Host specialization may structure parasite distributions.
Our analyses provided mixed support for a positive relationship between specialization and parasite occurrence on different hosts (Q2). Within a 50-km radius of all 27 locations with parasite emergence data, 15 locations had at least one parasite occurrence record with information about host of origin. Proportion of records on a particular host for these 15 Striga populations was also a statistically significant predictor of emergence for maize (p maize< 0.001; βoccurrence= 0.31; GLMM) but not millet (p millet =0.26; βoccurrence=0.14; GLMM) or sorghum (p sorghum=0.96; βoccurrence= -0.01; GLMM; Table S2). Availability of additional parasite occurrence and performance data could reveal a stronger relationship between specialization and patterns of parasite occurrence on different hosts.
Parasite abiotic niche reflects host environmental tolerance.
To characterize the abiotic basis of patterns of host specialization, we built environmental niche models (ENMs) for potentially host-specialized parasite populations by excluding all occurrences with unknown host or with a non-focal host (Fig. S1). Despite a smaller number of observations compared to the all-occurrence model, ENMs had good predictive accuracy for each of the host-specific models. Area under the receiver operating characteristic curve (AUC) for the test set was 0.848 for the all-occurrence model (n = 1049) compared to 0.850 for the sorghum-only model (n = 262), 0.908 for the millet-only model (n = 157), and 0.841 for the maize-only model (n = 74).
Modeled niches generally reflected known environmental tolerances of hosts, with millet-specific models predicting high habitat suitability in low nitrogen (N), low rainfall environments but maize-specific models predicting higher habitat suitability in environments characterized by more nitrogen-rich soils and higher rainfall (Fig. 4A-D). Across all host species, annual rainfall and soil N were generally among the most important predictors of parasite occurrence (Table S4). Annual rainfall was the most common factor limiting habitat suitability for maize-parasitizing S. hermonthica (59% of all grid cells; 10th-90th percentile for grid cells with habitat suitability ≥ 0.5: 854-1537 mm/yr) but was also strongly limiting for parasites of sorghum (41% of cells; 10th-90th percentile: 509-1197 mm/yr) and millet (32% of cells; 10th-90th percentile: 441-1164 mm/yr) (Fig. S3). Mean temperature of the wettest quarter was also an important predictor of S. hermonthica parasitizing millet (Table S4) and frequently limited habitat suitability for millet-parasitizing populations (Fig. S3). Soil clay content was an important predictor forS. hermonthica occurrence on sorghum (Table S4), but was only limiting for millet-parasitizing populations in eastern Sudan and sorghum-parasitizing populations in western Senegal (Fig. S3). Higher soil clay content has been anecdotally associated with parasitism on sorghum in general (Mohamed et al. 2001) and in Sudan (Wilson-Jones 1955).
In agricultural as well as natural ecosystems, more productive environments may be associated with increased availability of alternate hosts, favoring generalists (Thrall et al. 2007). Parasitism on maize was associated with increased environmental quality, with habitat suitability peaking in locations of higher crop productivity (Fig. 4E). Combined crop yield per area harvested was significantly greater whereS. hermonthica parasitizes maize compared to millet (p < 0.001, Wilcoxon rank sum test) but not compared to sorghum (p =0.2), consistent with greater niche overlap between maize and sorghum (Table S5). We observed a weak trend towards reduced parasite specialization with increasing environmental productivity (Fig. 4F;p = 0.15, Chi-square goodness-of-fit test for linear regression model). Taken together, parasite ENMs are highly sensitive to differences in multivariate environmental tolerance of hosts, consistent with host cultivation on a gradient from marginal, warm environments (pearl millet) to more productive, cooler environments (maize).