Accumulating evidence suggests that physical isolation can potentially make a significant contribution to microbial population structure and incipient speciation. However, the effect of geographic factors on population structure has not been explicitly studied for ubiquitous cyanobacteria such as a water bloom-forming Microcystis aeruginosa. To investigate whether a fine-scale geographic structure is developed within M. aeruginosa, 96 isolates from five Japanese lakes separated by < 55 km were analysed using multilocus sequence typing. The results of multilocus sequence typing analyses indicated that M. aeruginosa was not phylogeographically structured, although a high level of genetic differentiation was observed among locations (FST = 0.372). Most surprisingly, the highest levels of genetic differentiation were observed between populations at the same location at different times of the same year, and between those separated by only 3 km on the same day. The isolation-by-distance pattern was not supported, but not completely ruled out. Taken together, our results suggest that geographic factors, if present, have far less impact on the fine-scale spatial genetic diversity of M. aeruginosa than local genetic drift or, possibly, selection.
© 2009 Society for Applied Microbiology and Blackwell Publishing Ltd.