Molecular basis of odor coding in the malaria vector mosquito Anopheles gambiae

Abstract

A systematic functional analysis across much of the conventional Anopheles gambiae odorant receptor (AgOR) repertoire was carried out in Xenopus oocytes using two-electrode, voltage-clamp electrophysiology. The resulting data indicate that each AgOR manifests a distinct odor-response profile and tuning breadth. The large diversity of tuning responses ranges from AgORs that are responsive to a single or small number of odorants (specialists) to more broadly tuned receptors (generalists). Several AgORs were identified that respond robustly to a range of human volatiles that may play a critical role in anopheline host selection. AgOR responses were analyzed further by constructing a multidimensional odor space representing the relationships between odorants and AgOR responses. Within this space, the distance between odorants is related to both chemical class and concentration and may correlate with olfactory discrimination. This study provides a comprehensive overview of olfactory coding mechanisms of An. gambiae that ultimately may aid in fostering the design and development of olfactory-based strategies for reducing the transmission of malaria and other mosquito-borne diseases.

Fig. 1.

Tuning curves of AgORs. nonnormalized AgOR responses are presented according to ref . The 88 odorants are displayed along the x axis, with those eliciting the strongest response placed near the center, and those eliciting the weaker responses placed near the edges; as a result, the order of odorants is different for each receptor.

Fig. 2.

Structurally similar odorants are distinguished by combinations of AgORs. (Upper) Normalized responses to odorants with similar structure evoke differential AgOR responses. (Lower) Structures of odorants evoking strong responses to given odorant receptor are indicated. (A) 4mp, 4-methylphenol; 2mp, 2-methylphenol; 2ep, 2-ethylphenol; 4ep, 4-ethylphenol. (B) C7, heptanoic acid; C8, octanoic acid; C10, decanoic acid; C11, undecanoic acid.

Fig. 3.

An. gambiae odor space as visualized by PCA. (A) Odor space relationships of the adult AgOR repertoire. (B) Odor space relationships across different odorant concentrations. In all graphs, vectors quantifying the responses of the 37 antennal receptors to each tested odor were projected onto a 3D coordinate system determined by PCA as described in ref. . The 3D representation captures ∼68% of the variation in the original 37-dimensional data set. PCA was performed using the responses of each receptor to each of the pure odorants.