Stochastic Modelling of Salpingoeca rosetta Chain Colonies

Abstract

Choanoflagellates are single-celled eukaryotes found in aquatic environments. Even though they are unicellular, several species of choanoflagellates can form multicellular colonies. What makes these microorganisms unique is that they are the closest living relatives of animals, making them an interesting model organism to study various aspects of the origin of animals. In this thesis, Salpingoeca rosetta (S. rosetta), a choanoflagellate species, is used to study the properties of simple multicellularity. S. rosetta cells are able to form chain colonies: linear chains of cells connected by intercellular bridges. One important aspect of these colonies is their size, i.e. how many cells are present per colony. This thesis presents a model that describes how S. rosetta chain colonies grow and split, revealing what factors determine colony sizes in simple multicellular colonies, leading to a better understanding of the development of more complex multicellular organisms. The model is built on two key processes that determine the colony sizes in chain colonies: cell division, which increases colony size, and bridge breakage, where connections between cells break, leading to new smaller colonies. The outputs of this model include the colony sizes after specific time points and the average number of cells per chain per time point. By doing various analyses with this model, we came to the following conclusions: The cell division in S. rosetta cells follows a specific pattern best described by a gamma distribution, which effectively means that division is age-dependent. We were not able to come to definitive conclusions on the lifetimes of bridges, but we were able to conclude that this process involves some randomness and is not deterministic. Furthermore, we found that after some time, the colonies in the simulation reach a steady state, where they stabilize in their size. The mean colony size at which they stabilize is dependent on the bridge break and division rate. These findings highlight how a simple model of two events can make accurate predictions on colony sizes. Our results contribute to the knowledge on the mechanics of colonies of choanoflagellates, the sister group to animals, which could in turn contribute to the knowledge on the transition from single-celled organisms to multicellular ones, which was a crucial step in the life history of animals.