Evolution uncovers a general tradeoff between recovery after heat shock and growth at elevated temperatures

Abstract

The maintenance of microbial diversity depends upon fitness tradeoffs between environments. While the importance of tradeoffs is clear, it has been surprisingly difficult to predict which traits they will occur between and at how granular a level. For example, it is unclear whether performance between a constant versus a pulsed exposure of the same stress tend to be positively correlated, independent of each other, or negatively correlated. Empirically, it has been shown that a critical feature structuring environments is temperature. However, the compatibility between strategies to deal with different forms of heat stress is unclear. For instance, are strains that grow well at higher temperatures also stronger at withstanding heat shock? To understand how environmental microbes can adapt to better deal with different forms of heat stress, we performed an evolution experiment using a dominant phyllosphere microbe Methylobacterium extorquens in a regime of intermittent heat shock. We identified the genetic basis of adaptation, discovering a large number of loci capable of mediating adaptation to heat shock, most of which previously had not been linked to heat stress. Despite the genetic divergence, we discovered a general tradeoff between heat shock resistance and growth at consistently elevated temperatures. We found this tradeoff was not limited to evolved isolates, but was also represented across a sample of environmentally isolated Methylobacterium strains. These findings indicate a generic conflict between strategies to deal with heat shock recovery and growth at elevated temperatures, suggesting even variation in intensities of a stressor can drive diversity in microbial strategies.