Divergence in the Saccharomyces Species' Heat Shock Response Is Indicative of Their Thermal Tolerance.

The Saccharomyces species have diverged in their thermal growth profile. Both Saccharomyces cerevisiae and Saccharomyces paradoxus grow at temperatures well above the maximum growth temperature of Saccharomyces kudriavzevii and Saccharomyces uvarum but grow more poorly at lower temperatures. In response to thermal shifts, organisms activate a stress response that includes heat shock proteins involved in protein homeostasis and acquisition of thermal tolerance. To determine whether Saccharomyces species have diverged in their response to temperature, we measured changes in gene expression in response to a 12 °C increase or decrease in temperature for four Saccharomyces species and their six pairwise hybrids. To ensure coverage of subtelomeric gene families, we sequenced, assembled, and annotated a complete S. uvarum genome. In response to heat, the cryophilic species showed a stronger stress response than the thermophilic species, and the hybrids showed a mixture of parental responses that depended on the time point. After an initial strong response indicative of high thermal stress, hybrids with a thermophilic parent resolved their heat shock response to become similar to their thermophilic parent. Within the hybrids, only a small number of temperature-responsive genes showed consistent differences between alleles from the thermophilic and cryophilic species. Our results show that divergence in the heat shock response is mainly a consequence of a strain's thermal tolerance, suggesting that cellular factors that signal heat stress or resolve heat-induced changes are relevant to thermal divergence in the Saccharomyces species.