Phenotypic Plasticity and Genetic Variation of Vaccinium macrocarpon, the American Cranberry. I. Reaction Norms of Clones from Central and Marginal Populations in a Common Garden Author(s): C. Neal Stewart Jr. and Erik T. Nilsen Source: International Journal of Plant Sciences, Vol. 156, No. 5 (Sep., 1995), pp. 687-697 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/2475048 . Accessed: 15/04/2011 16:44 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. 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REACTION NORMS OF CLONESFROM CENTRAL AND MARGINALPOPULATIONS IN A COMMONGARDEN C. NEAL STEWART JR. AND ERIK T. NILSEN Departmentof Biology, Virginia Polytechnic Institute and StateUniversity, Blacksburg, Virginia 24061-0406 Vaccinium macrocarponAiton (Ericaceae) cranberry,a dwarfshrub and a typicaltaxon in temperate peatbogs, has its central distribution incool temperate regions in eastern North America. Isolated southern marginalpopulations are distributedalong the Appalachian corridor and on theNorth Carolina coastal plain.A commongarden (Blacksburg, Virginia) was utilizedto determinewhether marginal cranberry clonesexhibit greater phenotypic plasticity than central clones. Three central clones from Massachusetts (MA), Wisconsin(WI), and New York (NY) and threemarginal clones from North Carolina (NC), Tennessee(TN), and WestVirginia (WV) weretested. A suiteof phenotypictraits was measuredin responseto edaphicvariation in thecommon garden. An analysisof reaction norms took the form of an analysisof covarianceto testfor significant differences among clones and to estimateregression slopes (plasticity)when compared with environmental (nutrient) variation. There was no regionalvariation in phenotypicplasticity, but there was significantclonal differentiation for77% ofnonintercorrelated traits. However,in mostcases the differences were seemingly random, with little biological importance. Hence littledifferentiation in relation to populationorigin was observedamong clones. Matrix comparisons wereperformed using a Manteltest to checkfor pairwise correlations among the followingmatrices: geographicdistances, trait means, plasticity, and molecularvariation assessed by randomamplified polymorphicDNA (RAPD) profiling.No correspondencewas foundamong matrices. The recentpost- glacialdistribution of cranberry may account for the absence of phenotypic and geneticheterogeneity. Introduction and Bazzaz 1983; Silander1984, 1985a; Mac- Donald and Chinnappa1989; Thompsonet al. Geneticdifferentiation among ecologically di- 1991). An alternativeviewpoint is thatpheno- versepopulations has been widelydocumented typicplasticity and geneticvariation may be pos- (Turesson 1922a, 1922b; Clausen et al. 1940; itivelyassociated. If strongdirectional or stabi- Quinn 1978; Silander1985b). Phenotypicplas- lizingselection were pervasivein populations, ticity,the morphological and/or physiological re- thenphenotypic plasticity could shieldgenetic sponsesof a genotypeto spatialor temporalen- vironmentalheterogeneity, has been hypothe- variationfrom the effectsof selection,thereby maintaininggenetic variation (Gillespie and Tur- sizedto be an importantaspect of genetic differ- entiation elli 1989; Goldsteinand Holsinger1992). withinpopulations (Bradshaw 1965; DeKroon and Schieving(1990) have provided Sultan that 1987). Gause (1947) proposed phe- a frameworkfor characterizing clonal plantlife notypicplasticity could be an alternativemode histories.Most vegetatively spreading facultative of adaptationcompared to "genoadaptation"or intrapopulationalgenetic differentiation. This in- clonalshrubs are classified as conservativegrowth verserelationship between phenotypic plasticity plants.These are homologousto thecategory of "stress variationhas been on em- tolerators"(Grime 1979; Chapin 1980). and genetic rejected This categoryincludes arctic, boreal, and tem- piricalgrounds (Moran et al. 1981; Hume and peratepeatland bog dwarfshrubs, such as Vac- Cavers 1982; Scheinerand Goodnight1984; Woodand Degabriele1985; Schlichting and Lev- cinium