Ontogenetic production of a chemical defense in Bugula neritina (Bryozoa): Chemistry, quantification, symbiont-mediated biosynthesis, and gene expression

Bugula neritina (Bryozoa) inhabits temperate marine environments throughout the world where it often co-occurs with the bacterial endosymbiont Candidatus Endobugula sertula. Many populations of B. neritina produce a biologically active class of polyketide-derived secondary metabolites called bryostatins, which have shown significant activity in cancer therapy. Their ecological role is less well-defined; however, because adult B. neritina are palatable to sympatric predators but their larvae are not, it has been suggested that B. neritina larvae are chemically defended by deterrent bryostatins. The objectives my research were to (i) identify the compounds that defend B. neritina larvae; (ii) quantify the concentrations of these compounds in all life stages of B. neritina; (iii) determine if the symbiont, E. sertula, is involved in the production of these metabolites; and (iv) quantify biosynthetic gene expression changes through development.; Using bioassay-guided fractionation, I have isolated and identified the deterrent secondary metabolites from B. neritina as bryostatins, and established that they are produced by the symbiont, E. sertula. I also examined the palatability of reproductive and non-reproductive adult and juvenile tissue, as well as the concentrations of a deterrent bryostatins in these extracts. Reproductive adult tissue was not palatable, but the same adult tissue without larvae was palatable; bryostatin concentrations were low and undetectable in reproductive adult and non-reproductive adult tissue respectively. Palatability increased and bryostatin concentrations decreased as the juveniles aged. I found a polyketide synthase (PKS) biosynthetic gene in larval tissue and quantified the expression of that gene in larvae, juveniles, and reproductive and non-reproductive adults. Expression of the PKS gene did not change within larval, juvenile or adult tissue. These data indicate that low concentrations of bryostatins may be produced continually throughout the tissues, and that these deterrent compounds may somehow be concentrated on the larva. This host/symbiont system is unique in that the symbiont produces secondary metabolites that protect the host's highly vulnerable larval stage from predators.