Genomic and phenotypic evolution of in a novel citrate-only resource environment.

Evolutionary innovations allow populations to colonize new ecological niches. We previously reported that aerobic growth on citrate (Cit) evolved in an population during adaptation to a minimal glucose medium containing citrate (DM25). Cit variants can also grow in citrate-only medium (DM0), a novel environment for . To study adaptation to this niche, we founded two sets of Cit populations and evolved them for 2500 generations in DM0 or DM25. The evolved lineages acquired numerous parallel mutations, many mediated by transposable elements. Several also evolved amplifications of regions containing the gene. Unexpectedly, some evolved populations and clones show apparent declines in fitness. We also found evidence of substantial cell death in Cit clones. Our results thus demonstrate rapid trait refinement and adaptation to the new citrate niche, while also suggesting a recalcitrant mismatch between physiology and growth on citrate.