title: Macroevolutionary patterns of glucosinolate defense and tests of defense-escalation and resource availability hypotheses.
abstract: We explored macroevolutionary patterns of plant chemical defense in Streptanthus (Brassicaceae), tested for evolutionary escalation of defense, as predicted by Ehrlich and Raven's plant-herbivore coevolutionary arms-race hypothesis, and tested whether species inhabiting low-resource or harsh environments invest more in defense, as predicted by the resource availability hypothesis (RAH). We conducted phylogenetically explicit analyses using glucosinolate profiles, soil nutrient analyses, and microhabitat bareness estimates across 30 species of Streptanthus inhabiting varied environments and soils. We found weak to moderate phylogenetic signal in glucosinolate classes and no signal in total glucosinolate production; a trend toward evolutionary de-escalation in the numbers and diversity of glucosinolates, accompanied by an evolutionary increase in the proportion of aliphatic glucosinolates; some support for the RAH relative to soil macronutrients, but not relative to serpentine soil use; and that the number of glucosinolates increases with microhabitat bareness, which is associated with increased herbivory and drought. Weak phylogenetic signal in chemical defense has been observed in other plant systems. A more holistic approach incorporating other forms of defense might be necessary to confidently reject escalation of defense. That defense increases with microhabitat bareness supports the hypothesis that habitat bareness is an underappreciated selective force on plants in harsh environments.
authors.affiliation: Center for Population Biology, and Department of Evolution of Ecology, University of California, One Shields Avenue, Davis, CA, 95616, USA.|Instituto de Biología, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, 04510, Mexico City, Mexico.
authors.forename: N Ivalú
authors.initials: NI
authors.lastname: Cacho
authors.affiliation: Department of Plant Sciences, University of California. One Shields Avenue, Davis, CA, 95616, USA.|DynaMo Center of Excellence, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Kliebenstein
authors.affiliation: Center for Population Biology, and Department of Evolution of Ecology, University of California, One Shields Avenue, Davis, CA, 95616, USA.
authors.forename: Sharon Y
authors.initials: SY
authors.lastname: Strauss
mesh_terms: D005075:Biological Evolution; D019607:Brassicaceae / Q000235:genetics* / Q000378:metabolism; D017753:Ecosystem; D005961:Glucosinolates / Q000378:metabolism*; D060434:Herbivory; D046948:Secologanin Tryptamine Alkaloids; D012987:Soil / Q000737:chemistry
chemical_list: D005961:Glucosinolates; D046948:Secologanin Tryptamine Alkaloids; D012987:Soil; C009244:serpentine (alkaloid)
keywords: Streptanthus; bareness; coevolutionary arms race; edaphic specialization; glucosinolates; herbivory; plant defense; resource availability hypothesis (RAH)