The genetic and evolutionary basis for somatic cell differentiation in the multicellular alga Volvox carteri: investigations into the regulation of regA expression
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Date
2014
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University of New Brunswick
Abstract
The evolution of life is characterized by a series of major transitions in the complexity of
biological systems. One such transition was from unicellular to multicellular organisms
and involved the evolution of sterile somatic cells-a premier example of cooperation.
The goal of this thesis was to investigate the genetic and evolutionary basis for somatic
cell differentiation in Volvox carteri, a simple multicellular green alga composed of ca.
2,000 somatic cells and up to 16 reproductive cells (gonidia). In V carteri, the terminal
differentiation of small somatic cells involves the expression of regA, a gene coding for a
transcription factor thought to repress nuclear genes required for chloroplast biogenesis
and, thus, for cell growth and division. regA induction is likely dependent on cell size,
but the molecular mechanism whereby cell size is translated into regA expression remains
to be elucidated. This study focused on the regulation of regA expression by employing
mechanistic and evolutionary approaches. Using a regA-/gonidialess double mutant strain
characterized in this study, I showed for the first time that in addition to its
developmental expression, regA can be induced by environmental stimuli, and this
induction is also dependent on cell size. These findings provide support for a previously
proposed hypothesis that regA evolved from an ancestral stress-response gene.
Furthermore, in mutants expressing a non-functional RegA protein, the conditions that
trigger regA expression also induce programmed cell death, which points towards a dual
function for regA in cell fitness: to decrease cell reproduction (by repressing cell growth)
and to increase cell survival (by conferring resistance to stress). Genes with antagonistic
pleiotropic effects on fitness have been proposed to stabilize cooperation, and regA is the
first such example in multicellular organisms with unitary development (i.e., developed from a single cell). To identify transcription factors binding to cis-regulatory elements of
regA I have used yeast one-hybrid assays. No potential candidates were identified,
suggesting that cooperative binding of proteins or multi-protein complexes are involved
in the regulation of regA. Overall, this study provides novel insights into our
understanding of somatic cell differentiation, from both a mechanistic and an
evolutionary perspective.