Chemo-ecological and evolutionary aspects of the pollinator reward system in Clusia grandiflora (Clusiaceae)

Abstract

The dioecious, neotropical plant species Clusia grandiflora produces two major pools of secondary metabolites, latex and floral resin. Latex serves as a defense against herbivory and floral resin as a pollinator reward. This reward is gathered by bees who use it as a nest construction material. Nuclear magnetic resonance and mass spectral studies show that C. grandiflora latex and resin share two classes of compounds, poly-isoprenylated benzophenones and tocotrienoic acids as major or minor components. The close chemical correspondence between latex and resin indicate that they are evolutionarily related. Though the tocotrienes were not found to have biological activity, benzophenones with potent bactericidal activity were isolated from both latex and resin. Bioassays show that these compounds are especially toxic to the honeybee pathogens Paenibacillus larvae and P. alvei. In pollinator reward resin, bioactive benzophenones are major components, whereas in latex they are minor components. Since latex is ancestral to resin in Clusia, it appears that the pollinator reward system may have arisen from the defensive system through modification of latex-producing tissues in the flower. Subsequent selective pressure from bee pollinators, who benefited from bactericidal activity in the reward, caused resin chemistry to diverge from its latex-like ancestral form. This selection has resulted in the high levels of activity observed in modern resin. In populations of C. grandifora growing in southeastern Venezuela, male and female resins exhibit a pronounced dimorphism in chemical makeup. Female resin possesses a single benophenone and male resin numerous benzophenones. Female resin is more bioactive and hardens quickly; male resin remains pliable for weeks. Female resin composition more closely resembles modern latex than does male resin. Assuming that latex has not evolved substantially since the point at which resin and latex diverged, it appears that male resin has undergone more extensive evolutionary modification than female, a result of selection for progressively earlier flowering times and of necessity a lower viscosity resin reward. The research presented here provides the first detailed chemical evidence of a functional switch in secondary metabolite pathway from the production of defensive compounds to the production of a pollinator reward.