The evolution of multicellularity and somatic cell differentiation in volvocine green algae
University of New Brunswick
Multicellularity evolved independently and repeatedly in all three domains of life, suggesting that multicellular phenotypes can provide many benefits and are easy to evolve. However, complex multicellular bodies (with distinct cell types) evolved in only a handful of lineages (e.g., red, green, and brown algae, land plants, animals, fungi). In complex multicellular organisms, a distinction can be made between cells involved in survival functions (somatic cells) and those specialized in reproduction (germ cells). Despite the major role that both simple and complex multicellularity played in the diversification of life, the exact fitness consequences and genetic basis for their early evolution remain unclear. The general goal of my research was to use the volvocine green algae (comprising both unicellular – such as Chlamydomonas reinhardtii, and multicellular – e.g., Volvox carteri, species) to address the following questions: 1. What are the costs and the genetic basis for the evolution of simple multicellularity in volvocine algae? 2. What is the genetic basis for the evolution of somatic cells in the volvocine lineage? To explore the first question, I took advantage of an experimentally evolved multicellular C. reinhardtii strain. My data show that the survival benefits of simple multicellularity are paralleled by costs in both reproduction and cell viability. These findings suggest that the early evolution of simple multicellularity is subjected to life history trade-offs. Furthermore, transcriptomic data from both the multicellular and unicellular C. reinhardtii strains suggest a novel mechanism underlying the evolution of simple multicellularity, involving the constitutive activation of a plastic multicellular phenotype. To understand the evolution of somatic cell differentiation, I investigated the adaptive and mechanistic role that RLS1 – the homolog of the gene involved in somatic cell differentiation in V. carteri, has in C. reinhardtii. Using an RLS1 mutant and transcriptomic data, I provided direct evidence for RLS1 acting as a life history trade-off gene that downregulates photosynthesis to increase survival, albeit at a cost to immediate reproduction. Overall, my work provides a better understanding of the genetic basis for the early evolution of both simple and complex multicellularity in the volvocine green algal lineage, including the role of stress and life history trade-offs.