Robertson, H.M. and K.W. Wanner. 2006 The chemoreceptor superfamily in the honey bee, Apis mellifera: expansion of the odorant, but not gustatory, receptor family. Genome Research. 16:1395-1403.

Honey bees (Apis mellifera) use olfaction for locating flowers (their food source) and for social communication. Foraging bees are able to distinguish between many olfactory clues and can detect differences between plant genotypes. The authors report here on a study of receptor proteins to better understand the range of olfaction and gustation and their underlying mechanisms in honey bees.

Olfactory receptor proteins (Ors) are a group of highly divergent proteins with seven transmembrane domains that are related to g-protein-coupled receptors (GPRCs). As discussed in class, these olfactory proteins are similar to rhodopsin, which also has seven transmembrane domains. These proteins tend to evolve quickly and can change a great deal even between species of insect. Another group of seven transmembrane domain GPRCs are the gustatory receptors (Grs), which are so named because believed to be involved in taste. Delineation of Grs and Ors is typically conducted through genomic research, as in this study.

Researchers analyzed the honey bee genome to find sequences that were similar to those already identified as Ors and Grs. Honey bees were then collected during the summer and the insects were dissected, separating antennae, proboscis, legs, heads (without antennae or proboscis) and bodies. RNA was isolated, quantified, and used for cDNA synthesis. Researchers found 170 Or genes in honey bees which is much larger than observed in flies (62) and mosquitoes (79). Just 10 Gr genes were present; compared with more than 60 for both D. melonagester and Anopheles gambiae.

Researchers used real-time quantitative PCR for 10 randomly selected Or genes and 9 Gr genes to determine expression pattern in different body parts. 7/10 Or genes were significantly enriched in worker bee antennae compared with legs, proboscis, head (without proboscis and antennae) and body. The other three Or genes were more likely to be expressed in antennae as well. 7 out of 9 Gr genes were enriched in gustatory organs including the labial palps and glossa (which has ~50-70 taste sensilla). This suggests that at least 7 Gr genes code for gustatory receptors. The Or family probably detects diverse floral odors and pheromones. This work strongly implies that olfaction sense in bees is much more varied than that of other well-studied insects. Lack of Gr genes in honey bees provides evidence that these insects do not have a very great tasting capability. Researchers believe this indicates that there are few or no Gr proteins which are responsible for detecting food toxins. Bees use antennae for contact chemoreception making it possible that some of Or proteins are in fact used for contact chemoreception similar to taste.

While not specifically mentioned in the paper, the large number of Or genes indicates that labeled-line coding is probably important in honey bee olfaction. The large number of Or genes, which are likely specifically tuned to individual molecules or portions of molecules, probably allow honey bees to detect exceedingly low levels of a relatively large number of chemicals. While the number of Or genes in honey bees appears to be large for insects it is much smaller than the nearly 350 such genes in humans and 1296 genes in mice. In class we discussed that the large number of Or genes in mice probably exist to detect potentially dangerous food sources; in bees this does not appear to be the case, particularly with the small number of Gr genes.

Another subject from class relevant to this paper is the importance of olfactory signals for colony insects. We observed that ants have a large array of scent glands and other studies indicate the same for honey bees. Thus it stands to reason that honey bees, as a colony insect, would have a greater number of Or genes than mosquitoes or fruit flies.