Macroevolution of defense syndromes in Ficus (Moraceae)

Abstract

AbstractRecursive adaptations and counter‐adaptations of plant‐feeding insects are thought to have driven chemical and physical diversity in plant defenses. Among existing theories on defensive diversity, the syndromes hypothesis predicts that plants may evolve suites of covarying defense traits along evolutionary arms races with herbivores. We use the large, phenotypically diverse angiosperm genus Ficus (Moraceae) to test the major predictions of this hypothesis: (1) plant species will form distinctive combinations of defensive traits; (2) these traits will be correlated within each syndrome. Trait combinations need not map well onto phylogenies because plant species can converge onto similar trait values, but strong phylogenetic signal driven by selection (as opposed to drift) suggests roles for escalation and coevolution. Finally, Ficus species with complementary combinations of defenses will be less susceptible to insect damage and harbor distinct insect communities. We quantified susceptibility to insect herbivory and nine leaf traits related to resource acquisition and defense in 36 Ficus species growing in a common‐garden setting in dry and wet seasons over 2 yr. We recovered a set of three syndromes defined by relatively small sets of trait combinations. Broadly speaking, these syndromes grouped fig species with different life forms. For example, epiphytic figs had nutrient‐poor, tough, tannin‐rich leaves, while free‐standing trees tended to have leaves covered in trichomes and full of alkaloid‐rich latex. When season and species identity were accounted for, the combination of two traits, higher C:N and higher latex tannin content, provided significantly stronger defense than did either trait taken singly. Several individual traits (C:N, latex tannin, and trichome density) were significantly negatively correlated with herbivore damage, while alkaloid content was positively correlated (perhaps as a result of feeding by adapted herbivores). Several defensive traits influenced insect herbivore community structure. Finally, traits followed different evolutionary trajectories. While latex tannin, C:N, and leaf tannin fit a Brownian‐motion model of evolution, the first two escalating across Ficus phylogeny, others appeared to have more limited phylogenetic signal or tended to de‐escalate. Overall, the patterns we detected support the concept of coordinated defense syndromes, demonstrating that evolutionary arms races can drive combinations of traits in this genus.