Mg. Milgroom et al., INTERCONTINENTAL POPULATION-STRUCTURE OF THE CHESTNUT BLIGHT FUNGUS, CRYPHONECTRIA-PARASITICA, Mycologia, 88(2), 1996, pp. 179-190
The population structure of the chestnut blight fungus, Cryphonectria
parasitica was analyzed using restriction fragment length polymorphism
s (RFLPs). A total of 791 isolates were sampled from four regions, Chi
na, Japan, North America and Europe, and assayed for alleles at eight
RFLP loci. Alleles at all eight loci segregated in simple Mendelian ra
tios, and most loci were unlinked. Gene diversity was decomposed hiera
rchically: 56% of gene diversity was attributable to diversity within
subpopulations, compared to 7% among subpopulations within regions, an
d 37% among regions. Subpopulations of C. parasitica in China have dis
tinctly different allele frequencies from subpopulations in other regi
ons, including Japan. DNA fingerprint genotypes in China were also sig
nificantly different from those in the other regions. There was an ave
rage of 3.2 restriction fragments hybridizing to a DNA fingerprinting
probe in Chinese isolates from 11 subpopulations compared to 8.6 in is
olates from Japan; the exception was in one subpopulation in northeast
ern China where there was an average of 11.1 fragments per isolate. No
rth American and European subpopulations were similar to each other, a
nd more similar to Japan than to China, for both RFLP allele frequenci
es and DNA fingerprints. The results suggest that C. parasitica was in
troduced into North America from Japan, not China. The origin of C. pa
rasitica for European subpopulations cannot be determined from these r
esults, but eastern China was not a likely source. Analysis of populat
ion structure within China showed moderate differentiation, with 11% o
f gene diversity attributable to differences among subpopulations (G(S
T) = O.11). Pairwise estimates of gene flow between subpopulations wer
e negatively correlated to geographic distances between subpopulations
in China. This result suggests that Chinese populations are in equili
brium, and that restricted gene flow and genetic drift shape these pop
ulations.