Investigating the evolutionary changes in Crabtree-negative yeasts during a long-term evolution experiment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Genetics at Massey University, Albany, New Zealand
The Crabtree effect is a metabolic strategy that allows yeast to ferment in the presence of oxygen.
This is of interest as not all yeasts display this strategy, and nearly 100 years after its discovery it
is still unclear what the overall benefit is. Two key theories attempt to explain the emergence of
this phenomenon, the make-accumulate-consume theory and the rate/yield trade-off theory. The
aim of this thesis was to investigate whether a trade-off between rate and yield develops in
Crabtree-negative yeasts over the course of 1500 generations in a high sugar environment.
Chapter Two demonstrates that growth rate is more likely to increase than decrease while growth
yield is more likely to decrease than increase in the isolate-derived populations of yeast. We find
that species that started out relatively fast, changed little while the slower species had more
significant gains in growth rate. With growth yield, the species with initially high yield lost more
significantly than the already low yield species. This could suggest there is an overall optimum
growth rate and growth yield, that the species are evolving towards. In Chapter Three, ethanol
production was measured using colorimetric tests and no change was observed to support the
development of the Crabtree effect in these populations after 1500 generations. In Chapter Four
growth yield was investigated using flow cytometry and it was found that several yeast
populations both increased in cell size and decreased in growth yield. This is an interesting
observation that has been observed in several previous experimental evolution experiments. In
Chapter Five, as cell size is often associated with ploidy changes, DNA content was measured
using DAPI and SYTOX DNA stains, detected by flow cytometry. This did not provide any
statistically significant conclusions but highlighted the importance of employing further
techniques to analyse the DNA content of these populations. This thesis has illustrated the
importance of studying the competitive behaviours of microorganisms in isolation, where selfish
traits appear to thrive.