Yeast prions [URE3] and [PSI+] are diseases.

Viruses, plasmids, and prions can spread in nature despite being a burden to their hosts. Because a prion arises de novo in more than one in 10(6) yeast cells and spreads to all offspring in meiosis, its absence in wild strains would imply that it has a net deleterious effect on its host. Among 70 wild Saccharomyces strains, we found the [PIN+] prion in 11 strains, but the [URE3] and [PSI+] prions were uniformly absent. In contrast, the "selfish" 2mu DNA was in 38 wild strains and the selfish RNA replicons L-BC, 20S, and 23S were found in 8, 14, and 1 strains, respectively. The absence of [URE3] and [PSI+] in wild strains indicates that each prion has a net deleterious effect on its host.