title: Coevolution can explain defensive secondary metabolite diversity in plants.
abstract: Many plant species produce defensive compounds that are often highly diverse within and between populations. The genetic and cellular mechanisms by which metabolite diversity is produced are increasingly understood, but the evolutionary explanations for persistent diversification in plant secondary metabolites have received less attention. Here we consider the role of plant-herbivore coevolution in the maintenance and characteristics of diversity in plant secondary metabolites. We present a simple model in which plants can evolve to invest in a range of defensive toxins, and herbivores can evolve resistance to these toxins. We allow either single-species evolution or reciprocal coevolution. Our model shows that coevolution maintains toxin diversity within populations. Furthermore, there is a fundamental coevolutionary asymmetry between plants and their herbivores, because herbivores must resist all plant toxins, whereas plants need to challenge and nullify only one resistance trait. As a consequence, average plant fitness increases and insect fitness decreases as number of toxins increases. When costs apply, the model showed both arms race escalation and strong coevolutionary fluctuation in toxin concentrations across time. We discuss the results in the context of other evolutionary explanations for secondary metabolite diversification.
authors.affiliation: Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, Faculty of Health & Life Sciences, University of Liverpool, Liverpool, L69 7ZB, UK.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Speed
authors.affiliation: Department of Evolution, Ecology and Behaviour, Institute of Integrative Biology, Faculty of Health & Life Sciences, University of Liverpool, Liverpool, L69 7ZB, UK.
authors.forename: Andy
authors.initials: A
authors.lastname: Fenton
authors.affiliation: Functional and Comparative Genomics, Institute of Integrative Biology, Faculty of Health & Life Sciences, University of Liverpool, Liverpool, L69 7ZB, UK.
authors.forename: Meriel G
authors.initials: MG
authors.lastname: Jones
authors.affiliation: School of Biology, University of St Andrews, St Andrews, KY16 9TH, UK.
authors.forename: Graeme D
authors.initials: GD
authors.lastname: Ruxton
authors.affiliation: Department of Biology, University of York, York, YO10 5DD, UK.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Brockhurst
mesh_terms: D000818:Animals; D005075:Biological Evolution; D056084:Genetic Fitness; D060434:Herbivory; D007313:Insecta / Q000235:genetics*; D010641:Phenotype; D010935:Plant Diseases; D010944:Plants / Q000235:genetics* / Q000378:metabolism; D064210:Secondary Metabolism; D014118:Toxins, Biological / Q000378:metabolism*
chemical_list: D014118:Toxins, Biological
keywords: chemical defence; coevolution; herbivore; secondary metabolite; specialized metabolism; theoretical modelling