A shift to parasitism in the jellyfish symbiont <i>Symbiodinium microadriaticum</i>

TLDR

Coral, hydra, and jellyfish form mutualistic symbioses with algae that are typically acquired infectiously, yet evolutionary models predict that such algae should evolve toward parasitism. The study aimed to test how changing algal transmission modes influences virulence evolution in the jellyfish–algae symbiosis. The experiment compared vertical transmission, which ties symbiont fitness to host reproduction, with horizontal transmission that decouples it, to assess effects on symbiont cooperation. Horizontal transmission caused a shift to parasitism, with algae growing faster, dispersing more, and reducing host reproduction and growth, while a trade‑off between host harm and symbiont fitness was observed.

Abstract

One of the outstanding and poorly understood examples of cooperation between species is found in corals, hydras and jellyfish that form symbioses with algae. These mutualistic algae are mostly acquired infectiously from the seawater and, according to models of virulence evolution, should be selected to parasitize their hosts. We altered algal transmission between jellyfish hosts in the laboratory to examine the potential for virulence evolution in this widespread symbiosis. In one experimental treatment, vertical transmission of algae (parent to offspring) selected for symbiont cooperation, because symbiont fitness was tied to host reproduction. In the other treatment, horizontal transmission (infectious spread) decoupled symbiont fitness from the host, potentially allowing parasitic symbionts to spread. Fitness estimates revealed a striking shift to parasitism in the horizontal treatment. The horizontally transmitted algae proliferated faster within hosts and had higher dispersal rates from hosts compared to the vertical treatment, while reducing host reproduction and growth. However, a trade-off was detected between harm caused to hosts and symbiont fitness. Virulence trade-offs have been modelled for pathogens and may be critical in stabilising ‘infectious’ symbioses. Our results demonstrate the dynamic nature of this symbiosis and illustrate the potential ease with which beneficial symbionts can evolve into parasites.