The olfactory basis of orchid pollination by mosquitoes.

Mosquitoes are important vectors of disease and require sources of carbohydrates for reproduction and survival. Unlike host-related behaviors of mosquitoes, comparatively less is understood about the mechanisms involved in nectar-feeding decisions, or how this sensory information is processed in the mosquito brain. Here we show that Aedes spp. mosquitoes, including Aedes aegypti, are effective pollinators of the Platanthera obtusata orchid, and demonstrate this mutualism is mediated by the orchid's scent and the balance of excitation and inhibition in the mosquito's antennal lobe (AL). The P. obtusata orchid emits an attractive, nonanal-rich scent, whereas related Platanthera species-not visited by mosquitoes-emit scents dominated by lilac aldehyde. Calcium imaging experiments in the mosquito AL revealed that nonanal and lilac aldehyde each respectively activate the LC2 and AM2 glomerulus, and remarkably, the AM2 glomerulus is also sensitive to N,N-diethyl-meta-toluamide (DEET), a mosquito repellent. Lateral inhibition between these 2 glomeruli reflects the level of attraction to the orchid scents. Whereas the enriched nonanal scent of P. obtusata activates the LC2 and suppresses AM2, the high level of lilac aldehyde in the other orchid scents inverts this pattern of glomerular activity, and behavioral attraction is lost. These results demonstrate the ecological importance of mosquitoes beyond operating as disease vectors and open the door toward understanding the neural basis of mosquito nectar-seeking behaviors

Olfactory Preferences of the Parasitic Nematode Howardula aoronymphium and its Insect Host Drosophila falleni

Many parasitic nematodes have an environmental infective stage that searches for hosts. Olfaction plays an important role in this process, with nematodes navigating their environment using host-emitted and environmental olfactory cues. The interactions between parasitic nematodes and their hosts are also influenced by the olfactory behaviors of the host, since host olfactory preferences drive behaviors that may facilitate or impede parasitic infection. However, how olfaction shapes parasite-host interactions is poorly understood. Here we investigated this question using the insect-parasitic nematode Howardula aoronymphium and its host, the mushroom fly Drosophila falleni. We found that both H. aoronymphium and D. falleni are attracted to mushroom odor and a subset of mushroom-derived odorants, but they have divergent olfactory preferences that are tuned to different mushroom odorants despite their shared mushroom environment. H. aoronymphium and D. falleni respond more narrowly to odorants than Caenorhabditis elegans and Drosophila melanogaster, consistent with their more specialized niches. Infection of D. falleni with H. aoronymphium alters its olfactory preferences, rendering it more narrowly tuned to mushroom odor. Our results establish H. aoronymphium-D. falleni as a model system for studying olfaction in the context of parasite-host interactions

The olfactory basis of orchid pollination by mosquitoes.

Mosquitoes are important vectors of disease and require sources of carbohydrates for reproduction and survival. Unlike host-related behaviors of mosquitoes, comparatively less is understood about the mechanisms involved in nectar-feeding decisions, or how this sensory information is processed in the mosquito brain. Here we show that Aedes spp. mosquitoes, including Aedes aegypti, are effective pollinators of the Platanthera obtusata orchid, and demonstrate this mutualism is mediated by the orchid's scent and the balance of excitation and inhibition in the mosquito's antennal lobe (AL). The P. obtusata orchid emits an attractive, nonanal-rich scent, whereas related Platanthera species-not visited by mosquitoes-emit scents dominated by lilac aldehyde. Calcium imaging experiments in the mosquito AL revealed that nonanal and lilac aldehyde each respectively activate the LC2 and AM2 glomerulus, and remarkably, the AM2 glomerulus is also sensitive to N,N-diethyl-meta-toluamide (DEET), a mosquito repellent. Lateral inhibition between these 2 glomeruli reflects the level of attraction to the orchid scents. Whereas the enriched nonanal scent of P. obtusata activates the LC2 and suppresses AM2, the high level of lilac aldehyde in the other orchid scents inverts this pattern of glomerular activity, and behavioral attraction is lost. These results demonstrate the ecological importance of mosquitoes beyond operating as disease vectors and open the door toward understanding the neural basis of mosquito nectar-seeking behaviors

Diverse host-seeking behaviors of skin-penetrating nematodes.

