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
Loading...
Date
2014
DOI
Open Access Location
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Massey University
Rights
The Author
Abstract
Major 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.
Description
Keywords
Evolution (Biology), Cells, Evolution (Biology), Biocomplexity