Munching their way through ears and stems of maize, the larvae of the cereal stem borer (Sesamia nonagrioides) harm maize harvests from southern Europe to sub-Saharan Africa. ‘The level of damage inflicted on a crop by an agricultural pest species depends on the pest's ability to colonise the field area’, says Laure Kaiser and colleagues from the Institut de Recherche pour le Développement (IRD) at CNRS of Paris Sud, Gif sur Yvette, France. So, understanding the molecular mechanisms of how an insect seeks out food is an essential step in the ongoing war between agriculture and hungry pests. Explaining that one gene, for, which encodes a cGMP-dependent kinase (PKG) in Drosophila, has been shown to play a crucial role in that insect's foraging behaviour, Kaiser and her colleagues decided to investigate whether the same gene could be identified in the cereal stem borer and whether it lies at the heart of the pest's voracious appetite (p. ).
Knowing the sequence of the for gene in a few other moth species, the team was able to identify the gene in the larvae of the cereal stem borer and build a family tree showing that the gene is extremely similar to for genes in other closely related moth species. The team then compared the for gene sequences of 15 individual larvae and found one point mutation in some of the larvae, which substitutes a serine amino acid for a glycine in the protein sequence. And when they identified the location of the mutation in the PKG protein encoded by the gene, they found that it occurred in the region that regulates cellular functions.
Having confirmed that the for gene exists in the maize pest, Floriane Chardonnet then tested how the protein that is encoded by the gene affects larval behaviour. Using a drug that activates the for gene's PKG protein, Chardonnet monitored how often larvae that had been fed low levels of the drug foraged and moved between two plates of food. She found that these larvae foraged much more than the larvae that had not received the drug. The team also tested how the mutation in the protein affected the larvae's behaviour and found that the larvae that had two copies of the serine mutation switched between the plates of food more than larvae that had only one copy of the serine mutation or no copies. They point out that this is similar to the behaviour patterns found in Drosophila, where flies carrying one form of the for gene (rovers) roam between well-stocked plates of food when food is plentiful. When food is scarce, rovers also fare better than flies that carry the other form of the gene, known as sitters, which remain stationary while eating.
