The genetic and evolutionary basis for somatic cell differentiation in the multicellular alga Volvox carteri: investigations into the regulation of regA expression

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.