| Abstract: | Geographic variation in the genetic structure of natural enteric
populations of Escherichia coli was assessed at both the single-locus and
dilocus levels from allozyme genotypes at 12 enzyme loci in 178 cell lines
isolated from human hosts in Sweden, Iowa, and Tonga. Although there was
significant heterogeneity in allele frequencies at six of the 12 loci,
geographic variation accounted for only 2.0% of the total genetic diversity
(HT = 0.518). Ohta's D-statistics were used to partition the total variance
of dilocus linkage disequilibrium into within-population and
between-population components. The observed total variance in
disequilibrium (0.0339), averaged over 66 locus-pairs, was significantly
greater than would be expected (0.0103) if alleles were randomly associated
in an unstructured total population; and both within-locality and
between-locality components made substantial contributions to the total
variance. Half the locus-pairs exhibited the specific dual relationship
among components expected when random factors are generating
disequilibrium, but 20% of the locus-pairs showed the opposite
relationship, reflecting systematic allele associations. The magnitude of
dilocus disequilibrium apparently is unrelated to the chromosomal distance
between loci. This and other evidence indicates that substitutive
recombination rates in natural populations are sufficiently low to permit
indirect periodic selection to play a prominent role in generating
multilocus genetic structure.
|
