A multi-level understanding of the evolutionary trade-offs in thermal adaptation

The hypothesis that increased fitness within a selective environment must be accompanied by a loss of fitness in other non-selective environments leads to the notion of evolutionary tradeoffs. Experimental evolution provides an approach to test the existence of evolutionary tradeoffs, characterize their general quality, and reveal their genetic origins. To examine the underlying mechanism for a fitness trade-off, we constructed the evolutionary trajectories of Escherichia coli K-12 at increasing temperatures up to 45.3°C, and found diverging mutational histories that led to adaptive phenotypes with and without fitness trade-offs at low temperatures. We identified genetic changes in cellular respiration, iron metabolism and methionine biosynthesis that regulated gene expression to achieve thermal adaptation and determined the presence and absence of a fitness trade-off. Our results suggested that evolutionary trade-off could be generated by a regulatory protein mutation that was beneficial in the selective conditions but forced suboptimal proteome allocation under non-selective environments. Expression profiling data for nine thermally adapted strains at three temperatures were generated by RNA-seq, in duplicate