Macroevolutionary chemical escalation in an ancient plant–herbivore arms race

Abstract

A central paradigm in the field of plant-herbivore interactions is that the diversity and complexity of secondary compounds in plants have intensified over evolutionary time, resulting in the great variety of secondary products that currently exists. Unfortunately, testing of this proposal has been very limited. We analyzed the volatile chemistry of 70 species of the tropical plant genus Bursera and used a molecular phylogeny to test whether the species' chemical diversity or complexity have escalated. The results confirm that as new species diverged over time they tended to be armed not only with more compounds/species, but also with compounds that could potentially be more difficult for herbivores to adapt to because they belong to an increasing variety of chemical pathways. Overall chemical diversity in the genus also increased, but not as fast as species diversity, possibly because of allopatric species gaining improved defense with compounds that are new locally, but already in existence elsewhere.Bursera ͉ chemical complexity ͉ chemical diversity ͉ evolution of secondary compounds ͉ coevolution

Do multiple herbivores maintain chemical diversity of Scots pine monoterpenes?

“…Specifically, many inactive compounds may have served as active deterrents of current or now-extinct herbivores in earlier iterations of the coevolutionary process, but have since become redundant. A recent study, which takes a phylogenetic approach, confirms that chemical diversity increases through evolutionary time [4]. This suggests that the retention or disappearance of redundant compounds may well be a key determinant of PSM diversity.…”

“…The widely accepted hypothesis of a coevolutionary arms race between the plant and its herbivores or pathogens [2,3] suggests that new defensive compounds evolve progressively by mutation and natural selection that favours new genotypes, until their enemies evolve counteradaptations and the processes is repeated [4][5][6]. The 'screening hypothesis' suggests that phytochemical diversity evolves and is maintained because it maximizes the probability of evolving active compounds when defence requirements demand them [7,8].…”

Macroevolution and the biological diversity of plants and herbivores

“…), phenolic compounds and tolerance to herbivory appear to have increased, but the toxic cardenolides for which these plants are renowned have declined during the diversification of the genus (43,44). In this Special Feature, Becerra et al (45) report a pattern of escalation in the Bursera system, in which the diversity of terpenoid defenses has increased.…”

Four Central Points About Coevolution

“…How natural selection balances all these conflicting demands varies among environments, leading sometimes to complex geographic patterns in levels of defense and counter-defense in interacting species (Hanifin et al 2008). Over longer geological time scales, these coevolutionary arms races have fueled some of the diversification of life (Ehrlich and Raven 1964;Farrell 1998;Becerra et al 2009;McKenna et al 2009;Segraves 2010).…”