The cation channel TRPA1 tunes mosquito thermotaxis to host temperatures

1Thermotaxis is a sensory-motor behavior that guides animals toward a preferred temperature. This type of sensory navigation allows animals to avoid environments of noxious cold and heat, with the goal of remaining in physiologically suitable ambient temperatures. For ectotherms, such as most insects, thermotaxis behavior is the primary method of thermoregulation. Terrestrial invertebrates are vulnerable to temperature extremes, facing the risk of desiccation at elevated temperatures, and rapid hypothermia at low temperatures. Therefore, mechanisms to detect environmental temperatures and trigger appropriate approach or avoidance behaviors are extremely important for their survival. For instance, adult Caenorhabditis elegans worms migrate preferentially toward a specific thermal environment determined by the temperature of their cultivation. Adult Drosophila melanogaster flies prefer a narrow range of air temperatures around 24–25°C and rapidly avoid air temperatures of ~31°C.

2Interestingly, some hematophagous (blood-feeding) arthropods have evolved a specialized mode of thermotaxis to locate endothermic (warm-blooded) hosts. Such thermophilic behavior is seen in kissing bugs [Triatoma infestans and Rhodnius prolixus], the bedbug [Cimex lectularius], the tick [Ixodes ricinus], and many species of mosquito including Ae. aegypti, a major tropical disease-vector. Female Ae. aegypti require a vertebrate blood meal for the production of eggs, and finding a suitable warm-blooded host is therefore an essential component of reproduction. Mosquitoes use a variety of physical and chemical senses to locate hosts in their environment. When host-seeking, these animals become strongly attracted to inanimate warm objects, eagerly probing at them as if they were hosts.

3In nature, mosquitoes thermotax in a complex thermal landscape in which ambient air temperature, host body temperature, and surrounding surface temperatures can vary widely. For mosquitoes such as Ae. aegypti, host-seeking behavior can be activated by an increase in ambient carbon dioxide (CO2). This activation elicits flight activity and results in an array of behaviors including attraction to visual stimuli and host olfactory cues, and landing on warm objects. Ae. aegypti flying in a wind tunnel can detect a warmed stimulus from a distance, eliciting attraction and thermotaxis.

4What are the mechanisms by which animals detect thermal stimuli, and how might these be adapted for the specialized needs of heat-seeking female mosquitoes? Thermotaxis is typically initiated by thermosensitive neurons that sample environmental temperature to inform navigational decision-making. Such neurons must be equipped with molecular thermosensors capable of detecting and transducing thermal stimuli. Diverse molecular thermoreceptors have been identified in the animal kingdom, many of which are members of the transient receptor potential (TRP) superfamily of ion channels. Different thermosensitive TRPs show distinct tuning spanning the thermal spectrum from noxious cold to noxious heat. Among these is TRPA1, which is a heat sensor in multiple insects, including the vinegar fly D. melanogaster and the malaria mosquito Anopheles gambiae. Neurons in thermosensitive sensilla of An. gambiae female antennae express TRPA1. In D. melanogaster, TRPA1 is expressed in internal thermosensors located in the brain, and DmelTRPA1-/- mutants fail to avoid high air temperature in a thermal gradient. Interestingly, some snakes and vampire bats express thermosensitive TRP channels in organs used to sense infrared radiation from warm-blooded prey. This raises the possibility that AaegTRPA1 may be used by mosquitoes to find hosts. Recently, a structurally distinct insect thermosensor, Gr28b, was identified in D. melanogaster. Gr28b, a gustatory receptor paralog, is expressed in heat-sensitive neurons of D. melanogaster aristae and is an important component of thermotaxis during rapid avoidance of heat. It is also highly conserved among Drosophila species, and has a clear ortholog in Ae. aegypti, AaegGr19. A functional role for these thermosensors has never been investigated in the mosquito.

5Here, we use high-resolution quantitative assays to examine the behavioral strategies underlying mosquito heat-seeking behavior. Our results show that by seeking relative warmth and avoiding both relative cool and high temperatures, female mosquitoes selectively localized to thermal stimuli that approximate warm-blooded hosts. Using genome editing, we generated mutations in the candidate thermoreceptors, AaegTRPA1 and AaegGr19. We found that TRPA1 is required for tuning mosquito thermotaxis during host-seeking. AaegTRPA1-/- mutants lacked normal avoidance of thermal stimuli exceeding host body temperatures, resulting in a loss of preference for biologically relevant thermal stimuli that resemble hosts. This work is important because it identifies a key mechanism by which mosquitoes tune their thermosensory systems toward human body temperatures.

