[Show abstract][Hide abstract] ABSTRACT:
Plants and their herbivores constitute more than half of the organisms in tropical forests. Therefore, a better understanding of the evolution of plant defenses against their herbivores may be central for our understanding of tropical biodiversity. Here, we address the evolution of antiherbivore defenses and their possible contribution to coexistence in the Neotropical tree genus Inga (Fabaceae). Inga has >300 species, has radiated recently, and is frequently one of the most diverse and abundant genera at a given site. For 37 species from Panama and Peru we characterized developmental, ant, and chemical defenses against herbivores. We found extensive variation in defenses, but little evidence of phylogenetic signal. Furthermore, in a multivariate analysis, developmental, ant, and chemical defenses varied independently (were orthogonal) and appear to have evolved independently of each other. Our results are consistent with strong selection for divergent defensive traits, presumably mediated by herbivores. In an analysis of community assembly, we found that Inga species co-occurring as neighbors are more different in antiherbivore defenses than random, suggesting that possessing a rare defense phenotype increases fitness. These results imply that interactions with herbivores may be an important axis of niche differentiation that permits the coexistence of many species of Inga within a single site. Interactions between plants and their herbivores likely play a key role in the generation and maintenance of the conspicuously high plant diversity in the tropics.
Proceedings of the National Academy of Sciences 09/2009; 106(43):18073-8. DOI:10.1073/pnas.0904786106 · 9.81 Impact Factor
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Background/Question/Methods Genus-level analysis of plant defense chemistry can give important insights into the nature of plant-herbivore interactions at both ecological and evolutionary time scales, thus providing direct evidence of chemotype evolution under herbivore selection. We are pursuing just such a large-scale analysis of chemical defenses in the Neotropical legume genus Inga, a speciose taxon which has undergone explosive radiation since the Miocene. To date, we have explicitly characterized chemical defenses in 40 species of Inga. Using a generalist lepidopteran as a bioassay organism, we identified active metabolite classes, isolated abundant metabolites from each class and solved their structures by NMR. With the structural information thus gained, we made inferences about the composition of the metabolite class as a whole using HPLC-mass spectrometry.
We have found that Inga chemical defenses are largely based on two chemically complex metabolite classes, phenolics and saponins. Phenolics include flavan-3-ols (condensed tannins), flavone glycosides, phenolic acids, and the primary amino acid tyrosine (and derivatives). High structural variability is generated in this group through modifications to stereochemistry, oxidation pattern, glycosylation and polymerization. Inga saponins are composed of one of several triterpene cores which are moderately to highly substituted with an array of saccharide, terpene and phenolic acid moieties. Among the 40 species analyzed, there was very little overlap in chemistry: more than 50% of chemotypes were represented by a single species. True structural novelty was almost never encountered: those previously undescribed structures we did encounter were recombinations of common metabolites. Two major conclusions can be drawn from this analysis. 1) Evolutionary innovation in Inga chemical defenses occurs almost entirely at the level of regulation, i.e. pathway switching, modulation of the timing and location of expression of structural genes and modulation of end-product accumulation levels. 2) Switches in defense chemotypes appear to happen rapidly. Given Inga’s rapid radiation, this can be inferred from both the rarity of intermediate defense chemotypes and the high percentage of species with narrowly defined chemical defenses.