2. Evolution of Plant Defence

Our lab seeks to understand the genetic and phenotypic mechanisms of how plants defend themselves against a diversity of enemies, and how these defenses evolve over microevolutionary and macroevolutionary timescales. In addressing these problems we have sought to answer four specific questions.

How much do herbivores eat? This is the most basic and important question to the study of plant-herbivore interactions. Plants are the basal resource to virtually all food webs. Therefore, how much herbivores eat determines the amount of energy supporting animal communities, but also the importance of herbivores in driving the evolution of plant defenses. It is therefore surprising that there has been no large-scale comprehensive analysis on the annual rate of herbivore damage (i.e., herbivory) across vascular plants. PDF Martin Turcotte, former technician Christina Thomsen, and a collaborator from McGill University (Jonathan Davies) are performing phylogenetically explicit analyses of herbivory across more than 1000 plant species that represent the lineages of vascular plants. Stay tuned for results!

What drives major biogeographical patterns in herbivory? It is widely recognized that species diversity frequently increases towards the equator. This observation has led to the prediction that the strength of species interactions (e.g., herbivory) and the intensity of coevolution are greatest at lower latitudes. Ph.D. student Daniel Anstett is studying whether these predictions can explain biogeographic patterns in herbivory and plant defenses. Studying the plant Oenothera biennis (common evening primrose), he found that the relationship between herbivory and latitude exhibits every possible pattern (positive, negative and no relationship at all), depending on the herbivore species inflicting the damage. He also found that gradients in temperature explain the most variation in biogeographic patterns of herbivory. Daniel’s future research on this topic will dissect the roles of genetic variation among plant populations and environmental variation in explaining the observed biogeographic patterns in plant defenses and herbivory.

Do herbivores drive plant defense evolution? Herbivores are thought to have driven the diversification of many of the plant traits and species we see on earth today. Despite this common belief, including studies that show herbivores impose natural selection on plant traits, there is little experimental evidence that this natural selection by herbivores causes plants to evolve adaptations. This represents a fundamental gap in our understanding of the evolution of life on earth. Through the work of Ph.D. student Nash Turley, and international collaborators (Anurag Agrawal and Amy Picard-Hastings – Cornell; John Maron – Montana, Juha-Pekka Salminen – Turku), we have provided solid evidence in support of the long-held prediction that herbivores drive very rapid evolution of plant defenses (see Agrawal et al. 2012 Science; Turley et al. 2012 Am Nat).

Why do plant species vary so dramatically in defense? One of the greatest unsolved problems in the study of plant defense evolution is why species vary so dramatically in the levels and types of defenses they employ against herbivores. To answer this question we combined experiments, analytical chemistry and comparative phylogenetics to understand how plant defense evolves in as single plant lineage (Oenothera –evening primrose genus) diversifies. We discovered that that the evolution of plant defenses in Oenothera has been rapid yet constrained by tradeoffs in the expression of traits. In other words, plants adapt to express the defense traits that maximize fitness in a given environment, but this prevents the plants from expressing other chemical and morphological defense traits that are favoured in other environments. This work also shows that reductions in sexual reproduction, which are common in plants, also constrains plant defense evolution and further explains why plant species vary in defense.

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