Data from multilocus enzyme electrophoresis
of bacterial populations were analyzed using a statistical test
designed to detect associations between genes at different loci.
Some species (e.g., SalmoneUa) were found to be clonal at all
levels of analysis. At the other extreme, Neisseria gonorrhoeae
is panmictic, with random association between loci. Two
intermediate types of population structure were also found.
Neisseria meningitidis displays what we have called an "epidemic"
structure. There is significant association between loci,
but this arises only because of the recent, explosive, increase in
particular electrophoretic types; when this effect is eliminated
the population is found to be effectively panmictic. In contrast,
linkage disequilibrium in a population of Rhizobium meliloti
exists because the sample consisted of two genetically isolated
divisions, often fixed for different alleles: within each division
association between loci was almost random. The method of
analysis is appropriate whenever there is doubt about the extent
of genetic recombination between members of a population. To
illustrate this we analyzed data on protozoan parasites and
again found panmictic, epidemic, and clonal population structures.