The role of macrozamin and cycasin in cycads (Cycadales) as antiherbivore defenses1

Abstract

fenses. We evaluated the association between herbivory and the amount of azoxyglycosides in the Cycadales using phylogenetic independent contrasts. We hypothesized that herbivory types should be related to the presence of macrozamin and cycasin, thus herbivory should be lower in species with higher concentrations of azoxyglycosides. We gathered information available on the literature of these two characters as well as life form, geographic distribution, height, and seed volume for the majority of cycad species, in order to assess correlated evolution and control for possible allometric effects. Herbivory types and macrozamin were negatively correlated, suggesting a possible defensive function for macrozamin against herbivores. No significant correlation was observed between cycasin percent and herbivory type. However, when analysed using phylogenetic independent contrasts and thus removing the historical effect, the association did not hold. This suggests that the presence of metabolites in plants may have evolved for some other reason, and has been mantained among cycads perhaps by phylogenetic inertia. The presence of macrozamin should then be explained as an exaptation, playing today an important role in defense against herbivores. Furthermore, this analysis showed that macrozamin has independently and repeatedly (Bowenia, Macrozamia, Stangeria) increased over evolutionary time.