Transcriptome and differentially expressed genes of Busseola fusca (Lepidoptera: Noctuidae) larvae challenged with Cry1Ab toxin

Abstract

Busseola fusca (Fuller) (Lepidoptera: Noctuidae), a major insect pest of maize in sub-Saharan Africa, has de-veloped high levels of non-recessive resistance to Cry1Ab toxin expressed in genetically modified Bt maize.Multiple resistance mechanisms to various Cry toxins have been identified in Lepidoptera, but no study has yetbeen done to determine the mechanism of Cry1Ab resistance inB.fusca. Therefore, the larval transcriptome ofB.fuscawas sequenced, de novo assembled and characterized. Differential expression analysis was performed tocompare gene expression profiles of Cry toxin challenged and unchallenged neonate larvae to assess the mo-lecular basis of the defence mechanism employed by this insect. Several genes associated with Cry toxin re-sistance in other lepidopteran pests were detected inB.fusca. Results suggest that differential expression ofmetabolic and immune-related genes might explain Cry1Ab toxin defence in this pest (supplementalfile).Transcript expression profiles of neonates demonstrated that 33.59% and 60.31% of the 131 differentially ex-pressed genes were upregulated and downregulated in the toxin-challenged neonate larvae, respectively.Transcripts were grouped into two subclusters according to the similarity of their expression patterns.Transcripts in subcluster 1 were moderately upregulated in the toxin-challenged neonate larvae, and, conversely,downregulated in the unchallenged neonate larvae. The solute carrier organic anion transporter, which is in-volved in insecticide detoxification, was upregulated in the toxin-challenged neonate larvae. Conversely, most ofthe transcripts in subcluster 2 were moderately downregulated in the toxin-challenged neonate larvae, andupregulated for neonates feeding on non-challenged maize. Four unidentified transcripts were extremely down-regulated in the toxin-challenged neonate larvae, and upregulated in the unchallenged neonate larvae. Furtherstudies are recommended to establish if there is a direct correlation between these differentially expressed genesand the observed resistance. Elucidation of such defence mechanisms is crucial for developing insect resistancemanagement strategies to ensure sustainable use of genetically modified maize in Africa. Nevertheless, this is thefirst study on gene expression profiles ofB.fuscastrains challenged with Cry toxin. The transcriptome char-acterized in this study provides a significant resource base for future studies onB.fuscaand contributes tounderstanding some of the gene regulation and signalling networks involved in the defence ofB.fuscaagainstCry1Ab toxin