The evolution of animals from their unicellular ancestors was a major transition in evolutionary history that enabled the diversity and complexity of extant animals. Due to the lack of a clear fossil record of the progenitors of animals, we know relatively little about the first animals and the developmental events that predicated their evolution. My doctoral research utilized Salpingoeca rosetta, a model choanoflagellate and one of the closest living relatives of animals, to investigate the molecular changes that might have led to the origin of multicellularity in animals.
S. rosetta facultatively forms multicellular “rosettes” through serial cell division in a process reminiscent of early animal embryogenesis. To determine the genetic underpinnings of rosette development in S. rosetta, I performed a forward genetic screen for rosette defect mutants (Chapter 2). I identified a new class of mutants that aggregate promiscuously into amorphous clumps of cells, but that do not develop into orderly rosettes. Two clumpy mutants, named Jumble and Couscous, mapped to lesions in genes encoding predicted glycosyltransferases.
The mutations in the jumble and couscous genes were shown to disrupt glycosylation patterns at the basal pole of the extracellular matrix (ECM). The only previously identified gene required for rosette formation, rosetteless, was found to encode a protein that localizes at the basal pole of cells in the ECM-filled center of rosettes. Thus, all three genes known to be required for rosette development in S. rosetta play a role in establishing the ECM of rosettes and implicate the ECM in the regulation of multicellular development.
Beyond the specific genes required for cell adhesion and cell signaling, animal multicellular development relies on transcriptional regulation of specific genes for cell differentiation. To examine the role of transcriptional regulation, I generated an improved S. rosetta genome assembly that allows for better annotation of regulatory regions and analyzed chromatin accessibility using an assay for transposase-accessible chromatin (ATAC-seq) in distinct S. rosetta cell types, including unicellular slow swimmers and rosettes (Appendix). Slow swimmers and rosettes were found to have nearly identical chromatin accessibility profiles—consistent with previous transcriptome sequencing that showed remarkably similar expression profiles between slow swimmers and rosettes.
Taken together, S. rosetta may rely on translational and/or post-translational regulation, including modification of the ECM, to build multicellular rosettes. Continuation of the forward genetic screen for rosette defects to saturation and utilization of recently developed methods for reverse genetics will allow future scientists to more fully elucidate the genetic basis of rosette development in S. rosetta and its possible homology to animal development.