We next asked whether the An. gambiae
and D. melanogaster
Or repertoires differ in their coverage of odour space. As an initial means of addressing this issue, we considered the 53 odourants that were tested against both the AgOr and the DmOr repertoires12
and constructed odour spaces of the type described above for each receptor repertoire.
The two species differed in their relative coverage of odour space. The mosquito allocated greater relative coverage to the aromatics (; dark blue in lower right quadrant). By contrast, the fly devoted greater relative activity to some of the esters (dark green) and one of the two aldehydes that were compared (gray) (, Supplementary Figure 5b
). We then compared the two species with respect to the strong responses (≥100 spikes/second). In the mosquito, 15% of the receptor-aromatic combinations yielded strong responses, compared to 7% of the receptor-ester combinations. By contrast, in the fly, 9% of the receptor-aromatic combinations yielded strong responses, compared to 20% of receptor-ester combinations ().
We next considered whether the species differed with respect to another kind of odour space, a biological odour space that relates odourants based on the primary sensory signals they generate. We created a space in which each axis represents the response magnitude (in spikes/second) for one odourant receptor, as described previously12
. Odourants that elicit similar patterns of activity across the receptor repertoire map close together. Odourants that are close may be similar in their perceptual qualities, and may be more difficult for the animal to discriminate because they generate similar patterns of ORN activity. Experiments conducted with Drosophila
larvae have provided evidence to support a relationship between such odour space distances and perception10
We constructed such spaces for the mosquito and the fly and depicted them in three dimensions by applying PCA (). Odourants of the same chemical class tend to cluster together (), as previously observed in the fly12
. However, some chemical classes were differentially distributed in the odour spaces of the two insects. Esters were more widely distributed in D. melanogaster
space than in An. gambiae
space, while aromatics were more widely distributed in An. gambiae
space (). To quantify these differences, we calculated the Euclidean distance for every pair of odourants with the same functional group, within the odour space of each species. We found that the mean inter-odourant distance for esters is significantly higher for D. melanogaster
than An. gambiae,
while the mean inter-odourant distance for aromatics is higher for An. gambiae
(p<0.001 for esters, p=0.01 for aromatics, Mann-Whitney); there were no differences in distances among alcohols or ketones, the other groups that could be compared. These results suggest that mosquitoes may be better able than fruit flies to discriminate among aromatics, while fruit flies may be better able to discriminate among esters, perhaps reflecting the biological relevance of these classes of compounds to the animals.
Distribution of odourants in a receptor activity-based odour space
Are the functional differences between the mosquito and fruit fly Or repertoires due to a particular clade of mosquito or fly receptors? When the AgOr
family was first identified, phylogenetic analysis revealed a clade of An. gambiae
odourant receptors with no close D. melanogaster
relatives, and a clade of Drosophila
odourant receptors with no close An. gambiae
(see also Supplementary Figure 6
). Do these species-specific clades of receptors respond to odourants of a particular kind? We used matrix analysis to generate a dendrogram of receptors (Supplementary Figure 7
), and PCA to create a “receptor space,” based on the responses of the receptors to odorants (Supplementary Figure 8
). In neither case did we observe clustering of receptors of the species-specific clades. Thus, the observed functional differences between the An. gambiae
and D. melanogaster
odourant receptor repertoires may reflect evolutionary changes distributed across the receptor repertoires rather than concentrated in a specific branch of the phylogenetic tree.
We note finally that no AgOr showed even a modest response of 50 spikes/second to any carboxylic acid or to any amine in our system. By contrast, some DmOrs respond strongly to certain carboxylic acids12, 41
. Recently, a set of variant-ionotropic glutamate receptors that respond to amines and carboxylic acids have been identified in D. melanogaster42
. Receptors of this class may detect these compounds in An. gambiae