Of late a lot of hubbub has brewed over Richard Lenski’s latest paper on the laboratory evolution of citrate-eating E. coli (PNAS 105(23):7899-7906 (2008)). Lenski figures very prominently in Carl Zimmer’s overview of E. coli research entitled Microcosm, and for good reason. For those that don’t know of the content of the current paper, for the last 20 years Lenski and his students have been growing bacteria (starting from a single bacterium and creating 12 lines) and periodically freezing samples (every 500 generations or so) to see if these bacteria evolve in the lab. At the time this paper was published, over 44,000 generations had passed.
Periodically, the frozen ‘fossil’ bacteria can be thawed and DNA compared over time. Since the bacteria are cloned and reproduce asexually, only natural selection and genetic drift can act on the gene populations, greatly simplifying analysis. Previous to this paper, changes in growth rates, reduced lag phases when the bacteria were transfered to a fresh culture medium, reduced peak population densities and larger cell sizes relative to their ancestors were reported. Most populations evolved increased DNA supercoiling.