Skin-penetrating parasitic nematodes infect approximately one billion people worldwide and are responsible for some of the most common neglected tropical diseases. The infective larvae of skin-penetrating nematodes are thought to search for hosts using sensory cues, yet their host-seeking behavior is poorly understood. We conducted an in-depth analysis of host seeking in the skin-penetrating human parasite Strongyloides stercoralis, and compared its behavior to that of other parasitic nematodes. We found that Str. stercoralis is highly mobile relative to other parasitic nematodes and uses a cruising strategy for finding hosts. Str. stercoralis shows robust attraction to a diverse array of human skin and sweat odorants, most of which are known mosquito attractants. Olfactory preferences of Str. stercoralis vary across life stages, suggesting a mechanism by which host seeking is limited to infective larvae. A comparison of odor-driven behavior in Str. stercoralis and six other nematode species revealed that parasite olfactory preferences reflect host specificity rather than phylogeny, suggesting an important role for olfaction in host selection. Our results may enable the development of new strategies for combating harmful nematode infections

Diverse Host-Seeking Behaviors of Skin-Penetrating Nematodes

<div><p>Skin-penetrating parasitic nematodes infect approximately one billion people worldwide and are responsible for some of the most common neglected tropical diseases. The infective larvae of skin-penetrating nematodes are thought to search for hosts using sensory cues, yet their host-seeking behavior is poorly understood. We conducted an in-depth analysis of host seeking in the skin-penetrating human parasite <i>Strongyloides stercoralis</i>, and compared its behavior to that of other parasitic nematodes. We found that <i>Str. stercoralis</i> is highly mobile relative to other parasitic nematodes and uses a cruising strategy for finding hosts. <i>Str. stercoralis</i> shows robust attraction to a diverse array of human skin and sweat odorants, most of which are known mosquito attractants. Olfactory preferences of <i>Str. stercoralis</i> vary across life stages, suggesting a mechanism by which host seeking is limited to infective larvae. A comparison of odor-driven behavior in <i>Str. stercoralis</i> and six other nematode species revealed that parasite olfactory preferences reflect host specificity rather than phylogeny, suggesting an important role for olfaction in host selection. Our results may enable the development of new strategies for combating harmful nematode infections.</p></div

Foraging behaviors of skin-penetrating nematodes.

<p><b>A</b>. IJ motility in the absence of chemosensory stimulation. Motility varies across species (<i>P</i><0.0001, one-way ANOVA), with <i>Str. stercoralis</i> being the most active (<i>P</i><0.01, one-way ANOVA with Tukey-Kramer post-test). n = 6–9 trials for each species. For this graph and subsequent graphs with multiple species, red = skin-penetrating; gold = passively ingested; blue = entomopathogenic. Of the three entomopathogenic species, <i>Ste. carpocapsae</i> is considered an ambusher, <i>Ste. glaseri</i> is considered an active cruiser, and <i>He. bacteriophora</i> is considered a less active cruiser <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305-Downes1" target="_blank">[15]</a>. Statistical analysis is shown in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305.s007" target="_blank">Table S1</a>. <b>B</b>. Unstimulated vs. heat-stimulated mean speeds of mammalian-parasitic IJs. Heat-stimulated IJs were exposed to an acute 37°C stimulus and tracked at 37°C. ***, <i>P</i><0.001; *, <i>P</i><0.01, unpaired t test or Mann-Whitney test. n = 5–10 trials for each species. <b>C–D</b>. Heat stimulates local search behavior. <b>C</b>. Representative tracks for <i>Str. stercoralis</i> and <i>Str. ratti</i> from 20 s recordings at room temperature versus 37 s recordings at room temperature versus 37°C. <b>D</b>. Movement patterns at room temperature versus 37°C. Distance ratios were calculated as the total track length divided by the maximum displacement attained during the 20 s recording period. A distance ratio of 1 indicates travel in a straight line s recording period. A distance ratio of 1 indicates travel in a straight line; a distance ratio of >1 indicates a curved trajectory. ***, <i>P</i><0.001; **, <i>P</i><0.01, Mann-Whitney test. n = 5–10 trials. <b>E</b>. Nictation frequencies of IJs. Nictation was defined as standing or waving behavior of at least 5 s in duration over the course of a 2 min period. Nictation frequencies varied among species (<i>P</i><0.0001, chi-square test). <i>N. brasiliensis</i> showed a nictation frequency comparable to <i>Ste. carpocapsae</i> (<i>P</i>>0.05, chi-square test with Bonferroni correction) and greater than <i>Str. stercoralis</i> or <i>Str. ratti</i> (<i>P</i><0.01, chi-square test with Bonferroni correction). Statistical analysis is shown in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305.s010" target="_blank">Table S4</a>. n = 20–28 IJs for each species. For all graphs, error bars indicate SEM.</p

