The Origin and Evolution of the REG Cluster in the Volvocine Green Algae: A Model System for the Evolution of Cellular Differentiation

Abstract

The evolutionary transition from single-celled to multicellular life has occurred dozens of times, yet the evolution of the genetic and developmental mechanisms underlying these transitions is not well understood. The volvocine green algae provide an excellent model system to study questions about the evolution of multicellularity because different species within this group span a range from single-celled, to undifferentiated multicellularity, to multicellularity with two distinct cell-types. This dissertation focuses on how the genes and developmental mechanisms that control cellular differentiation in the volvocine green algae evolved. I show that a suite of transcription factors, known as the reg cluster, which are thought to be involved in the development of cellular differentiation, arose early in the evolution of multicellularity in this group and are present in many species that lack cellular differentiation as well (Appendix A). I then study the development and genetics of Volvox powersii, a species that represents an ideal model organism for studying the early stages of cellular differentiation and multicellular complexity because it is thought to possess an ancestor-like developmental program. I show that a morphological mutant of this species possesses a mutation in the rlsB reg cluster gene and I developed a novel transformation procedure to test the causality of this mutation (Chapter 3). I also used laser scanning confocal fluorescence microscopy to test hypotheses about how cell fate is determined in V. powersii. I find that cell size is unlikely to play a role in cell fate in V. powersii but provide evidence that chloroplast DNA nucleoid number may be involved in cellular differentiation (Chapter 4). This result suggests that differential inheritance of chloroplast nucleoids may be an early arising mechanism for cell fate specification in the volvocine algae. I also demonstrate a mutagenesis procedure of Volvox ferrisii, a species that unambiguously evolved cellular differentiation and the Volvox morphology independently of V. carteri and V. powersii (Chapter 5). Collectively, this work establishes new techniques for studying the development and genetics of volvocine green algae, shows the origin and distribution of a key suite of transcription factor genes within this model system, and provides initial evidence for a previously unconsidered mechanism of cellular differentiation in this group.