6We previously described an assay to model heat-seeking behavior in the laboratory by monitoring mosquitoes landing on a warmed Peltier element in the context of a cage supplemented with CO2 (Figure 1A,B). This assay has the advantages that it is simple in design, produces robust behaviors, and enables the collection of data from large numbers of animals in a short experimental timeframe. Using this system, we can examine mosquito responses to diverse thermal stimuli and measure thermotaxis in different ambient temperature environments. We first needed to determine whether heat-seeking behavior habituates over multiple thermal stimulations. In our heat-seeking assay, Ae. aegypti mosquitoes reliably responded to 12 serial presentations of a 3-minute long 40°C stimulus over the course of more than 2.5 hr, with no evidence of habituation (Figure 1—figure supplement 1).

7Ae. aegypti can feed on a variety of hosts with core body temperatures ranging from ~37°C (humans) to ~40–43°C (chickens) (Figure 1C). It is unknown whether there are minimal or maximal temperature thresholds constraining mosquito heat-seeking, and whether responses to thermal stimuli depend on the background ambient temperature.

8To investigate these questions, we measured attraction to thermal stimuli produced by heating the Peltier to temperatures ranging from ambient (set to 26°C in these experiments) to 60°C (Figure 1D, E). We found that mosquitoes were highly sensitive to thermal contrast and were attracted to stimuli 2.5°C above ambient (Figure 1D–F). Furthermore, mosquito occupancy on the Peltier increased monotonically with stimulus temperatures up to 40°C. However, for higher temperature stimuli, we observed a dramatic reduction in Peltier occupancy. A 50°C stimulus resulted in approximately half as many animals on the Peltier compared to a 40°C stimulus (Figure 1D–F). Stimuli of 55°C or greater resulted in occupancy rates indistinguishable from an ambient thermal stimulus (26°C) (Figure 1D–F). Spatial analysis of mosquito occupancy on or near the Peltier revealed that while mosquitoes were still attracted to high-temperature stimuli, they populated the area peripheral to the Peltier, and strongly avoided the Peltier itself for stimuli ≥ 55°C (Figure 1F).

9Female mosquitoes searching for a warm-blooded host may be responding to the absolute temperature of a stimulus or may instead be evaluating relative warmth, defined as the differential between a stimulus and background ambient temperature. To investigate the thermotaxis strategies constituting mosquito heat-seeking behavior, we conducted experiments at three ambient temperatures: 21, 26, and 31°C (Figure 2A–F). We found that Peltier occupancy for stimuli 21–40°C depended on the differential between the Peltier and ambient temperature (Figures 2B,C), rather than the absolute temperature of the Peltier (Figure 2A). For example, at all ambient temperatures tested, a stimulus 5°C above ambient was sufficient to elicit significant heat-seeking, and elicited approximately half as much Peltier occupancy as a stimulus 10°C above ambient. On the other hand, heat-seeking to targets 50–55°C was inhibited at all ambient temperatures tested (Figures 2D,F), despite the fact that the temperature differential varied widely in these situations (Figure 2E).

10These results show that Ae. aegypti thermotaxis is driven by seeking relative warmth, but restricted by an absolute upper threshold of ~50–55°C. Because female mosquitoes are attracted to relative warmth, we hypothesized that they may also avoid relative cool. This complementary behavior would serve to improve host-seeking thermotaxis. We examined mosquito responses to cooling by analyzing the rate at which animals left the Peltier when it cooled at the conclusion of a stimulus period. We found that mosquitoes left the Peltier at similar rates regardless of the absolute temperature of the stimulus (Figure 2G,H, Figure 2—figure supplement 1; based on analysis of data in Figure 1D,E), demonstrating that mosquitoes avoid relative cool during heat-seeking.

