FINE-SCALE GENETIC AND PHENOTYPIC STRUCTURE IN NATURAL POPULATIONS OF BACILLUS SUBTILIS AND BACILLUS LICHENIFORMIS:...

The genetic and phenotypic structure of sympatric populations of wild bacteria traditionally identified as Bacillus subtilis and B. licheniformis was analyzed. Small soil samples were taken from a single, tiny site in the Sonoran Desert of Arizona, USA, to provide a true population analysis, in contrast to many analyses of genetic structure using bacterial strain collections of widely heterogeneous origin. Genetic analyses of isolates used multilocus enzyme electrophoresis, mismatches in restriction fragment length polymorphism, and variants from Southern hybridization with B. subtilis DNA probes. Phenotypic analyses of isolates used the API test system for detection of growth and acid production on specific carbon sources. The two species were distinct both phenotypically and genetically, despite their known potential for genetic exchange in laboratory experiments. Genic and genotypic diversity were high in both species, and only 16% of observed allozyme variants might possibly be common to both species. Hence, there is probably modest genetic exchange, if any, between the species in nature. Clear hierarchies of population-genetic structure were found for both species. Different types of genetic data yield concordant population structures for B. subtilis. For both species, two-locus and multilocus statistical analyses of linkage demonstrated modest to strong disequilibrium at the species level but truly panmictic subunits within each species. The evidence for extensive genetic recombination within these fine-scale subdivisions is unequivocal, indicating that the sexuality of these bacteria can be well expressed in nature. The relation of these results to processes of bacterial evolution and speciation is discussed.