In my thesis I present data collected from a long-term selection experiment using the
freshwater model organism Chlamydomonas reinhardtii. The selection experiment
was designed to disentangle the effects of the number of multiple environmental
drivers (MEDs) and the identity of those environmental drivers including high CO2,
high temperature, general nutrient depletion, reduced light intensity, reduced
phosphate availability, the addition of a herbicide, UV radiation and reduced pH.
Using up to eight environmental drivers, I show how simple organisms such as C.
reinhardtii evolve in response to MEDs.
The first step in this investigation is to examine the short-term response of MEDs.
Data collected at the beginning of the selection experiment will provide insight into
the early stages of microevolution by investigating key differences in the short-term
(plastic) responses to few vs. many MEDs. Here, I focus on how the data collected
from the responses to single environmental drivers can help us predict the responses
to MEDs by using ecological models (additive, comparative, multiplicative). I show
that the short-term plastic responses to single environmental drivers can predict the
effect of MEDs using the comparative model because the response is largely driven
by the single dominant driver present. I also demonstrate the importance of the
number of environmental drivers (NED) for making predictions from the single
environmental drivers and show that predictions become more reliable as the NED
increases. The results gathered from short-term responses provide evidence that
single environmental driver studies are useful for predicting the effect of MEDs.
After evolution, I found that the strength of selection varies with NED in a
predictable way, which connects the NED to the evolutionary response (size of the
direct response) through the strength of selection. Here, I used statistical models to
quantify the effect of NED on the evolutionary response to MEDs and then
interpreted this by considering the possible genetic constraints on adaptation to
MEDs.
A subset of populations evolved in environments with five environmental drivers and
all populations evolved in the single environmental driver environments are used to
examine how adapting to single vs. many environmental drivers affect local
adaptation. I examine how populations selected in environments with one
environmental driver, five environmental drivers and the evolved control, differ in
their response to new environments with the same NED, environments with different
NED, and a novel environment. I found that there is a relationship between local
adaptation and the strength of selection in the local environment and patterns of local
adaptation are affected by the NED of new environments. Lastly, I present the
phenotypic consequences of evolution under MEDs. I found that before evolution,
measures of chlorophyll content and cell size decline with increasing NED.
However, after evolution the relationship between chlorophyll content and cell size
with NED is weaker because populations converge on the same phenotypes as they
evolve. I also present a case-study of how mass spectrometry methods can be used
to better understand underlying molecular mechanisms of two phenotypes
(chlorophyll positive and chlorophyll negative cells).
This selection experiment is a good example of how laboratory investigations and
model organisms can be used to design experiments with enough replication to have
high statistical power in order to make more accurate predictions on the short- long-term
effects of MEDs. Whilst there have been some studies on the effects of MEDs,
these studies rarely have more than three environmental drivers (sometimes 5
environmental drivers) and there are only a handful of long-term MED studies. This
study can be used to develop a priori hypotheses for investigating how
environmental change will shape natural microbial communities, and is especially
useful for organisms where long-term studies with multiple environmental drivers
are unfeasible