Roller, B. R. 2015. Life histories of bacteria: Genomic foundations and ecological implications. Dissertation, Michigan State University, East Lansing, Michigan.

Life history tradeoffs are of great interest to biologists because they are central to biodiversity theory and have the power to explain the outcomes of competition. One particular tradeoff has been a frequent target of study, the relationship between rapid and efficient reproduction. Researchers have pursued evidence for the existence of this tradeoff in many study systems and the literature surrounding this topic is extensive. While theoretical studies have indicated that a rate-efficiency tradeoff should play an important role in the evolution of bacterial populations, experiments have frequently found conflicting evidence for its existence and influence on bacterial evolution. In this dissertation I explore the physiological and ecological conditions favoring rapid and efficient bacterial growth.

Microbial ecologists have long observed that the richness of cultivation medium leads to the growth of different types of bacteria. Copiotrophic bacteria have a higher relative fitness under conditions of resource abundance, while oligotrophic bacteria are favored when resources are scarce. The central topic of my dissertation research is to explore if copiotrophic bacteria employ rapid growth life history tactics and if oligotrophic bacteria employ efficient growth life history tactics. It has been noted that rapidly growing bacteria tend to possess multiple copies of the ribosomal RNA operon (rrn) in their genomes, while oligotrophic bacteria typically encode few rrn copies. I examined if rrn copy number was related to rapid and efficient growth tactics using physiological experiments and comparative genomics.

The major findings from my research suggest that copiotrophs tend to utilize rapid growth tactics, while oligotrophs utilize efficient growth tactics. This evidence is consistent with a rate-efficiency tradeoff underlying divergence on this life history axis. I also demonstrate that rrn copy number is quantitative predictor of life history tactics and that it explains a large fraction of variation in the genome content of diverse bacterial species. Finally, I have explored particular features of bacterial genomes which play a role in life history adaptation.

It is increasingly recognized that bacteria directly influence the health of our planet. However, the scale of bacterial diversity is immense and there is much more to learn before we can manage bacterial communities to improve wellbeing on Earth. Applying life history theory to bacteria holds promise for improving our understanding of the ecological and evolutionary forces acting on bacterial populations and communities.

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