11Our characterization of Ae. aegypti heat-seeking revealed multiple behavioral components contributing to selective thermotaxis during host-seeking: 1) the seeking of relative warmth; 2) the avoidance of relative cool; and 3) the avoidance of thermal stimuli exceeding host temperature. Each of these sensory-motor functions may rely on the same molecular thermosensors, or may instead use distinct thermosensors. We considered the possibility that thermoreceptors ordinarily dedicated to the behavioral thermoregulation typical of most ectotherms such as D. melanogaster, may have evolved a function in host-seeking by mosquitoes and other hematophagous arthropods. Using this reasoning, we generated AaegTRPA1-/- mutants using zinc-finger nuclease-mediated genome editing (Figure 3A, Figure 3—figure supplement 1A).

12In addition to its function as a thermoreceptor, TRPA1 is a highly conserved chemosensor of electrophile irritants such as N-methylmaleimide. Using a modified capillary feeding (CAFE) assay (Figure 3B), we found that wild-type Ae. aegypti mosquitoes strongly avoided consumption of N-methylmaleimide (Figure 3C), as well as the bitter compound denatonium benzoate (Figure 3D). AaegTRPA1-/- mutants rejected denatonium benzoate (Figure 3D) but did not avoid consumption of N-methylmaleimide (Figure 3C). We interpret this result as a loss of N-methylmaleimide detection in AaegTRPA1-/- mutants, leading to no preference between sucrose and sucrose containing N-methylmaleimide. We note that this simple CAFE assay could be used to discover additional mosquito anti-feedants to repel Ae. aegypti, beyond the two chemicals identified here.

13Because TRPA1 is important in insect thermoregulation, we used a modified thermal gradient assay to assess thermal preference in wild-type and AaegTRPA1-/- mutant mosquitoes (Figure 3—figure supplement 1B). AaegTRPA1-/- mutants were impaired in avoidance of high air temperature, leading to significant mortality (Figure 3—figure supplement 1C–G). Together, these data indicate that TRPA1 has a conserved chemosensory and thermosensory function in Ae. aegypti.

14We next asked if AaegTRPA1 is required for mosquito heat-seeking behavior. AaegTRPA1-/- mutants showed normal attraction to stimuli at or below 45°C, but strikingly lacked normal avoidance of higher temperature stimuli (50°C and 55°C) (Figures 3E,F). A detailed analysis of Peltier occupancy over time revealed that AaegTRPA1-/- mutants persisted on the Peltier during 50, 55, and 60°C stimulus presentations, whereas control animals rapidly left these high-temperature stimuli (Figure 3G). Therefore, AaegTRPA1 is not required for initial attraction to warmth but is required for normal avoidance of high-temperature stimuli that exceed host body temperature.

15Because AaegTRPA1-/- mutants retained attraction to warm stimuli, we used targeted mutagenesis to test a requirement for AaegGr19, the Ae. aegypti ortholog of Gr28b, in heat-seeking (Figure 3—figure supplement 2A,B). Although this thermosensor is required for mediating rapid avoidance of warmth in D. melanogaster, AaegGr19-/- mutant mosquitoes showed no thermotaxis defects (Figure 3—figure supplement 2C). The different phenotypes of these mutations in Ae. aegypti and D. melanogaster may reflect differences in expression patterns of these genes. DmelTRPA1 is expressed in internal thermosensors of the brain, while thermosensitive isoforms of DmelGr28b are expressed in peripheral heat-sensors. In Ae. aegypti, TRPA1 RNA is expressed in numerous tissues including the antennae, where it may be associated with peripheral thermosensitive sensilla responding to rapid thermal fluctuations, as it is in An. gambiae. The cellular expression pattern of Gr19 in Ae. aegypti is not known, but its transcript is broadly expressed.

16Although AaegTRPA1-/- mutants did not show normal avoidance of high-temperature stimuli, they may still prefer host-temperature stimuli if presented with a choice. In a heat-seeking choice assay with two independently controlled Peltiers, we examined the importance of AaegTRPA1 in guiding mosquito thermotaxis in a more complex thermal landscape (Figure 4A). In this assay, mosquitoes were simultaneously presented with two thermal stimuli. When presented with two 40°C stimuli, both wild-type and mutant mosquitoes distributed equally between the Peltiers (Figure 4B), but in a choice between a 40°C and 50°C stimulus, wild-type mosquitoes strongly preferred the 40°C Peltier (Figure 4C) and avoided the 50°C Peltier (Figure 4D). Remarkably, in this choice scenario, AaegTRPA1-/- mutants failed to avoid the 50°C Peltier, resulting in no preference for the 40°C stimulus (Figure 4 C, D, Video 1).

