Accelerated evolution associated with genome reduction in a free-living prokaryote

Abstract

We have examined the evolutionary mechanisms involved in this process, which are different from those known from host-dependent organisms. Indeed, most substitutions that have occurred in Prochlorococcus have to be selectively neutral, as the large size of populations imposes low genetic drift and strong purifying selection. We assume that the major driving force behind genome reduction within the Prochlorococcus radiation has been a selective process favoring the adaptation of this organism to its environment. A scenario is proposed for genome evolution in this genus.