A yeast prion provides a mechanism for genetic variation and phenotypic diversity

A major enigma in evolutionary biology is that new forms or functions often require the concerted effects of several independent genetic changes. It i s unclear how such changes might accumulate when they are likely to be dele terious individually and be lost by selective pressure. The Saccharomyces c erevisiae prion [PSI+] is an epigenetic modifier of the fidelity of transla tion termination, but its impact on yeast biology has been unclear. Here we show that [PSI+] provides the means to uncover hidden genetic variation an d produce new heritable phenotypes. Moreover, in each of the seven genetic backgrounds tested, the constellation of phenotypes produced was unique. We propose that the epigenetic and metastable nature of [PSI+] inheritance al lows yeast cells to exploit pre-existing genetic variation to thrive in flu ctuating environments. Further, the capacity of [PSI+] to convert previousl y neutral genetic variation to a non-neutral state may facilitate the evolu tion of new traits.