17We have elucidated the basic thermotaxis strategies used by mosquitoes, and revealed an important role for TRPA1 in regulating this behavior (Figure 4E). Using a quantitative thermotaxis assay, we modelled Ae. aegypti heat-seeking behavior in the laboratory. We found that mosquitoes can search for hosts in a wide range of ambient temperatures by seeking relative warmth and avoiding relative cool. Remarkably, these animals can detect a stimulus with thermal contrast as small as 2.5°C. In an outdoor environment, however, hosts are often warmer than the surrounding air but cooler than sun-warmed soil, rocks, trees, and human-made objects (Figure 4F). For this reason, diurnal mosquitoes such as Ae. aegypti are poorly served by merely thermotaxing to the hottest object available. A more optimal strategy is to search specifically for biologically relevant stimuli, and to avoid thermal stimuli exceeding host temperature, as we have observed in our laboratory models of heat-seeking. Acquiring a blood meal is an essential component of reproduction for a female Ae. aegypti mosquito. To maximize her chances of success, a female mosquito should reject ‘distracting’ stimuli that exceed host temperatures. Our results demonstrate that AaegTRPA1 is critical for this selective thermotaxis.

18Mosquito heat-seeking behavior represents an excellent model system for further study of the genetics, neuroscience, and decision-making underlying thermosensation and thermotaxis. Until now, mechanistic studies of thermosensation have been largely restricted to traditional laboratory model organisms, such as domestic mice and Drosophila melanogaster flies, whose thermotaxis consists mainly of moving away from suboptimal thermal environments. Mosquitoes too, must undergo such behavioral thermoregulation, as we have found in our thermal gradient assay. However, their repertoire of thermotactic behaviors is expanded by the evolution of a specialized and highly tuned mode of thermotaxis to locate warm-blooded hosts. It may also be that these two thermotactic drives—host-seeking and thermoregulation—interact in mosquitoes. For instance, the avoidance of high-temperature stimuli during heat-seeking may be influenced by a nociceptive or thermoregulatory response independent of host-seeking behavior. It will be interesting to investigate the neural mechanisms that regulate divergent behavioral choices of thermoregulation and heat-seeking, and whether these systems are in behavioral conflict during mosquito host-seeking.

19Our work identifies TRPA1 as a gene regulating mosquito avoidance of high-temperature stimuli, which we have shown to be a major behavioral component of heat-seeking. However, because both AaegTRPA1-/- and AaegGr19-/- mutants retain normal attraction to warmth, this aspect of heat-seeking must rely on other thermoreceptors, still to be identified. Our study shows that this attraction must be mediated either by a single thermosensor that adapts to background temperature, or multiple thermosensors each tuned to a distinct absolute threshold. During interactions with a warm-blooded host or a 50°C Peltier, mosquitoes are likely experiencing a wide range of thermal fluctuations. For instance, there can be a 10°C thermal gradient in the air within 5 mm of a 37°C stimulus or human arm. This suggests that for a mosquito standing on a warmed Peltier, the antennae, brain, thorax, forelegs, and proboscis may all be experiencing different temperatures. The temperature of mosquito tissues will also be greatly influenced by their material and geometric properties, as well as thermal conduction due to contact with the Peltier. Furthermore, convective plumes forming in the air near a vertical heated plate can be highly dynamic and turbulent in their structure, with thermal air gradients differing considerably at the bottom and top of the plate. These features of thermal stimuli may explain why mosquitoes in our assay often appear to be differentially attracted to the bottom and top of the Peltier. Future studies characterizing this complex thermal microenvironment, and identifying the relevant thermosensory neurons and receptors will be required to define the thermal fluctuations experienced by mosquitoes during heat-seeking. Understanding the behavioral and molecular basis of thermotaxis in mosquitoes and other disease vectors is of great biomedical importance. Ae. aegypti mosquitoes are potent vectors of yellow fever, chikungunya, and dengue arboviruses, resulting annually in hundreds of millions of infections. Further study of mosquito heat-seeking behavior may aid in the design of next-generation traps, repellents, and control strategies.

