Functional analysis of the rlsA gene in Volvox carteri

Abstract

Multicellularity is arguably one of the most important evolutionary innovations of life on Earth, however, its origins are not fully understood. The volvocine green algae, a clade of algae that includes Volvox carteri, are ideal model organisms for studying the early development of multicellular life and how developmental mechanisms associated with multicellularity evolved. Here, I describe the results of experiments designed to shed light on the evolution of multicellularity in V. carteri, particularly on the evolution of cell differentiation. I set out to determine the function of the rlsA gene, one of the closest paralogs of regA, an essential gene for cellular differentiation. Like regA, rlsA is expressed exclusively or nearly so in somatic cells, with peak levels just before the onset of somatic cell differentiation. My hypotheses was that rlsA might also be essential for somatic cell differentiation. Using Cas9 gene-editing, I generated and imaged a V. carteri strain in which the reading frame of the rlsA gene was disrupted. I subsequently observed an increase in size in the mutant somatic cells relative to wild type, as well as an increase in somatic cell eyespot size relative to wild type toward the posterior end of mutant V. carteri spheroids. Therefore, I conclude that while rlsA is not essential for cellular differentiation under standard vegetative growth conditions, it appears to play a role in limiting somatic cell growth.