Figure 6. The sequence of events leading to the evolution of nontransitivity in population BYS1-D08.

Nontransitivity arises through multilevel selection requiring adaptive mutations in both the nuclear and viral genomes. The Early clone (orange) produces, and is resistant to, killer toxin. Step 1: after 335 generations, the Intermediate clone (green) fixed three nuclear mutations including a beneficial mutation in yur1 and lost the ability to produce killer toxin due to intracellular competition between viral variants. Step 2: after another 665 generations, the Late clone (purple) fixed an additional 10 nuclear mutations including a beneficial mutation in ste4 and lost immunity to the killer toxin, which is no longer present in the environment. Step 3: when brought into competition with the Early clone (1000 generations removed), the Late clone loses in a frequency-dependent manner due to killer toxin produced by the Early clone. Positive frequency-dependent selection (PFDS) emerges in the competition because the fitness disadvantage of the Late clone can be overcome if it starts the competition at high frequency relative to the Early clone.