20Ae. aegypti wild-type (Orlando), AaegGr19, and AaegTRPA1 mutant strains were maintained and reared at 25–28°C, 70–80% relative humidity with a photoperiod of 14 hr light:10 hr dark (lights on at 8 a.m.) as previously described. Adult mosquitoes were provided constant access to 10% sucrose solution for feeding, and females were provided with a blood source for egg production, either live mice or human volunteers. Blood-feeding procedures were approved and monitored by The Rockefeller University Institutional Animal Care and Use Committee and Institutional Review Board, protocols 14756 and LV-0652 respectively. Human volunteers gave their informed written consent to participate in mosquito blood-feeding procedures. Before behavioral assays, mosquitoes were sexed and sorted under cold anesthesia (4°C) and fasted for 15–24 hr in the presence of a water source.

21PCR was carried out using Novagen KOD polymerase (EMD Millipore, Billerica, MA), products were cloned using pCR4-TOPO (Invitrogen), and Sanger sequenced by Genewiz (South Plainfield, NJ).

22ZFNs targeting AaegTRPA1 or AaegGr19 (VectorBase Accession numbers AAEL009419 and AAEL011073, respectively) were designed and produced by the CompoZr Custom ZFN Service (Sigma-Aldrich, St. Louis, MO). The nucleotide sequences of the ZFN-binding sites (upper case) and nonspecific cut site for wild-type heterodimeric Fok1 endonuclease (lower case) are:

23AaegTRPA1: 5’-GTCGTTTTCGTCCATACCgatgtcGTTGCTTAGGACGTT-3’.

24AaegGr19: 5’-ACCAACCTTTCACTGCaaatgacCCACCGGAAAGTGGCA-3’.

25Donor plasmids were generated as previously described. All donor plasmids included homologous arms cloned from wild-type genomic DNA, and consisting of sequence flanking or partially overlapping the ZFN recognition sites.

26AaegTRPA1 was targeted using pSL1180-HR-PUbECFP-TRPA1. The left homologous arm (1608 bp, primers: forward, 5’-GCATGCATGGGTAACAAGAAGGGTTGT-3’ and reverse, 5’-CGACAAGTGGTTTACTTGTGTGTCAATC-3’) and right homologous arm (3037 bp, primers: forward, 5’-CGTTTTCCATGATGCTCGGC-3’ and reverse, 5’-CGAAGACCAACGCGATGTAGTTCCA-3’) were cloned into the EcoRI and NotI sites of pSL1180-HR-PUbECFP (Addgene #47917), respectively.

27AaegGr19 was targeted using pSL1180-HR-PUBdsRED-Gr19. The left homologous arm (1257 bp, primers: forward, 5’-AGGTATGCCTGGATTGGACGTAAGAAA-3’ and reverse, 5’-GCAGTGAAAGGTTGGTTAACTG-3’) and right homologous arm (1203 bp, primers: forward, 5’-CCACCGGAAAGTGGCATTTACCGC-3’ and reverse, 5’-GCGACGGTCCCTTGCGATGTCGTTAT-3’) were cloned into the XmaI and NotI sites of pSL1180-HR-PUBdsRED (Addgene #49327), respectively.

28To generate AaegTRPA1 homologous recombination mutant alleles, 2000 pre-blastoderm stage wild-type embryos were microinjected (Genetic Services Inc., Cambridge, MA) with AaegTRPA1 ZFN mRNA (200 ng/µl) and pSL1180-HR-PUbECFP-TRPA1 (750 ng/µl). To generate AaegGr19 homologous recombination mutant alleles, 1000 pre-blastoderm stage wild-type embryos were microinjected (IBBR Insect Transformation Facility, Rockville, MD) with AaegGr19 ZFN mRNA (200 ng/µl) and pSL1180-HR-PUBdsRED-Gr19 (700 ng/µl). Injected G0 animals were crossed to wild-type in multiple batches to generate G1 lines with independent ZFN mutagenesis events. G1 homologous recombination mutant individuals were recovered via fluorescence as previously described, and outcrossed separately to wild-type for five generations before establishing four independent homozygous lines: AaegTRPA1ECFP-1, AaegTRPA1ECFP-2, AaegGr19DsRed-1, and AaegGr19DsRed-2.

