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).