Dissertation Proposal - The University of Arizona Campus Repository

ABSTRACT
The fitness of any evolutionary unit can be understood in terms of its two basic
components: fecundity and viability. The trade-offs between these fitness components
drive the evolution of a variety of life-history traits in extant multicellular lineages. Here,
I show evidence that the evolution of germ-soma separation and the emergence of
individuality at a higher level during the unicellular-multicellular transition are also
consequences of these trade-offs. The transition from unicellular to larger multicellular
organisms has benefits, costs, and requirements. I argue that germ-soma separation
evolved as a means to counteract the increasing costs and requirements of larger
multicellular colonies. Volvocalean green algae are uniquely suited for studying this
transition since they range from unicells to undifferentiated colonies, to multicellular
individuals with complete germ-soma separation. In these flagellated organisms, the
increase in cell specialization observed as colony size increases can be explained in terms
of increased requirements for self-propulsion and to avoid sinking. The collective
flagellar beating also serves to enhance molecular transport of nutrients and wastes.
Standard hydrodynamic measurements and concepts are used to analyze motility (self-
propulsion) and its consequences for different degrees of cell specialization in the
Volvocales as colony size increases. This approach is used to calculate the physical
hydrodynamic limits on motility to the spheroid colony design. To test the importance of
collective flagellar beating on nutrient uptake, the effect of advective dynamics on the
productivity of large colonies is quantified. I conclude first, that when colony size
exceeds a threshold, a specialized and sterile soma must evolve, and the somatic to
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