Maintaining unused traits by interconnecting trait networks

Evolution allows living systems to produce novel properties that improve fitness when encountering new environments. At the same time organisms face the challenge of maintaining traits that might be dispensable under current circumstances but contribute to fitness when the environment changes. Such evolutionary decay of unused traits has important implications for the ability of organisms to cope with environmental changes. To investigate how organisms can deal with this, we performed experimental evolution with a strain of S. cerevisiae that has a cell polarity defect as well as a reduced capability to adjust to growth on different carbon sources. In two separate evolution experiments, we subjected this strain to an environment where diauxic growth on different carbon sources was relevant to fitness and one where it was not. We found that in both environments, repair of the cell polarity defect was accompanied by a better ability to perform diauxic growth. This suggests that cells could circumvent the loss of an unused trait through its pleiotropic coupling to the molecular network that regulates cell polarity. By analyzing mutations that drive adaptation in both environments, we give a first insight into what the molecular details of such pleiotropic couplings might look like.