Hg. Welz et Kj. Leonard, GAMETIC PHASE DISEQUILIBRIA IN POPULATIONS OF RACE-2 AND RACE-3 OF COCHLIOBOLUS-CARBONUM, European journal of plant pathology, 101(3), 1995, pp. 301-310
Over 3 years 15 samples of C. carbonum were collected from 11 corn fie
lds in North Carolina and Tennessee. Among 514 isolates of race 2 and
319 isolates of race 3, five phenotypic characters (mating type, produ
ction of pseudothecia, production of asci and ascospores, tolerance to
cycloheximide and carboxin, respectively) that are controlled by sing
le genes at unlinked loci (Mat, Psu, Asc, Cyh, Crb) were examined. Gam
etic phase disequilibrium (GPD) was analyzed by three methods. First,
observed and expected four-locus haplotype frequencies were compared i
n G-tests for goodness of fit (method 1) and second, four-locus disequ
ilibrium was calculated using an index of association which is based o
n the variance of the number of loci at which two isolates in a given
population differ (method 2). Third, observed and expected frequencies
of allele pairs were compared in 2 x 2 contingency tables (method 3).
Each test was performed on individual and pooled samples which also a
llowed assessment of heterogeneity among samples. In general this hete
rogeneity was low, i.e. the consistency of associations among samples
was high. Four-locus disequilibrium was significant in race 2 with met
hod 1 but not with method 2. In race 3 we found no disequilibrium with
any of the two methods. Method 3 indicated that several allele pairs
were significantly associated in race 2 but not race 3. Thus GPD was s
ignificant in race 2 but not in race 3. Mating type frequencies were c
lose to 0.50 in race 2 and race 3. This and the non-significant index
of association indicate that the sexual stage of C. carbonum is operat
ing within each of the two race populations. It is suggested that sele
ction favours different haplotypes in the asexual than in the sexual s
tage of the pathogen. Such disruptive selection may have maintained po
lymorphisms of genes related to sexual fertility and stabilized gene f
requencies in C. carbonum over a period of 15 years (1972-1987).