Knockout of OR39 reveals redundancy in the olfactory pathway regulating the acquisition of host seeking in Anopheles coluzzii

The attraction of anthropophilic mosquitoes to human host cues, such as body odour and carbon dioxide, gradually increases during adult maturation. This acquisition of host-seeking behaviour correlates with age-dependent changes in odorant receptor (OR) transcript abundance and sensitivity of olfactory sensory neurons (OSNs). One OR gene of the human malaria vector, Anopheles coluzzii, AcolOR39, is significantly downregulated in mature females, and a cognate ligand of AcolOR39, sulcatone, a major component of human emanations, mediates the observed behavioural inhibition of newly emerged (teneral) females to human body odour. Knockout of AcolOR39, using CRISPR-Cas9 mutagenesis, selectively abolished sulcatone detection in OSNs, housed in trichoid sensilla. However, knockout of AcolOR39 altered neither the response rate nor the flight behaviour of teneral females in a wind tunnel, indicating the involvement of other genes, and thus a redundancy, in regulating the acquisition of host seeking in mosquitoes.