The secret to a successful relationship: lasting chemistry between ascidians and their symbiotic bacteria

Bioactive secondary metabolites are common components of marine animals. In many cases, symbiotic bacteria, and not the animals themselves, synthesize the compounds. Among marine animals, ascidians are good models for understanding these symbioses. Ascidians often contain potently bioactive secondary metabolites as their major extractable components. Strong evidence shows that similar to 8% of the known secondary metabolites from ascidians are made by symbiotic bacteria, and indirect evidence implicates bacteria in the synthesis of many more. Far from being secondary to the animals, secondary metabolites are essential components of the interaction between host animals and their symbiotic bacteria. These interactions have complex underlying biology, but the chemistry is clearly ascidian species-specific. The chemical interactions are ancient in at least some cases, and they are widespread among ascidians. Ascidians maintain secondary metabolic symbioses with bacteria that are phylogenetically diverse, indicating convergent solutions to obtaining secondary metabolites and reinforcing the importance of secondary metabolism in animal survival.