Consequences of interspecific variation in defenses and herbivore host choice for the ecology and evolution of Inga, a speciose rainforest tree

Abstract

We summarize work on a speciose Neotropical tree genus, Inga (Fabaceae), examining how interspecific variation in anti-herbivore defenses may have evolved, how defenses shape host choice by herbivores and how they might regulate community composition and influence species radiations. Defenses of expanding leaves include secondary metabolites, extrafloral nectaries, rapid leaf expansion, trichomes, and synchrony and timing of leaf production. These six classes of defenses are orthogonal, supporting independent evolutionary trajectories. Moreover, only trichomes show a phylogenetic signature, suggesting evolutionary lability in nearly all defenses. The interspecific diversity in secondary metabolite profiles does not arise from the evolution of novel compounds, but from novel combinations of common compounds, presumably due to changes in gene regulation. Herbivore host choice is determined by plant defensive traits, not host phylogeny. Neighboring plants escape each other's pests if their defenses differ enough, thereby enforcing the high local diversity typical of tropical forests. Related herbivores feed on hosts with similar defenses, implying that there are phylogenetic constraints placed on the herbivore traits that are associated with host use. Divergence in defensive traits among Inga appears to be driven by herbivore pressure. However, the lack of congruence between herbivore and host phylogeny suggests that herbivores are tracking defenses, choosing hosts based on traits for which they already have adaptations. There is, therefore, an asymmetry in the host-herbivore evolutionary arms race.

From plants to herbivores: novel insights into the ecological and evolutionary consequences of plant variation

“…While changes that occur during leaf development are often strong (see Coley et al 2018 for a nice overview of this topic), ontogenetic shifts are likewise important. Quintero and Bowers (2018) and Ochoa-López et al (2018) provide new perspectives to the ecology and evolution of ontogeny.…”

“…Lämke and Unsicker (2018) indicate that genome duplication, followed by functional diversification as well as hybridization are major drivers of variation. Coley et al (2018) review and integrate their years of research on Inga, a speciose genus of tropical trees. They show that at an interspecific level, diversity in secondary metabolite profiles does not necessarily arise from the evolution of novel compounds, but can rather result from novel combinations of common compounds, presumably due to changes in gene regulation.…”

“…Intrinsic mechanisms underlying withinand among-plant variation Two reviews (Coley et al 2018;Lämke and Unsicker 2018) examine the underlying molecular mechanisms and consequences of variation in plant defenses to long-lived trees, which are often understudied. Lämke and Unsicker (2018) indicate that genome duplication, followed by functional diversification as well as hybridization are major drivers of variation.…”

“…The studies of this special topic cover a broad range of plant life forms, from marine algae in an aquatic system (Ledet et al 2018) to terrestrial herbs (Calf et al 2018;Chiriboga et al 2018;Cuny et al 2018;Howard et al 2018;Humphrey et al 2018;Kergunteuil et al 2018;Quintero and Bowers 2018), to shrubs (Nell et al 2018;Ochoa-López et al 2018), to long-lived trees (Albrectsen et al 2018;Bagchi et al 2018;Cipollini and Peterson 2018;Coley et al 2018;Falk et al 2018;Lämke and Unsicker 2018). In twelve case studies and four review papers, selected aspects of the huge variation in phenomena and underlying mechanisms are highlighted and new research questions opened, which may inspire further research in this exciting field.…”

“…Micro-or macro-scale spatial heterogeneity in the abiotic and biotic environment can generate significant within-and among-plant variation (e.g., Schaeffer et al 2018), and recent studies have provided novel insights into how plant chemistry impacts herbivores, microbes and natural enemies (Albrectsen et al 2018;Calf et al 2018;Coley et al 2018;Cuny et al 2018). Moreover, the effects of plant chemical traits are often considered in isolation, but they may act together with other features of growth and defense to form distinct trait combinations or syndromes (Coley et al 2018;Kergunteuil et al 2018;Tewes and Müller 2018). Yet it is important not to just focus on plant traits, as environmental change such as habitat fragmentation and climate change also alter the relative abundances of herbivores and predators (van der Putten 2012;Genua et al 2017).…”