The evolution of cooperation and conflict, experimental model systems and theory

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Date

2004

Authors

Sachs, Joel Lawrence

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Abstract

I present three different studies in three chapters. In chapter 1, I describe a general theoretical framework for the evolution of cooperation both within and between species. Three general models are distinguished by which cooperation can evolve and be maintained: (i) directed reciprocation—cooperation with individuals who give in return; (ii) shared genes—cooperation with relatives; and (iii) byproduct benefits—cooperation as an incidental consequence of selfish action. In chapter 2, I investigate the origins of cooperation at the genotypic and phenotypic levels. While theory and empirical work enlighten the maintenance of cooperation, few studies explore its origins. Here, I examine the origins of cooperation by experimentally evolving two antagonistic bacteriophages. I experimentally enforced the two bacteriophages, f1 and IKe, to undergo fifty iterated cycles of co-infection, paired vertical transmission, and infectious transmission in Escherichia coli cells. Phenotypic and genomic analysis then characterized the outcome. Strikingly, the two bacteriophages evolved to co- package their genomes into one symbiotic unit, ensuring co-transmission during the infectious stage. Furthermore, one bacteriophage evolved a minimal genome with the inability to infect cells independently, becoming an obligate viral symbiont. These results parallel a wide variety of natural systems: evolution of reduced genomes, co-transmission of partners, and obligate coexistence between cooperating species. In chapter 3, I examine a puzzling example of cooperation between species, the symbiotic interaction that occurs in corals, hydras, and jellyfish and their dinoflagellate algae. These algae are mostly acquired infectiously, and according to models of virulence evolution should be selected to exploit the host. However, symbiont cheating is virtually unknown. I experimentally manipulated transmission mode of algal symbionts in jellyfish hosts to determine if altering symbiont transmission mode selects for cheating within symbiont populations. Cheating symbionts evolved under experimentally enforced horizontal transmission. Fitness estimates revealed that cheater algae had faster within-host growth, higher dispersal rates, and caused lower host growth compared to algae which underwent repeated vertical transmission. A trade-off was detected between harm caused to hosts and symbiont fitness. Such trade-offs have been modeled for pathogen evolution and may be critical in stabilizing ‘infectious’ symbioses.

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