Transcriptome alterations of field-evolved resistance inPectinophora gossypiellaagainst Bt Bollgard II cotton in India

Pink bollworm (PBW),Pectinophora gossypiella, is one of the most destructive insect pests of cotton worldwide. In India, PBW has developed widespread resistance to the Cry1Ac and Cry2Ab toxins of Bt Bollgard II cotton. To understand the molecular mechanism of Bt resistance, the midgut transcriptome of an Indian strain of Bt-susceptible (PBW-BtS) and Bt-resistant (PBW-BtR) PBW larvae were sequenced using Illumina platform to identify the genes and pathways associated with Bt resistance. A total of 31,764 unique unigenes were assembled from all the four samples, and 1,741 unigenes were found to be differentially expressed between PBW-BtS and PBW-BtR, out of which 1,024 unigenes were down-regulated and 717 were up-regulated. Down-regulated genes associated with already known Bt resistance genes in different organisms included APN, ABCA, ABCG8 and cadherin. On the other hand, unigenes related to metabolic resistance such as cytochrome P450, GST and carboxylesterase were up-regulated in PBW-BtR when compared with PBW-BtS. Moreover, real-time quantitative results of randomly selected genes confirmed the transcriptome response. This is the first comparative transcriptome study to identify the candidate genes involved in Bt BG II resistance in pink bollworm. We believe that the assembled and annotated transcriptome will serve as a valuable resource for future studies to discern the molecular basis of PBW Bollgard II resistance.