Genetic studies of Busseola fusca (Lepidoptera : Noctuidae)

Abstract

Genetically modified (GM) Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) crops have been adopted widely in many countries around the world due to its success in reducing pest damage to crops and the associated insecticides used to control certain pests. However, evolution of resistance by pests threatens the continued sustainable use of these crops. Busseola fusca Fuller (Lepidoptera: Noctuidae), a major insect pest in sub-Saharan Africa, has developed high levels of non-recessive resistance to the Cry1Ab toxin expressed in Bt maize within eight years after the initial cultivation. Limited population genetic and phylogeography data are available for B. fusca and African insects in general. To address this issue, B. fusca larvae were sampled from maize across the maize production area of South Africa (SA) and statistical and network analyses of mitochondrial gene sequences were performed. Statistical results and haplotype networks of partial cytochrome c oxidase I (COI) and cytochrome b (cyt b) sequences indicated that South African B. fusca populations have low genetic diversity. This low degree of diversity together with non-recessive inheritance and high dispersal capabilities suggest that resistance might evolve rapidly in susceptible B. fusca populations if they are subjected to the same selection pressure as their resistant counterparts. On the other hand, the biology of B. fusca is also poorly understood, which, in combination with larval movement and feeding behaviour, also contributes to development and spread of resistant populations of this pest. A repertoire of resistance mechanisms to various Cry toxins has been identified from laboratory, greenhouse and field studies in Lepidoptera. However, no study has yet been done to determine the molecular mechanism of Cry1Ab resistance in B. fusca. As part of on-going research into resistance evolution, the abovementioned mechanisms of resistance were reviewed in this current study in addition to the transcriptome of B. fusca that was sequenced and de novo assembled. Several genes that have been associated with Cry toxin resistance in lepidopteran pests were detected in B. fusca. These genes include Cry toxin receptors (alkaline phosphatase (ALP), aminopeptidase N (APN) and cadherin (CDH)), ATP-binding cassette (ABC) transporters and mitogen-activated protein kinases (MAPKs). In order to investigate potential mechanisms of resistance in B. fusca, differential expression (DE) analysis was performed on larvae that fed on Bt and non-Bt maize, respectively. The DE results suggest that differential expression of metabolic and immune-related genes might explain resistance to the Cry1Ab toxin in this pest. Further studies are recommended to establish if there is a direct correlation between these differentially expressed genes and the observed resistance. Elucidation of such resistance mechanisms is crucial for developing insect resistance management (IRM) strategies to ensure sustainable use of GM crops. Nevertheless, the transcriptome characterized in this study provides a significant resource base for future studies on B. fusca and contributes to understanding some of the gene regulation and signalling networks involved in Bt resistance in this pest