Isolation by distance in the spore-forming soil bacterium Myxococcus xanthus

Genetic differentiation between spatially separated populations within a species is commonly observed in plants and animals, but its existence in microbes has long been a contentious issue[1-5]. Traditionally, many microbial ecologists have reasoned that microbes are not limited by dispersal as a result of their immense numbers and microscopic size [2, 6]. In this view, the absence of barriers to gene flow between populations would prevent differentiation of populations by genetic drift and hinder local adaptation. Myxococcus xanthus is a globally distributed, spore-forming bacterium that offers a robust test for genetic differentiation among populations because sporulation is expected to enhance dispersal. Using multi-locus sequence data, we show here that both diversity and the degree of differentiation between populations increase as a function of distance in M. xanthus. Populations are consistently differentiated at scales exceeding 10(2)-10(3) km, and isolation by distance, the divergence of populations by genetic drift due to limited dispersal, is responsible. Our results provide new insights into how genetic diversity within species of free-living microbes is distributed from centimeter to global scales.