Experimental Evolution of Cellular Response Networks in Yeast

Abstract

Organisms respond to environmental changes using complex signaling networks, whose evolution has been studied by comparing their structure and function in related organisms. We complemented this approach by using experimental evolution in the budding yeast, Saccharomyces cerevisiae, to explore how existing response networks change under selection. We selected for cell cycle arrest in response to an environmental change by alternating between an environment that favors cell proliferation and one that contains high salt (0.5 M NaCl) and hydroxyurea, a replication inhibitor that kills a genetically engineered ancestor if it attempts DNA replication. This selection yielded strains that arrest division in high NaCl. Causal mutations were identified by whole-genome sequencing and confirmed by genetic reconstruction. We identified four different forms of rewiring: mutations that alter three different hexose transporters, allowing them to admit sodium ions into cells; inactivation of HAL5, which encodes a protein kinase that regulates potassium channels; inactivation of HOG1, which encodes a MAP kinase that detects high external osmolarity; and loss-of-function mutations in IMP2′, a poorly characterized transcriptional activator also implicated in ion homeostasis.