1b) consisted of 5,552 nucleotides (nt), with an open reading frame (ORF) of 4,776 nt, encoding a protein of 1,592 amino acid residues, plus 685- and 88-nt of 5′- and 3′-noncoding regions, respectively. The alternative splicing site was located between positions 3,784-3,960 and formed a pair of mutually exclusive exons (a/b) that were same in size (177 nt), but showed only 65% identity at the nucleotide level. During B. dorsalis growth and development, BdCHS1 and BdCHS1a were both mainly expressed during the larval-pupal and pupal-adult transitions, while BdCHS1b was mainly expressed during pupal-adult metamorphosis and in the middle of the pupal stage. BdCHS1a was predominately expressed in the integument whereas BdCHS1b was mainly expressed in the trachea. The 20-hydroxyecdysone (20E) induced the expression of BdCHS1 and its variants. Injection of dsRNA of BdCHS1, BdCHS1a, and BdCHS1b into third-instar larvae significantly reduced the expression levels of the corresponding variants, generated phenotypic defects, and killed most of the treated larvae. Furthermore, silencing of BdCHS1 and BdCHS1a had a similar result in that the larva was trapped in old cuticle and died without tanning completely, while silencing of BdCHS1b has no effect on insect morphology. These results demonstrated that BdCHS1 plays an important role in the larval-pupal transition and the expression of BdCHS1 in B. dorsalis is regulated by 20E.