Olfactory responses of <i>Strongyloides</i> species vary across life stages.

<p><b>A–B</b>. Responses of either <i>Str. stercoralis</i> (<b>A</b>) or <i>Str. ratti</i> (<b>B</b>) IJs, free-living adults, and free-living larvae to host odorants and fecal odor. *, <i>P</i><0.05; ***, <i>P</i><0.001, two-way ANOVA with Tukey's post-test. n = 4–12 trials for <i>Str. stercoralis</i> and n = 6–26 trials for <i>Str. ratti</i> for each condition. Error bars indicate SEM.</p

Olfactory responses of mammalian-parasitic nematodes.

<p><b>A</b>. <i>Str. stercoralis</i> is attracted to a number of human-emitted odorants. Red = attractants for <i>Str. stercoralis</i> that also attract anthropophilic mosquitoes <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305-Qiu1" target="_blank">[31]</a>, <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305-Mukabana1" target="_blank">[52]</a>–<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305-Millar1" target="_blank">[58]</a>. n = 6–23 trials per odorant. <i>Str. stercoralis</i> did not respond to the chemotaxis controls (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305.s003" target="_blank">Figure S3</a>). *, <i>P</i><0.05; ***, <i>P</i><0.001 relative to control, t-test (CO₂ vs. air and L-lactic acid vs. H₂O) or one-way ANOVA with Bonferroni post-test (all other odorants vs. paraffin oil). <b>B</b>. Olfactory responses across species. Response magnitudes are color-coded according to the scale shown to the right of the heat map, and odorants are ordered based on hierarchical cluster analysis. n = 6–14 trials for each odorant-species combination. Each species exhibited a unique odor response profile (<i>P</i><0.0001, two-way ANOVA with Tukey's post-test). Data for responses of EPNs and <i>C. elegans</i> to 10% CO₂ are from Dillman <i>et al.</i>, 2012 <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004305#ppat.1004305-Dillman1" target="_blank">[22]</a>. Red = skin-penetrating; gold = passively ingested; blue = insect-parasitic; green = free-living. <b>C</b>. Responses of <i>Ha. contortus</i> to grass odor. Responses to the odors of two different grass samples were examined. n = 8–17 trials for each sample. <b>D</b>. Olfactory preferences reflect host specificity rather than phylogeny. The behavioral dendrogram was constructed based on the odor response profiles of each species. Hierarchical cluster analysis was performed using UPGMA (Unweighted Pair Group Method with Arithmetic Mean). Euclidean distance was used as a similarity measure. Hosts (humans, ruminants, rodents, or insects) for each species are indicated. Coph. Corr. = 0.96. For all graphs, error bars indicate SEM.</p