Outlook
Yeasts are colonising organisms: a few cells arriving in a compatible environment must rapidly reproduce, likely in competition with other yeasts and microorganisms, then cells or spores from this population colonise new environments to complete the life cycle. The capacity for adaptive evolution is required to survive environmental differences and emerging threats from competing organisms as well as, where the colony forms in a living organism, host defences. However, populations arise from a few cells and have inherently low genetic heterogeneity, which limits evolutionary potential unless genetic heterogeneity is acquired during colony growth. Yet wild-type yeast cells in the lab have low enough rates of all mutation classes that populations grown from single cells are essentially clonal (Serero et al., 2014).
We suggest that the yeast genome has evolved to undergo mutation linked to transcription of environmentally responsive genes, since focusing mutations on such genes has a higher chance of yielding an adaptive genetic change. However, under ideal conditions this mutation rate is very low so transcriptionally-induced mutations are hard to detect, and the replisome is regulated to minimise replication fork stalling even under adverse conditions (Duch et al., 2018; Duch et al., 2013). However, long term exposure to environmental toxins such as ethanol does increase replication stress (Voordeckers et al., 2020), and we predict that genetic heterogeneity will then emerge in the population, particularly at environmentally responsive genes through increasing use and decreasing processivity of BIR forks.