UC Berkeley

Soritid foraminifera host dinoflagellate symbionts of the genus Symbiodinium, the same algae that power the formation and persistence of coral reefs through mutualism with cnidarians, molluscs, sponges, and other reef-dwelling hosts. This dissertation examines the interactions between these foraminiferal hosts and their symbionts, placing those interactions in the context of the overall reef photosymbiotic system. Given the finding of multiple symbiont types within an individual, the general phenomenon of multiple Symbiodinium infections is examined in light of extant theory on the evolution of mutualism.

The first chapter reviews the scientific literature that reports the incidence of mixed Symbiodinium infections. This trait has a wide phylogenetic distribution across major host groups. A similarly wide distribution of this trait is also found across the phyletic diversity of scleractinian corals. Extant theory suggests that a mixed population of symbionts can be disadvantageous to the host, and that hosts should evolve mechanisms to control mixing of their symbionts. Stability of Symbiodinium-host mutualisms is maintained though any of three model mechanisms: partner fidelity feedback, cooperator association, and partner choice. Which operates depends on the particular biological processes that mediate the interaction.

The second chapter examines the distribution of multiple symbiont types within individual foraminifer, reporting the finding that multiple types of Symbiodinium are distributed differentially across the radius of the foraminifer. Multiple hypotheses could explain this phenomenon, including: processing of the symbionts as they move into the host, partitioning of symbiont functional roles, or differential competition of symbionts within a heterogeneous internal host environment.

The third chapter explores symbiont acquisition by soritid foraminifera. I report that soritid foraminifera typically do not acquire new symbiont types as adults. Symbionts move from internal chambers to the newly formed outermost chambers. Foraminifera transmit their symbionts vertically through rounds of asexual reproduction and horizontally through rounds of sexual reproduction, and thus may optimize these different symbiont acquisition strategies for different environmental conditions.