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Variation in Sex Expression in Canada Yew (Taxus Canadensis) Author(S): Taber D

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Variation in Sex Expression in Canada Yew (Taxus Canadensis) Author(S): Taber D Variation in Sex Expression in Canada Yew (Taxus canadensis) Author(s): Taber D. Allison Source: American Journal of Botany, Vol. 78, No. 4 (Apr., 1991), pp. 569-578 Published by: Botanical Society of America Stable URL: http://www.jstor.org/stable/2445266 . Accessed: 23/08/2011 15:56 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Botanical Society of America is collaborating with JSTOR to digitize, preserve and extend access to American Journal of Botany. http://www.jstor.org AmericanJournal of Botany 78(4): 569-578. 1991. VARIATION IN SEX EXPRESSION IN CANADA YEW (TAXUS CANADENSIS)1 TABER D. ALLISON2 JamesFord Bell Museumof Natural History and Departmentof Ecology and BehavioralBiology, Universityof Minnesota, Minneapolis, Minnesota 55455 Sex expressionwas measuredin severalCanada yew (Taxus canadensisMarsh.) populations of theApostle Islands of Wisconsinand southeasternMinnesota to determinethe extent of variationwithin and among populations. Sex expression was recorded qualitatively (monoecious, male,or female) and quantitatively (by male to female strobilus ratios or standardized phenotypic gender).No discernibletrends in differencesin sex expressionamong populations or habitats wererecorded. Trends in sexexpression of individuals within populations were complex. Small yewstended to be maleor, if monoecious, had female-biasedstrobilus ratios. Large yews were monoeciousbut had male-biased strobilus ratios. Phenotypic gender, recorded as relativemale- ness,however, was negatively,but weakly, correlated with plant size. Genderdistribution in fourof five populations was bimodal, suggesting that cosexual populations consist of male and femalemorphs. Strobilus ratios of individuals in ApostleIsland populations showed significant annualvariation, but genderfor these same plantswas significantlycorrelated from year to year.Annual adjustments in genderwere most pronounced in smallyews. The resultsindicate thatrelative investment in maleand femalereproductive structures by Canada yew individuals is responsiveto environmentalvariation, but sex expressionalso has a proximategenetic component. Severalstudies have shownthat plants of McKone and Tonkyn,1986) in responseto cosexualspecies (containing individuals pro- environmentalvariation or changesin plant ducingboth pollen and ovules)vary widely in size orvigor. Such variations in genderamong theirrelative male and femalereproductive individualsare notnecessarily genetic. Alter- effort.Lloyd (1980) developedthe concept of natively,bimodal, or dimorphic,gender dis- standardizedphenotypic gender to providea tributionsalso have been recordedin angio- quantitativemeasure of this variation.Ac- spermswhere a cosexual plant population cordingto this measure, a plant'sgender ranges consistsof male and femalemorphs (Lloyd, from0.0 (female)to 1.0 (male)and is thecon- 1980).Female morphs, for example, may pro- tributionof male function (pollen) and female duceboth pollen and seeds,but they comprise function(ovules and seeds)to a plant'sfitness a discretegroup of the population that repro- relativeto othermembers of the population. ducesprimarily through seeds. The variationin genderamong individuals Dimorphicplant species are of special in- withina population,or genderdistribution, terest because they represent extremes in gen- suggestsmechanisms by which sex expression derspecialization. Lloyd (1980), forexample, in a plantpopulation is determined.For ex- has suggestedthat dimorphic or discontinuous ample, in manyplant speciesgender distri- genderdistributions indicate genetic differen- butionis unimodalor monomorphic;func- tiationwithin a populationreflecting incipient tionalgender may vary widely and continuously or stalledevolution of dioecy.Studying di- amongindividuals (Primack and Lloyd,1980; morphicplant species shouldtherefore illu- minate conditionsby which dioecy might 1 Receivedforpublication5 July 1990; revisionaccepted evolvefrom monoecy or vice versa. 