The ability to detect and discriminate between chemical cues in the environment is key to successful host finding in mosquitoes. The recent availability of the genome sequence of Anopheles gambiae, the principle vector of human malaria in Africa, has allowed for the identification and annotation of the genes believed to encode odorant (AgOr) and gustatory (AgGr) receptor genes in the mosquito. These genes are members of the G-coupled protein receptor (GCPR) superfamily of chemoreceptors, characterized in part by their seven-transmembrane domains. Phylogenetic analysis reveals that only a few orthologous pairs of the genes have been conserved between Drosophila and Anopheles. Otherwise, most Ors and Grs form species-specific gene subfamilies.
We have begun studies of the expression patterns of the putative AgGr genes, based upon an RT-PCR assay. In this way, we can examine the conservation of expression of Grs in A. gambiae over the roughly 250 million years since it diverged from the lineage containing Drosophila melanogaster. Patterns of similarity across the two species are apparent. Three of the genes (AgGr22, AgGr23, AgGr24) with high sequence similarity to two genes in Drosophila melanogaster (DmGr21a and DmGr63a) are expressed in antennae and maxillary palps, which matches the expression patterns seen in Drosophila. Interestingly, DmGr21a has recently been identified as a candidate carbon dioxide receptor in Drosophila. We are in the process of determining whether the closely related anopheline genes share the same function.
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