Natural variation in a Drosophila melanogaster odorant receptor locus

Olfaction plays a large role in an insect’s life. Adults use their sense of smell to locate the best mates and food, as well as the oviposition sites that will give their progeny the best head start in life. An insect’s ability to detect odorants is determined by olfactory receptor neurons (ORNs), which express members of three olfactory receptor families. The odorant receptors (Ors) are the largest of these three families. Despite their extensive study since their discovery in 1999, little is known about how the ligand-binding properties of the insect Ors are defined.
   
   The Ors are a large multigene family thought to have originated from a single ancestral gene. An interesting example of more recent evolution in this family is the case of Or22a and Or22b in Drosophila melanogaster. These receptors share 78% amino acid identity and it has been shown that Or22a determines the response of the ab3A ORN in the Canton S laboratory strain, while Or22b appears to be non-functional in this strain. This locus has undergone a deletion event within D. melanogaster, generating a single chimaeric gene called Or22ab that consists of the first exon of Or22a and the remaining three exons from Or22b. This chimaera is found with high frequency in the north of Australia and is completely absent from the south of Australia, where the Or22a and Or22b variant is fully penetrant. This clinal variation indicates that this locus might be under selection.
   
   In this thesis the relationship between ORN response and Or sequence was evaluated for the naturally occurring receptor variants at the Or22 locus. Through in vivo rescue experiments in the ab3A neuron it is shown here that this receptor variation leads to variation in ab3A response properties. In order to better understand the spatial variation in allele frequency at the Or22 locus in Australia, Drosophila preference behaviours to key odorants were evaluated. From this it was found that some of the ab3A phenotypic variation correlated with altered olfactory behaviour. This behavioural variation is therefore potentially linked to the geographical distribution of the allele frequencies.
   
   The genetic changes underlying the ab3A phenotypic changes were also analysed, and it was found that one amino acid residue in the second extracellular loop in Or22b, residue 194, is important for overall functionality, while two other amino acid residues, 92 and 201 in the second and fourth transmembrane domains, are important for odorant specificity. To better understand the evolution of this locus the ab3A response variants in Drosophila were also compared to those of closely-related species, which have varying ab3A responses. This showed that the ab3A phenotype seen with a functional Or22b is likely the ancestral phenotype. Taken together the findings of this thesis suggest an important role for variation in odour specificity of the ab3A neuron in the evolution of odour-mediated behaviours in Drosophila, and determine the genetic basis of this variation in odour specificity. The results of this thesis show how the naturally occurring variation in ab3A response in Drosophila is directly linked to variation at the Or22 locus. The variation at the Or22 locus provided an opportunity to better understand how these receptors have evolved and how they function. This direct linkage of receptor variation to natural neuronal phenotype variation makes this locus useful for evaluating how insect Ors bind ligands.