Emergence of polyphagous herbivorous insects entails significant adaptation to recognize, detoxify and digest a variety of host-plants. Despite of its biological and practical importance - since insects eat 20% of crops - no exhaustive analysis of gene repertoires required for adaptations in generalist insect herbivores has previously been performed. The noctuid moth Spodoptera frugiperda ranks as one of the world’s worst agricultural pests. This insect is polyphagous while the majority of other lepidopteran herbivores are specialist. It consists of two morphologically indistinguishable strains (“C” and “R”) that have different host plant ranges. To describe the evolutionary mechanisms that both enable the emergence of polyphagous herbivory and lead to the shift in the host preference, we analyzed whole genome sequences from laboratory and natural populations of both strains. We observed huge expansions of genes associated with chemosensation and detoxification compared with specialist Lepidoptera. These expansions are largely due to tandem duplication, a possible adaptation mechanism enabling polyphagy. Individuals from natural C and R populations show significant genomic differentiation. We found signatures of positive selection in genes involved in chemoreception, detoxification and digestion, and copy number variation in the two latter gene families, suggesting an adaptive role for structural variation.
The mosquitocidal activity of Bacillus sphaericus is because of a binary toxin (Bin), which binds to Culex pipiens maltase 1 (Cpm1), an ␣-glucosidase present in the midgut of Culex pipiens larvae. In this work, we studied the molecular basis of the resistance to Bin developed by a strain (GEO) of C. pipiens. Immunohistochemical and in situ hybridization experiments showed that Cpm1 was undetectable in the midgut of GEO larvae, although the gene was correctly transcribed. The sequence of the cpm1GEO cDNA differs from the sequence we previously reported for a susceptible strain (cpm1IP) by seven mutations: six missense mutations and a mutation leading to the premature termination of translation. When produced in insect cells, Cpm1IP was attached to the membrane by a glycosylphosphatidylinositol (GPI). In contrast, the premature termination of translation of Cpm1GEO resulted in the targeting of the protein to the extracellular compartment because of truncation of the GPI-anchoring site. The interaction between Bin and Cpm1GEO and the enzyme activity of the receptor were not affected. Thus, Bin is not toxic to GEO larvae because it cannot interact with the midgut cell membrane, even though its receptor site is unaffected. This mechanism contrasts with other known resistance mechanisms in which point mutations decrease the affinity of binding between the receptor and the toxin. E nvironmentally safe toxins produced by Bacillus thuringiensis and͞or Bacillus sphaericus have been integrated in management programs to control crop pests such as Heliothis virescens and Plutella xylostella, and disease vectors such as the mosquitoes Anopheles gambiae and Culex pipiens (1, 2). However, the potential benefits of these biopesticides may be rapidly lost because of the proliferation of highly resistant insect populations (3-6). Control strategies to delay or prevent the development of resistance have been developed, based on several assumptions. The most important of these assumptions are that the resistance gene is recessive and that the rate of mutation to generate resistance alleles is low. Currently, it is difficult to evaluate the success of these strategies because we lack adequate methods for monitoring resistance alleles because of our very restricted knowledge of the mechanisms of resistance to bioinsecticides. Bacillus sphaericus is toxic to mosquitoes, mainly because it produces a binary toxin (Bin) in crystals during sporulation. Following the ingestion and solubilization of crystals by larvae, the released toxin is activated and interacts with the brushborder membrane of the midgut epithelium. In a previous study, we reported the partial purification of a Bin-binding protein from IP, a susceptible strain of C. pipiens. This receptor displayed sequence similarity to ␣-glucosidases and other maltase-like proteins, and was thus named Cpm1, for Culex pipiens maltase 1 (7). We recently isolated the cDNA encoding Cpm1 from IP larvae (cpm1 IP ) and showed that Cpm1 has ␣-glucosidase activity when produced in bacteria (8). ...
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