Female mosquitoes are major vectors of human disease and the most dangerous are those that preferentially bite humans. A ‘domestic’ form of the mosquito Aedes aegypti has evolved to specialize in biting humans and is the main worldwide vector of dengue, yellow fever, and chikungunya viruses. The domestic form coexists with an ancestral, ‘forest’ form that prefers to bite non-human animals and is found along the coast of Kenya. We collected the two forms, established laboratory colonies, and document striking divergence in preference for human versus non-human animal odour. We further show that the evolution of preference for human odour in domestic mosquitoes is tightly linked to increases in the expression and ligand-sensitivity of the odorant receptor AaegOr4, which we found recognizes a compound present at high levels in human odour. Our results provide a rare example of a gene contributing to behavioural evolution and provide insight into how disease-vectoring mosquitoes came to specialize on humans.
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Sequence Read Archive
Raw RNA-seq data are available for download at the NCBI Sequence Read Archive (accession number SRP035216). Coding sequences of AaegOr4 alleles are at GenBank (accession numbers KF801614, KF801615 and KF801617–KF801621).
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We thank M. K. N. Lawniczak, K. J. Lee, M. N. Nitabach, and the Vosshall laboratory for discussion and comments on the manuscript; J. E. Brown and J. R. Powell for discussion and coordination of field collections; W. Takken for advice regarding many aspects of this work; J.-P. Mutebi, B. Miller, and A. Ponlawat for live specimens from Uganda and Thailand; D. Beck, K. Nygaard, K. Prakash, and L. Seeholzer for expert technical assistance. We also thank X. Chen for pre-publication access to a draft Ae. albopictus genome assembly, and J. Liesch for access to Orlando strain RNA-seq data. We received valuable advice on collecting and working with forest and domestic forms of Ae. aegypti from M. Trpis, J. L. Peterson, and P. Lounibos, and on the design and use of two-port olfactometers from U. Bernier and V. Sherman. This work was funded in part by a grant to R. Axel and L.B.V. from the Foundation for the National Institutes of Health through the Grand Challenges in Global Health Initiative. This work was supported in part by the following National Institutes of Health grants: K99 award from NIDCD to C.S.M. (DC012069), an NIAID Vectorbase DBP subcontract to L.B.V. (HHSN272200900039C), and a CTSA award from NCATS (5UL1TR000043). R.I. received support from the Swedish Research Council and SLU: Insect Chemical Ecology and Evolution (IC-E3). L.B.V. is an investigator of the Howard Hughes Medical Institute.
The authors declare no competing financial interests.
Extended data figures and tables
a, Representative photograph used to measure scale colour (Fig. 1e, g). Red dots mark the approximate position of 4 points where the colour of dark scales on the scutum was assessed. b, Representative photograph used to measure cuticle colour (Fig. 1f, h). Red dots mark the approximate position of 4 points where the colour of bare cuticle on the circular postnotum was assessed. c, Representative photographs used to assess the extent of white scaling on the first abdominal tergite (Fig. 1i), outlined with the red rectangle. Each individual is representative of the scaling score shown at the bottom.
Extended Data Figure 2 Or4 coding sequence variation in human-preferring and guinea-pig-preferring colonies from around the world.
a, Geographical origin of colonies characterized in b and c. Circle fill colour indicates preference of strains. Circle outline colour indicates origin: Purple, laboratory strain derived from USA; blue, reference genome strain derived from West Africa; orange, Uganda; red, Kenya, green, Thailand. b, Host preference assayed in the live host olfactometer. Data for Thailand, K14, K2, K4, K27, K18, K19, and Uganda are reprinted from Fig. 2g. c, Frequency of non-synonymous single nucleotide polymorphisms (SNPs) in female antennal RNA-seq reads. SNPs are defined as differences from the A reference allele. SNPs with frequency ≤ 0.1 are not shown. Vertical black and red lines indicate SNPs that were present and absent, respectively, in the major alleles subject to functional analysis.
Dots represent amino acid differences with respect to the genome reference, not an inferred ancestor. Red dots indicate differences that are unique to the given allele. Blue dots indicate differences that are shared among multiple alleles. Snake plots are based on the predicted orientation and location of transmembrane domains. Extracellular loops are oriented up and cytoplasmic loops are oriented down. Allele names are indicated to the left of each snake plot.
a, Histogram showing the number of alleles represented in the Or4-derived PacBio reads obtained for each of 270 parent and F2 hybrid mosquitoes. Alleles were only considered if they received at least 5% of an individual’s reads. b, Histogram showing the fraction of reads from individual mosquitoes assigned to individual alleles. For all 270 mosquitoes, individual alleles were represented by either very few reads (grey bars, inferred to result from allele or barcode assignment errors or polymerase chain reaction contaminants), approximately half the reads (light blue bars, inferred to represent the two alleles in heterozygotes), or over 98% of all reads (dark blue bars, inferred to represent the single allele carried by homozygotes).
Extended Data Figure 5 Response of human-preferring mosquitoes to sulcatone-scented guinea-pig odour.
a, Olfactometer apparatus in which 50 mosquitoes per trial were given a choice between guinea-pig odour/CO2 mix supplemented with solvent on one side and sulcatone 10−4 on the other side. b, Corrected preference for sulcatone vs solvent ports is indicated. Data were corrected for the daily average left–right side bias observed across 2–3 solvent vs solvent tests conducted on each day of testing. An index value of 1 indicates strong preference for the sulcatone side, whereas −1 indicates strong preference for the solvent side. Neither mosquito colony showed a preference significantly different from zero (one-sample t-test P = 0.76 for ORL, P = 0.11 for K14). The trials for each colony were performed across 4–8 days (n = 40 for ORL and n = 22 for K14).
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McBride, C., Baier, F., Omondi, A. et al. Evolution of mosquito preference for humans linked to an odorant receptor. Nature 515, 222–227 (2014). https://doi.org/10.1038/nature13964
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