Naturally occuring polymorphisms in behavior are difficult to map genetically and thus are refractory to molecular characterization. An exception is the foraging gene (for), a gene that has two naturally occurring variants in Drosophila melanogaster food-search behavior: rover and sitter. Molecular mapping placed for mutations in the dg2 gene, which encodes a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Rovers had higher PKG activity than sitters, and transgenic sitters expressing a dg2 complementary DNA from rover showed transformation of behavior to rover. Thus, PKG levels affected food-search behavior, and natural variation in PKG activity accounted for a behavioral polymorphism.

Genes can affect natural behavioral variation in different ways. Allelic variation causes alternative behavioral phenotypes, whereas changes in gene expression can influence the initiation of behavior at different ages. We show that the age-related transition by honey bees from hive work to foraging is associated with an increase in the expression of the foraging (for) gene, which encodes a guanosine 3',5'-monophosphate (cGMP)-dependent protein kinase (PKG). cGMP treatment elevated PKG activity and caused foraging behavior. Previous research showed that allelic differences in PKG expression result in two Drosophila foraging variants. The same gene can thus exert different types of influence on a behavior.

Genes are understandably crucial to physiology, morphology and biochemistry, but the idea of genes contributing to individual differences in behaviour once seemed outrageous. Nevertheless, some scientists have aspired to understand the relationship between genes and behaviour, and their research has become increasingly informative and productive over the past several decades. At the forefront of behavioural genetics research is the fruitfly Drosophila melanogaster, which has provided us with important insights into the molecular, cellular and evolutionary bases of behaviour.

Two larval foraging strategies in Drosophila melanogaster were identified, "rover" and "sitter." "Rovers" traverse a large area while feeding whereas "sitters" cover a small area. The difference between "rovers" and "sitters" was analyzed genetically by chromosomal substitutions between isogenic stocks. Differences in larval locomotor behavior ("crawling behavior") can be attributed to the second chromosome, the "rover" strategy being dominant over the "sitter" strategy. Differences in feeding rate ("shoveling behavior") are affected additively by both the second and third chromosomes. Natural populations of Drosophila larvae were sampled three times over a 2-month period; "rovers" and "sitters" were at constant frequencies in these populations. The two foraging strategies are discussed in the light of resource utilization in environments where food is distributed continuously or discontinuously.