The Evolution of Symbiosis in Communities

Abstract

All organisms host a menagerie of symbionts. While harmful pathogens have historically held the attention of researchers, recent technological advances have revealed a cornucopia of benign, and even beneficial, symbionts. Observations that most organisms are party to a wide variety of harmless symbionts are at odds with theory that suggests that infections by multiple symbionts should lead to the evolution of harmful pathogens. Current theory regarding the evolution of symbionts is predicated on the assumption that symbionts receive a reproductive payoff for harming their hosts. Because harming the host, or virulence, indirectly decreases symbiont infection duration, increased symbiont reproduction comes at a cost and leads to a tradeoff. A consequence of this tradeoff is that when multiple symbionts infect the same host the most virulent symbiont receives the highest reproductive payoff while all symbionts suffer decreased infection duration. Consequently, multiple infections are predicted to select for higher virulence, a prediction that runs counter to observation of the plethora of relatively harmless symbionts observed co-infecting most organisms. The three chapters of this thesis seek to bring theory in line with observations of the commonality of co-infecting commensals. The first chapter of this thesis lays out a mathematical model that uses the virulence tradeoff hypothesis to show that multiple infections do not necessarily lead to increased virulence. The second chapter extends the model developed in the first chapter to show that symbiont defense of the host can lead to the evolution of lower virulence. Finally, the third chapter examines ¬genetic variation in virulence and inhibition between symbiont species for fungal symbionts isolated from two populations of maize. Together, this work furthers our understanding of how symbionts evolve in communities and is an important step toward resolving the paradox of ubiquitous benign symbionts.