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AgrawalEcolLet04

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REPORTEvolution of plant resistance and tolerance to frostdamageAnurag A. Agrawal,1*Jeffrey K. Conner2andJohn R. Stinchcombe31Department of Botany,University of Toronto, 25Willcocks Street, Toronto, ON,Canada M5S 3B22Kellogg Biological Station andDepartment of Plant Biology,Michigan State University, 3700East Gull Lake Drive, HickoryCorners, MI 49060, USA3Department of Ecology andEvolutionary Biology, Box G-W,Brown University, Providence, RI02912, USA*Correspondence and presentaddress: Anurag A. Agrawal,Department of Ecology andEvolutionary Biology, CorsonHall, Cornell University, Ithaca,NY 14853, USA. E-mail:[email protected] defence against any type of stress may involve resistance (traits that reducedamage) or tolerance (traits that reduce the negative fitness impacts of damage). Thesetwo strategies have been proposed as redundant evolutionary alternatives. A late-seasonfrost enabled us to estimate natural selection and genetic constraints on the evolution offrost resistance and tolerance in a wild plant species. We employed a genetic selectionanalysis (which is unbiased by environmental correlations between traits and fitness) on75 paternal half-sibling families of annual wild radish [Raphanus raphanistrum (Brassic-aceae)]. In an experimental population in southern Ontario, we found strong selectionfavouring plant resistance to frost, but selection against tolerance to frost. The selectionagainst tolerance may have been caused by a cost of tolerance, as we provide evidencefor a negative genetic correlation between tolerance and fitness in the absence of frostdamage. Although we found no evidence for the theoretically predicted trade-offbetween frost tolerance and resistance among our families, we did detect negativecorrelational selection acting on the two traits, indicating that natural selection favouredhigh resistance combined with low tolerance and low resistance coupled with hightolerance, but not high or low levels of both traits together. There were few geneticcorrelations between the measured traits overall, but frost tolerance was negativelycorrelated with initial seed mass, and frost resistance was positively correlated withresistance to insect herbivory. Periodic episodes of strong selection such as that causedby the late-season frost may be disproportionately important in evolution, and are likelybecoming more common because of human alterations of the environment.KeywordsAdditive genetic variance, Brassicaceae, cost of tolerance, genotypic selection analysis,paternal half-sibling design, plant temperature stress, Raphanus raphanistrum, wild radish.Ecology Letters (2004) 7: 1199–1208INTRODUCTIONBeginning with Darwin, evolutionary biol ogists have soughtto understand how natural selection produces adaptations.One goal is to observe on-going evolution and to be able topredict future responses to natural selection. Becauseadaptive traits are likely to have multiple functions andmay be energetically and ecologically costly, the net benefitsof each trait and the response to selection in realenvironments are complex.In response to any biotic or abiotic stress, organisms mayevolve adaptations that provide res istance or tolerance.Resistance traits reduce the level of damage by the stressor,while tolerance traits reduce the negative fitness impact for agiven amount of stress. For example, plant resistance toherbivory may be provided by toxic chemicals that have theeffect of reducing insect damage, whereas tolerance may beprovided by root storage, which allows regrowth followingdamage (Strauss & Agraw al 1999). Theory developed byworkers studying herbivory has predicted that plants shouldshow a trade-off, or negative correlation, between levels ofresistance and tolerance (Van Der Meijden et al. 1988;Fineblum & Rausher 1995). The logic behind this predictedtrade-off is that natural selection for resistance results in lowlevels of attack, and hence reduced selection for tolerance.Conversely, organisms with a high level of tolerance shouldnot experience selection for resistance, because attack doesnot reduce fitness (i.e. the organisms are tolerant). InEcology Letters, (2004) 7: 1199–1208 doi: 10.1111/j.1461-0248.2004.00680.x2004 Blackwell Publishing Ltd/CNRSaddition, costs associated with resistance and toleranceshould constrain their maximal expression and willcontribute to negative correlations between resistance andtolerance (Va n Der Meijden et al. 1988; Simms & Triplett1994; Fineb lum & Rausher 1995). Despite these well-developed theoretical predictions, empirical evidence fornatural selection favouring higher levels of either resistanceor tolerance, but not both traits, remains sparse (Pilson2000). Such negative correlational selection is predicted forany pair of redundant strategies, including resistance andtolerance to biotic and abiotic stressors.Here we present empirical evidence supporting two of themajor theoretical tenets of theory developed to explain theevolution of resistance and tolerance: fitness costs oftolerance and negative correla tional selection acting onresistance and tolerance. We took advantage of a hard late-season frost that damaged natural communities and ourexperimental population of wild radish [Raphanus raphani-strum (Brassicaceae)] in southern Ontario, Canada, to studythe evolution of resistance and tolerance to abiotic stress.Although understanding the genetics of plant defenceagainst frost continues to be a goal of breeding programs(Raymond et al. 1992; Ogren 1999; Kole et al. 2002) andmolecular analyses (Artus et al. 1996; Thomashow 1999;Smallwood & Bowles 2002), relatively little is known aboutvariation and selection on defence against frost in naturalpopulations (Daday 1965; Artus et al. 1996; Inouye 2000).Our results not only illustrate the utility of periodic intenseselection events, such as catastrophic frosts, for evaluatingmajor theoretical predictions in evolutionary ecology, butalso potential genetic cons traints and fitness costs of frosttolerance that may be encountered in agricultural breeding(Hsieh et al. 2002).MATERIALS AND METHODSRaphanus raphanistrum (Brassicaceae) is a widely distributed,self-incompatible annual plant found in disturbed sites onsix continents; the plant was introduced to our study areawell over 100 years ago and is naturalized in the community.We bred paternal half-sib families of R. raphanistrum from asingle wild population in upstate New York (Conner & Via1993), with the initial


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