29To confirm directed insertion of the PUbECFPnls-SV40 cassette into the AaegTRPA1 locus, a diagnostic PCR product (no wild-type band, 2075 bp mutant band) was amplified using a forward primer anchored outside the boundary of the left homologous arm (5’-CATGGGACAATTTGGCGTAGGCAGTAT-3’) and an ECFP reverse primer anchored inside the inserted cassette (5’- AGATCTCGACCCAAGAAAAAGCGGAAG-3’). To establish homozygous AaegTRPA1-/- lines, a diagnostic PCR product (679 bp wild-type band, 3305 bp mutant band) spanning the ZFN cut-site was amplified using primers: forward, 5-GGTTTCAAGGATGATTGACACACAAG-3’, and reverse, 5’-GCAGAGCTGATTTCTCGTAGTTTTCG-3’.

30To confirm directed insertion of the PUBdsRED-SV40 cassette into the AaegGr19 locus, a diagnostic PCR product (6362 bp wild-type band, 8787 bp mutant band) was amplified using primers anchored outside the boundaries of both homologous arms: forward, 5’-AGCTGATCAACGTTAACAACTACGATG-3’, and reverse, 5’-AGAGCATGGTGTAAACTTGACAGCTCAA-3’.

31The following lines were used for all experiments: AaegTRPA1ECFP-1/ECFP-2, AaegTRPA1ECFP-1/+, AaegGr19DsRed-1/DsRed-2, AaegGr19DsRed-1/+. Heteroallelic mutants were used to minimize fitness effects that might be present in homozygous mutants.

32All assays were carried out between ZT2-ZT12 at 26°C and 70–80% relative humidity unless stated otherwise. Whenever possible, time of day was randomized across conditions. All mosquitoes used were 10–21 day-old females, age-matched across conditions and genotypes.

33Assays were performed as previously described, in a 30 × 30 × 30 cm Plexiglas enclosure. All stimulus periods lasted 3 min and were presented on a single Peltier element (6 x 9 cm) covered with a piece of standard white letter size printer paper (extra bright, Navigator; Office Depot/Office Max, Cleveland, OH) cut to 15 × 17 cm and held taut by a magnetic frame. CO2 pulses (20 s) accompanied all stimulus period onsets. A second identical control Peltier element was situated on the wall opposite to the stimulus Peltier and was set to ambient temperature during all experiments. Peltier temperature set-point is reported throughout the paper. Measurements of Peltier temperature via a thermocouple embedded in the Peltier element are reported in Figure 1D, Figure 1—figure supplement 1A and Figure 2—figure supplement 1. For each trial, 45–50 mosquitoes were introduced into the assay, and only mosquitoes directly on the Peltier area (Figure 1B) during seconds 90–180 of stimulus periods were scored. The custom MATLAB scripts used to run and analyze these experiments are available for download as Source code 1 and 2 respectively. Heat maps are smoothed 2D histograms of mean mosquito occupancy during seconds 90–180 of stimulus periods, sampled at 1 Hz and binned into 12 × 16 image sectors.

34The assay was modified from the heat-seeking assay described above. For each trial, 45–50 mosquitoes were introduced into a custom-made Plexiglas box (38.5 × 28 × 12.5 cm) with two Peltier elements (4.6 × 6.5 cm surface area, ATP-O40-12, Custom Thermoelectric, Bishopville, MD) each covered with a taut piece of standard white letter size printer paper (extra bright, Navigator) cut to 9 × 11 cm. The Peltier elements were mounted on a single wall, 3.5 cm above the floor, 8.5 cm from either edge of the enclosure, and adjacent to one another separated by 6.5 cm. Mosquitoes were allowed to acclimate for 10 min, at which point the Peltier elements were both warmed to 40°C. After fully warming the Peltier elements for 60 s, a CO2 pulse (20 s) was added to the airstream and the Peltier elements were kept at 40°C for a 4-min stimulus period at which point they were returned to ambient temperature (26°C). After a 12-min interstimulus period, the stimulus was repeated, this time with the left Peltier element warmed to 50°C while the right Peltier element was warmed to 40°C. A camera (Point Grey Research, Richmond, BC, Canada) was used to measure mosquito landings, and only mosquitoes directly on either Peltier area during seconds 60–240 of stimulus periods were scored. Preference was calculated by subtracting the proportion of total mosquitoes on the left Peltier (40 or 50°C) from the proportion of total mosquitoes on the right Peltier (40°C). Heat maps are smoothed 2D histograms of mean mosquito occupancy during seconds 60–240 of stimulus periods, sampled at 1 Hz and binned into 15 × 52 image sectors.