28 December1990. To date,dimorphic gender has notbeen re- The authorthanks D. Thiede,S. Householder,and D. forany cosexual gymnosperm species, Blocksteinfor assistance in thefield; and P. Abrams,M. corded Davis, M. McKone,P. Morrow,P. Regal,A. Snow,and butthis could reflect the lack of application of anonymousreviewers for comments on themanuscript. Lloyd'sapproach to this taxonomic group. The Fundingwas providedby a CarolynCrosby Fellowship ratioof ovulate and staminatestrobili (referred fromthe Graduate School of the University of Minnesota, to hereafteras femaleand male strobili),how- The Dayton-WilkieNatural History Fund, the Minnesota ever, does vary widelywithin gymnosperm ZoologicalSociety, and SigmaXI. Denti, and 2 Currentaddress: Department of PlantBiology, The species (Sarvas, 1968; Schoen, OhioState University-Marion Campus, 1465 Mt. Vernon Stewart,1986), and these ratios vary according Avenue,Marion, OH 43302. to environmentaldifferences (Freeman et al., 569 570 AMERICAN JOURNAL OF BOTANY [Vol. 78 1981)and in responseto changesin plantsize temscan be tracedunder the litter layer, but and vigor (Matthews,1963; Smith, 1981; eventuallythese connections rot. In low-den- Whithamand Mopper,1985). In thispaper I sitypopulations, individual genets consisting describeannual and geographicvariation in of one or more branchesare readilydistin- sex expressionin Canada yew (Taxus cana- guished.In high-densitypopulations the ge- densisMarsh.), a shrubby,monoecious gym- neticrelatedness ofneighboring plants, or more nosperm.I recordedsex expressionin Canada appropriately,shoot system networks, is dif- yewqualitatively (e.g., male, female, and mon- ficultto determinedue to thislayering habit. oecious)as wellas quantitativelyby standard- In samplingreproductive effort in high-density ized phenotypicgender (Lloyd, 1980) to assess populations,I traced belowground connections the relativeusefulness of thesemeasures in to the rottedend. The sex expressionof an understandingsex expressionin Canada yew. individualgenet was estimatedby combining Speciesin the genus Taxus are principally the resultsfrom connected shoot systemsof dioecious,and the seeds are bird-dispersedthe plant. In high-densitypopulations, there- (Chamberlain,1966). Givnish(1980) hypoth- fore,a plantconstituted an unknownpropor- esizedthat dioecy in gymnospermswas linked tionof a genet. withbird-dispersal of seeds. Canada yew, how- Canada yewis a preferredwinter browse of ever,is monoecious,or cosexual,and is an white-taileddeer (Odocoileus virginianus Ra- exceptionto Givnish'shypothesis. Because of finesque)(e.g., Beals, Cottam, and Vogl, 1960). itsunique position in thegenus Taxus, it is of All dataon sexexpression in Canadayew were interestto examinevariation in sexexpression collectedfrom populations where current deer amongCanada yew individuals in orderto bet- browsingwas minimalor nonexistent.Brows- terunderstand the evolutionary and ecological ingby deersignificantly modifies sex expres- significanceof its matingsystem. sion in Canada yew(Allison, 1987). In addition,the studyof sex expressionin Canada yewis relevantto thedebate concern- ing the importanceof environmentalvs. ge- MATERIALS AND METHODS netic impactson gender.Taxus is cited as alteringsex expression in responseto environ- Reproductiveeffort in Canada yewwas es- mentalchanges (Freeman, Harper, and Char- timatedby countingthe number of male and nov, 1980). Carefulquantitative observations femalestrobili and seeds producedby indi- of the sex expressionof individualsover ex- vidual plants. This approach assumes that tendedperiods of timeis requiredto resolve variationamong individuals in thenumber of thisissue. pollengrains per strobilus or the mass of seeds Canada yewis a monoecious,or cosexual, is lowrelative to themean of these parameters evergreenshrub of the mixed conifer-hard-(e.g., Stanton,1984; Thompson,1984; Mc- woodforests of northeastern United States and Kone andTonkyn, 1986; McKone, 1989). The southeasternCanada (Martell, 1974). Male and size of male strobiliand seeds variedsignifi- femalestrobili are initiated during the summer cantlyamong individuals within populations, and typicallyreside singlyin leaf axils of butnot among the different populations sam- branchesproduced that year. The male stro- pled (Allison,1987). The rangeof variation bilusconsists of 5-14 peltatemicrosporophylls was small(C.V. = 13.3%for male strobiliand attachedto a centralaxis. Each microsporo- 11.1% forseed masses),however, indicating phyllcontains two to tenmicrosporangia (Du- thatcounts of male strobili,female strobili, pler,1 919). The femalestrobilus is uniovulate; and seedswere reasonable estimates for com- theovule is borneon a secondaryshoot of a parisonsofreproductive investment among in- shortprimary shoot (Dupler, 1920). After fer- dividualyews and yewpopulations. tilizationthe ovule develops into a stonyseed I sampledyew reproductive effort primarily thatis surroundedby a red, fleshy,aril-like at the Apostle Islands National Lakeshore, structure.Seed ripening in yew populations be- Wisconsin,USA (46?50'N latitude,90045'W ginsin lateJuly or earlyAugust and
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