Data from: The coexistence of generalist and specialist clonal lineages in natural populations of the Irish Famine pathogen Phytophthora infestans explains local adaptation to potato and tomato

Phytophthora infestans, causing late blight on Solanaceae, is a serious
threat to potato and tomato crops worldwide. P. infestans populations
sampled on either potato or tomato differ in genotypes and pathogenicity,
suggesting niche exclusion in the field. We hypothesized that such niche
separation can reflect differential host exploitation by different P.
infestans genotypes. We thus compared genotypes and phenotypes in 21
isolates sampled on potato (n = 11) or tomato (n = 10). Typing at 12
microsatellite loci assigned potato isolates to the 13_A2, 6_A1 and 1_A1
lineages, and tomato isolates to the 23_A1, 2_A1 and unclassified
multi-locus genotypes. Cross-inoculations on potato and tomato leaflets
showed that all isolates were pathogenic on both hosts. However: tomato
isolates performed much better on tomato than did potato isolates, which
performed better on potato than did tomato isolates, thus revealing a
clear pattern of local adaptation. Potato isolates were significantly
fitter on potato than on tomato, and are best described as
potato-specialists; tomato isolates appear to be generalists, with similar
pathogenicity on both hosts. Niche separation in the field may thus result
mainly from the large fitness gap on tomato between generalists and
unadapted potato-specialists, while the small, but significant fitness
difference on potato between both types of isolates may prevent population
invasion by generalists. Extreme specialization to potato seems very
costly relative to performance loss on the alternative host. This study
therefore shows that local adaptation and niche separation, commonly
expected to involve and generate specialists, can occur with generalists.