35This assay was adapted for the mosquito from the original assay developed for Drosophila For each trial, five mosquitoes were fasted with access to water for 24 hr, and placed in a polypropylene vial (#89092–742, VWR, Radnor, PA) with access to two 5-µl calibrated glass capillaries (#53432–706, VWR) containing 10% (w: v) sucrose with 5% (v: v) green McCormick brand food dye. Capillaries spaced ~0.5 cm apart traversed the cotton plug (#49–101, Genesee Scientific, San Diego, CA) of the vial and protruded into the vial < 1 mm to provide a flush surface for mosquitoes to access the liquid while resting on the plug surface. The control capillary contained only green sucrose solution, and the experimental capillary contained green sucrose solution supplemented with 0, 1, or 10 mM of denatonium benzoate (Sigma-Aldrich) or 0, 10, 50, or 100 mM of N-methylmaleimide (Sigma-Aldrich). The experimental capillary with 0 mM chemical was identical to the control capillary, and served as a zero-choice to test for side-bias in the assay. Because a small amount of liquid evaporated during preparation of the capillaries, all choice conditions and genotypes were prepared in a time-staggered format so that any measurement error due to evaporation was spread across the conditions. The levels of remaining liquid in both capillaries were measured after 18–20 hr and were compared to the known initial liquid level. Experiments started at ZT 8–10 and ended at ZT 4–6 the following day. Consumption values were compared to control capillaries in vials without mosquitoes to account for evaporation. We note that in cases where mosquitoes did not feed from a capillary and all liquid loss was due to evaporation, consumption values could be calculated to be negative due to very small variation in evaporation rates between experimental and control capillaries. Any negative consumption values were rounded to zero. Sucrose preference was calculated by dividing the amount consumed from the control capillary (not containing denatonium benzoate or N-methylmaleimide) by the total amount consumed from both capillaries. In experiments with 0 mM denatonium benzoate or 0 mM N-methylmaleimide, one capillary was arbitrarily designated ‘sucrose only’.

36This assay was adapted from one developed for Drosophila. A custom-built enclosure (6 mm tall) was affixed to an aluminum thermal gradient bar (50 × 30.5 cm, TGB-5030, ThermoElectric Cooling America Corp., Chicago, IL) driven by two Peltier elements (AHP-1200CPV, ThermoElectric Cooling America Corp., Chicago, IL). The enclosure was separated lengthwise into 4 lanes (each 50 × 6 cm) that were visually isolated from one another. Three lanes were for testing mosquito thermal preference, while the fourth lane was dedicated to measuring air temperature via an array of eight digital temperature sensors (DS18B20, Maxim, San Jose, CA; connected to an Arduino Uno, https://www.arduino.cc/) mounted to the top of the enclosure and distributed evenly across the length of the lane and centered in each analysis sector. An overhead camera (C910, Logitech, Lausanne, Switzerland) monitored mosquito position through the transparent lid of the enclosure. Images were acquired once per minute and analyzed using custom MATLAB scripts to count mosquitoes across eight analysis sectors of the lane. The MATLAB script is available for download as Source code 3. The assay was conducted in a room maintained at 80–90% relative humidity and 14°C to achieve low air temperatures within the gradient enclosure. At the beginning of each 3-hr trial, the air temperature throughout the enclosure was stabilized at ~26°C, and 25–30 mosquitoes were introduced into each lane of the assay. After 90 min, a thermal gradient was established (air temperatures: ~19°C to ~36°C) by heating the right Peltier element and cooling the left Peltier element. Mosquitoes were monitored for an additional 90 min while exposed to this thermal gradient. Mosquito distributions during minutes 60–90 were monitored in both the ‘no thermal gradient’ and ‘thermal gradient’ conditions. Dead mosquitoes were visually identified at the conclusion of each trial. All genotypes were tested in parallel, and their lane positions were randomized across trials.

37All thermal images were acquired with an infrared camera (E60, FLIR Systems, Wilsonville, OR).

38All statistical analyses were performed using Prism 5 software (GraphPad Software, Inc., La Jolla, CA).