Date of Award

12-15-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

George Pierce

Second Advisor

Nicole Lopanik

Third Advisor

Kuk-Jeong Chin

Fourth Advisor

Erik Sotka

Fifth Advisor

Sidney Crow

Abstract

Geographical differences in selective pressure may shift the relationship outcome in mutualistic symbioses from positive to negative. The marine bryozoan Bugula neritina is a colonial invertebrate common in temperate waters worldwide. Evidence suggests that an uncultured vertically transmitted symbiont, “Candidatus Endobugula sertula,” hosted by B. neritina produces the polyketide bryostatins, which protect vulnerable larvae from predation. Studies of B. neritina along the Western Atlantic coast revealed a complex of two morphologically similar sibling species separated by an apparent biogeographic barrier: the Type S sibling species was found south of Cape Hatteras, North Carolina, while Type N was found to the north. Interestingly, the Type N colonies lacked E. sertula and defensive bryostatins; their documented distribution was consistent with traditional biogeographical paradigms of latitudinal variation in predation pressure. Upon further sampling of B. neritina populations, we found that both host types occur in wider distribution, with Type N colonies living south of Cape Hatteras, and Type S to the north. Distribution of the symbiont, however, was not restricted to Type S hosts. Genetic and microscopic evidence demonstrates the presence of the symbiont in some Type N colonies and larvae, and they are apparently endowed with defensive bryostatins. Molecular analysis of the symbiont from Type N colonies suggests an evolutionarily recent acquisition, which is remarkable for a symbiont thought to be transmitted vertically only. Furthermore, most Type S colonies found at higher latitudes lack the symbiont, indicating that this relationship may be more flexible than previously thought. Transplant and common-garden experiments further suggest that the endosymbiont’s geographical range is mediated by a combination of environmental impact on symbiont growth and a fitness cost on the host imposed by association with E. sertula. These results provide insight into possible mechanisms of regulation of context-dependent mutualisms.

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