Figure 1.
Phylogenetic relationships and life cycles of parasitic nematodes.
A. Phylogeny of selected nematode species. Phylogenetic analysis is from Dillman et al., 2012 [22]. Species used in the present study are highlighted. Red = skin-penetrating mammalian-parasitic nematode; gold = passively ingested mammalian-parasitic nematode; blue = entomopathogenic nematode; green = free-living nematode. For each of the selected species, icons depict one of their common hosts (human, rat, beetle, or cow). Phylogenetic relationships are based on ML and Bayesian analyses of nearly complete SSU sequences. Values above each branch represent Bayesian posterior probabilities; ML bootstrap indices appear below each branch. Values lower than 75 are not reported. Priapulus caudatus and Chordodes morgani were defined as outgroups. Detailed methods for phylogenetic tree construction are provided in Dillman et al., 2012 [22]. B–C. Life cycles of skin-penetrating nematodes. B. Hookworms must infect a new host every generation. IJs infect hosts by skin-penetration. Nematodes develop to adulthood, reproduce, and lay eggs inside the host. Eggs are excreted in host feces and develop into IJs, which find and infect new hosts. C. Str. stercoralis and Str. ratti can develop through a single generation outside the host. Some larvae excreted in host feces develop into IJs; others develop into free-living adults that mate and reproduce outside the host. All progeny of free-living adults develop into IJs, which find and infect new hosts. L1–L4 are larval stages; IJ = infective juvenile. D. Ecology of selected nematode species.
Figure 2.
Foraging behaviors of skin-penetrating nematodes.
A. IJ motility in the absence of chemosensory stimulation. Motility varies across species (P<0.0001, one-way ANOVA), with Str. stercoralis being the most active (P<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, Ste. carpocapsae is considered an ambusher, Ste. glaseri is considered an active cruiser, and He. bacteriophora is considered a less active cruiser [15]. Statistical analysis is shown in Table S1. 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. ***, P<0.001; *, P<0.01, unpaired t test or Mann-Whitney test. n = 5–10 trials for each species. C–D. Heat stimulates local search behavior. C. Representative tracks for Str. stercoralis and Str. ratti from 20 s recordings at room temperature versus 37 s recordings at room temperature versus 37°C. D. 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. ***, P<0.001; **, P<0.01, Mann-Whitney test. n = 5–10 trials. E. 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 (P<0.0001, chi-square test). N. brasiliensis showed a nictation frequency comparable to Ste. carpocapsae (P>0.05, chi-square test with Bonferroni correction) and greater than Str. stercoralis or Str. ratti (P<0.01, chi-square test with Bonferroni correction). Statistical analysis is shown in Table S4. n = 20–28 IJs for each species. For all graphs, error bars indicate SEM.
Figure 3.
Olfactory responses of mammalian-parasitic nematodes.
A. Str. stercoralis is attracted to a number of human-emitted odorants. Red = attractants for Str. stercoralis that also attract anthropophilic mosquitoes [31], [52]–[58]. n = 6–23 trials per odorant. Str. stercoralis did not respond to the chemotaxis controls (Figure S3). *, P<0.05; ***, P<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. 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 (P<0.0001, two-way ANOVA with Tukey's post-test). Data for responses of EPNs and C. elegans to 10% CO2 are from Dillman et al., 2012 [22]. Red = skin-penetrating; gold = passively ingested; blue = insect-parasitic; green = free-living. C. Responses of Ha. contortus to grass odor. Responses to the odors of two different grass samples were examined. n = 8–17 trials for each sample. D. 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.
Figure 4.
Olfactory responses of Strongyloides species vary across life stages.
A–B. Responses of either Str. stercoralis (A) or Str. ratti (B) IJs, free-living adults, and free-living larvae to host odorants and fecal odor. *, P<0.05; ***, P<0.001, two-way ANOVA with Tukey's post-test. n = 4–12 trials for Str. stercoralis and n = 6–26 trials for Str. ratti for each condition. Error bars indicate SEM.