Interactions between the marine bryozoan Bugula neritina, its endosymbiont, and symbiont-produced bryostatins

The invasive marine bryozoan, Bugula neritina, is distributed in temperate habitats worldwide. B. neritina possesses an uncultured bacterial symbiont, “Candidatus Endobugula sertula” that is vertically transmitted from parent to offspring. It is also the source of the bryostatins, twenty bioactive polyketide metabolites with anticancer, anti-Alzheimer's disease, and anti-HIV activity. Antibiotic curing experiments showed that E. sertula is, in fact, the true source of the bryostatins, and in vitro biochemical assays with heterologously expressed portions of bry, the putative bryostatin polyketide synthase gene cluster, confirmed its role in bryostatin biosynthesis. Interestingly, the bryostatins are ecologically relevant, as some of the bryostatins are distasteful and defend the host from predators such as fish, indicating that this symbiosis is a tritrophic interaction among the host, symbiont, and predator. Bryostatin levels are higher on larval B. neritina, which are exceptionally vulnerable to predators, compared to adult colonies which rely more on physical defense. Previous research showed that the host is a complex of three closely related sibling species that vary both in their symbiont and bryostatins, with one (Type N) lacking the symbiont and bryostatins. Our studies along the western Atlantic showed that, instead of being restricted to higher latitudes where predation pressure is presumed to be lower, Type N B. neritina can be found at lower latitudes, and Type S colonies at higher latitudes, indicating a more widespread host distribution than previously thought. Curiously, some of those host colonies varied in their symbiotic status, with Type N colonies at low latitudes possessing the symbiont whereas Type S colonies collected from high latitudes are aposymbiotic. Furthermore, the symbiont in Type N colonies at low latitudes appears to be the same strain as that found in Type S. Together, these data indicate that the symbiont, but not the host, is restricted by biogeography, and that symbiont transmission is more flexible than previously thought. This system reflects the complexity and breadth of biological interactions resulting in symbiont-produced bioactive compounds.