Content area
Full text
CASTILLO-GUEVARA, C. AND V. RICO-GRAY (Departamento de Ecologia Vegetal, Instituto de Ecologia, A. C., Apdo. 63, Xalapa, Veracruz 91070, Mexico). The role of macrozamin and cycasin in cycads (Cycadales) as anliherbivore defenses. J. Torrey Bot. Soc. 130:206-217. 2003.-Macrozamin and cycasin are very toxic azoxyglycosides of the Cycadales. Caterpillars (Lepidoptera) and weevils (Coleoptera) feed on cycad root, stem, leaf, and reproductive tissues. Azoxyglycosides may have played an important ecological role as antiherbivore defenses. 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.
Key words: association of characters, azoxyglycosides, cycads, cycasin, herbivores, herbivory, independent contrasts, macrozamin.
Macrozamin and cycasin are characteristic and exclusive azoxyglycosides of the Cycadales (De Luca et al. 1980; Siniscalco 1990). This ancient gymnosperm group was abundant and widespread during the Mesozoic; however, it is currently isolated and confined to the tropics (Norstog and Nicholls 1997). They have remained relatively unchanged morphologically since the Mesozoic time (Norstog and Nicholls 1997; Jones 2000) and, according to most authors, extant cycads represent the end-points of a number of different lines, doubtless cognate with others now extinct (Siniscalco 1990).
The azoxyglycosides are mutagenic and carcinogenic only when deglucosylated. Its principal metabolite, methylazoxymethanol...