Results:
Complete
host specialization is rare.
We first characterized levels of host specialization observed inS. hermonthica populations across Africa. Degree of host
specialization was highly variable across the 27 populations studied
(Fig. 2). Populations were characterized by high emergence on either
millet or sorghum only, high emergence on two hosts, or high emergence
on all three hosts (Fig. 2). Hierarchical clustering with a cut-point at
0.5 showed five distinct groups: millet specialists (n=2),
millet/sorghum generalists (n=3), sorghum specialists (n= 7),
virulent generalists (n =3), and maize/sorghum specialists
(n= 10; Fig. 2). Besides the ‘virulent generalists’, all groups
included populations from at least two studies, suggesting that
differences among studies did not strongly bias clustering. These
results suggest that although parasites may become locally adapted to
commonly cultivated host species, complete specialization is rare.
Distribution of host communities shapes
specialization.
We then investigated whether the distribution of host communities across
environments might shape the evolution of host specialization (Q1).S. hermonthica- prone regions covered approximately 628 million
hectares, of which 80% was estimated to have nonzero production of
maize, millet, or sorghum. Crop production patterns generally followed
rainfall gradients, with greater land area planted to millet in more
arid regions of the Sahel, sorghum dominant at lower latitudes and in
eastern Sudan, and maize most common in more mesic regions (Fig. 3). In
2000, sorghum or maize were the dominant crop in most S.
hermonthica- prone areas (38% sorghum; 38% maize), with 24% of areas
characterized by millet as the major cereal crop.
Specialization observed in experiments was strongly associated with
spatial variation in host crop communities. For parasites on all three
hosts, relative host crop area harvested within a 50 km radius was a
strong predictor of relative emergence in experimental studies. This
finding was especially pronounced for millet(p millet<0.001;
βharvest_area=0.85; GLMM; Table S2-S3). Relative crop
area harvested was also a statistically significant predictor of
relative emergence on sorghum (p sorghum=0.03;
βharvest_area=0.25; GLMM) and maize
(p maize=0.02;
βharvest_area=0.25; GLMM; Table S2-S3). These results
are consistent with the conclusion that parasites adapt to the most
abundant host in a particular region and also suggest that relative
parasite emergence is a reasonable proxy for fitness (Fig. 1).