WILCOX, Thomas P.; SLOAN, Adrienne J.; BISSEL, J.: Experimental Evidence for the Evolution of Cheating in an Algal:Invertebrate Symbiosis

The role of cheating symbionts in algal:invertebrate symbiosis has been largely ignored, but theoretical studies suggest they may play a significant role in the evolution of these associations. This is particularly true in symbioses in which the symbionts are acquired from the environment by each new host generation (horizontal transmission), largely decoupling host and symbiont fitness. Theoretical work suggests that cheating symbionts can arise within an individual host via mutation and competition among resident algal strains. Theory also predicts that cheaters will not be able to invade the population if the juvenile hosts that acquire the cheating symbionts suffer significantly reduced fitness relative to hosts that acquire benevolent symbionts. In this study, we empirically examined two of the assumptions of these models: 1) juvenile hosts that survive contain benevolent symbionts and 2) that cheating symbionts can invade a host as it ages. Larvae from the jellyfish Cassiopea xamachana were reared to the scyphistomae stage and then infected with algae from either very young medusa (<1 year) or adult medusa (approx 5-7 years in age). Scyphistomae were divided into one of three treatments: controls (symbiont-free), infected with algae from young hosts, and infected with algae from old hosts. Results strongly indicate that algae isolated from old hosts cause significantly increased host mortality (p < 0.01) and reduced host growth rates (p < 0.01). These results are independent of polyp or algal source. Thus the results support the prediction that young hosts harbor benevolent symbionts and that algal populations within hosts can become less benevolent over time.