macrocarpon,which tolerate a suite of stressessuch lownutrient avail- in environmental as 1986;Taylor and Aarssen1988; Counts1993) and low has ability,physiological drought, temper- and on thefindings that phenotypic plasticity atures.Conservative growers respond plastically a geneticbasis (Bradshaw 1965; Schlichting 1986; to increasednutrient availability, potentially re- Scheinerand Lyman 1991; Scheiner 1994). How- sultingin rapidsite filling by one or a fewgenets, ofa ever,some research supports the hypothesis althoughChapin (1980, 1987) arguesthat phe- trade-offbetween phenotypic plasticity and ge- notypicplasticity would not be an importantmode neticvariation (Cook and Johnson1968; Ped- of adaptationfor these plants. Guerilla growth ersen1968; Jain 1978; Wu and Jain 1978; Zangerl (LovettDoust 1981) wouldallow a singleclone to spatiallyexclude possible competitors from a 'Authorfor correspondence and reprints.Present address: site,thereby rendering a possible selectivead- Departmentof Biology,University of North Carolina, vantageto plasticgenets. This scenarioindicates Greensboro,North Carolina 27412-5001. thatin long-lived perennial plants, plasticity could Manuscriptreceived January 1995; revisedmanuscript re- indeedbe an importantmode of adaptationin ceivedAnril 1995. locationswith heterogeneous microsites. Theo- 687 688 INTERNATIONAL JOURNAL OF PLANT SCIENCES Central populations selectionand driftwould decrease the amount of geneticvariation. A facultativelysexual popula- tionmay turn exclusively clonal if fecundity de- creasesto zerobecause of a lackof available ger- minationsites (Eriksson 1992). If a population was exclusivelyclonal, one couldenvisage a sin- gleadaptively plastic clone (or severalclosely re- lated plasticclones) excludingnonplastic intra- specificcompetitors in such a setting.It would not be surprisingthat, if plasticityof growthis an importanttrait, one wouldobserve few genets NC in old isolatedclonal populations. For this study we used V macrocarpon,the commerciallyimportant cranberry that occurs naturallyin peatlandsin thenortheastern United o h rn arg al populations States,the Great Lakes region,and southeastern Canada, withdistributionally marginal popula- tionspocketed in the centraland southernAp- palachian mountainsand the North Carolina coastal plain (fig.1). These two typesof popu- lations(central and marginal)correspond gen- Fig.1 The distributionof Vacciniummacrocarpon showing erallywith glaciation (central) and unglaciation thegeographic location of accessions. (marginal)of the last Pleistocene maximum. There are geneticand ecologicaldifferences as well. Amongcentrally distributed populations there are retically,phenotypic plasticity may be advanta- highergene flow and largersuitable habitats than geous in spatiallyor temporallyheterogeneous in marginalpopulations (Ogle 1984). Marginal habitats,whereas nonplastic or canalized phe- populationsare smaller,have lowersexual re- notypesmay be moreadvantageous in stableor production,and are thoughtto be relictsof the homogeneoushabitats (Sultan 1987). Opportu- Pleistoceneice age(Wieder et al. 1981;Ogle 1984; nityfor site filling by fewclones would be con- Stewart1993b). Moreover,marginal cranberry tingentupon longperiods between colonization populationshave been shownto have lowerge- events(i.e., low immigration),competitive ex- neticvariation than central populations (Stewart clusion,and smallsite area. and Excoffier,in press).Specifically, there were Althoughsubstantial research has recentlyin- fewerdiscernible clones and less interclonalmo- volvedthe nature of phenotypic plasticity and its lecularvariation in marginalpopulations com- rolein plant fitness, very little work has been done paredto central populations. Southern clones were usingclonal plants (Silander 198 5b; deKroon and largerand presumably older, indicating decreased Schieving1990). As a subsetof plasticity studies, recruitmentwithin marginal sites. Also, southern investigationsof geographicpatterning of clonal marginalsites are relativelymore heterogeneous plantplasticity