The expression and function of gustatory receptors in the honeybee (Apis mellifera)

Abstract

The ability to accurately assess the chemical environment is vital to the honeybee and as bees mature, their demands upon their chemical senses change. While honeybee olfaction has been intensively studied, the physiological mechanisms of bee gustation have only recently come to light. Robertson and Wanner (2006) were the first to identify honeybee gustatory receptors (Grs) and in comparison to other insect species, honeybees possess surprisingly few Gr genes (Apis: 10 Gr genes). The current project aimed to assess the expression of honeybee Gr genes and relate this to selection of the two most concentrated components of floral nectar; sugars and amino acids. The behavioural experiments demonstrated that bees are able to differentiate between the two major floral monosaccharides, with both newly emerged and forager bees exhibiting a slight fructose preference over glucose. Additionally, while no individual amino acid solution was preferred over sucrose alone, newly emerged bees were most willing to consume an eight amino acid mixture, probably due to its protein-resemblance, a major dietary component for young bees. Interestingly, the analysis of anatomical receptor gene expression discovered all 10 Gr genes in every gustatory appendage assayed (mouthparts, tarsi and antennae). All receptor genes were additionally expressed internally (gut and brain) indicating that, as in other insect species, honeybee Grs may play a role in nutrient sensing and feeding regulation. Some differential Gr gene expression was discovered between newly emerged and forager bees, indicating altered gustatory sensitivity with task differentiation. Finally, the expression of Gr genes in the forager brain were dependent on the nutritional status of the individual as well as nutritional experience. The current study demonstrated that AmGr3 may be acting as a nutrient sensor, with altered gene expression following starvation or changes in diet.