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.