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Item Theoretical investigation into the origins of multicellularity : a thesis presented in partial fulfilment of the requirements for the degree of PhD in Theoretical Biology at Massey University, Albany, New Zealand(Massey University, 2015) Pichugin, YuriyEvolution of multicellularity is a major event in the history of life. The first step is the emergence of collectives of cooperating cells. Cooperation is generally costly to cooperators, thus, non-cooperators have a selective advantage. I investigated the evolution of cooperation in a population in which cells may migrate between collectives. Four different modes of migration were considered and for each mode I identified the set of multiplayer games in which cooperation has a higher fixation probability than defection. I showed that weak altruism may evolve without coordination among cells. However, the evolution of strong altruism requires the coordination of actions among cells. The second step in the emergence of multicellularity is the transition in Darwinian individuality. A likely hallmark of the transition is fitness decoupling. In the second part of my thesis, I present a method for characterizing fitness (de-)coupling which involves an analysis of the correlation between cell and collective fitnesses. In a population with coupled fitnesses, this correlation is close to one. As a population evolves towards multicellularity, collective fitness starts to rely more on the interactions between cells rather than the individual performance of cells, so the correlation between particle and collective fitnesses decreases. This metric makes it possible to detect fitness decoupling. I used the suggested metric to investigate under which conditions fitness decoupling occurs. I constructed a model of a population defined by a linear traits-to-fitness function and used this to identify those functions that promote fitness decoupling. In this model, the fitness correlation is equal to the cosine of the angle between the gradients of fitnesses. Therefore, my results allow an estimation of the fitness (de-)coupling state before selection takes place. In the third section of my thesis, the accuracy of this estimation was tested on available experimental data and using a model simulating an experimental selection regime, which featured non-linear traits-to-fitness functions. The results obtained from the estimation of fitness correlations showed a close approximation to the fitness correlation calculated from experimental data and from simulations in a range of selection regimes.Item The evolution of multicellularity : a thesis presented in partial fulfillment of the requirements for the degree of PhD in Evolutionary Biology at Massey University, Albany, New Zealand(Massey University, 2014) Rose, CarolineMajor evolutionary transitions in Darwinian individuality are central to the emergence of biological complexity. The key to understanding the evolutionary transition to multicellularity is to explain how a collective becomes a single entity capable of self-reproduction – a Darwinian individual. During the transition from single cells to multicellular life, populations of cells acquire the capacity for collective reproduction; however, the selective causes and underlying mechanisms are unclear. This thesis presents long-term evolution experiments using a single-celled model system to address fundamental questions arising during the evolution of multicellularity. Populations of the cooperating bacterium Pseudomonas fluorescens were subjected to experimental regimes that directly selected on the capacity for collectives to differentially reproduce – an essential requirement for the evolution of collectives by natural selection. A crucial stage during an evolutionary transition to multicellularity occurs when the fitness of the multicellular collective becomes ‘decoupled’ from the fitness of its constituent cells. Before this stage, any differences in collective fitness are due to selection at the cellular level. In the present study, collectives that competed to reproduce via a cooperative propagule cell attained high levels of cooperation and also reached high levels of collective fitness. However, these improvements were shown to be a consequence of selection acting at the cell-level. In contrast, Darwinian individuality emerged in collectives that reproduced via a primitive life cycle that was fueled by conflict between cooperating cells and cheating cells that did not bear the cost of cooperation. Cheats were analogous to a germ line, acting as propagules to seed new collectives. Enhanced fitness of evolved collectives was attributable to a property selected at the collective-level, namely, the capacity to transition through phases of the life cycle, and was not explained by improvement in individual cell fitness. Indeed, the fitness of individual cells declined. In addition to providing the first experimental evidence of a major evolutionary transition in individuality, the work presented in this thesis highlights the possibility that the prevalence of complex life cycles among extant multicellular organisms reflects the fact that such cycles, on first emergence, had the greatest propensity to participate in Darwinian evolution.