Mating success in plants depends largely on the efficiency of pollen dispersal. For hermaphrodite plants, self-pollination, either within or among flowers, can reduce mating opportunities because of pollen and ovule discounting and inbreeding depression. Self-pollination may be particularly detrimental in plants such as orchids and asclepiads that package each flower's pollen into one or more pollinia which, together with accessory structures, comprise a pollinarium. Darwin proposed that physical reconfiguration of pollinaria serves as a mechanism for reducing the likelihood of self-pollination. To be effective, the time taken for pollinarium reconfiguration would need to exceed that spent by a pollinator on a plant. We investigated pollinarium reconfiguration (including pollinarium bending, pollinium shrinking and anther cap retention) in 19 species and found a strong positive relationship between reconfiguration time and the duration of pollinator visits. Reconfiguration times were also consistently longer than pollinator visit times. These results provide strong support for Darwin's idea that this mechanism promotes cross-pollination

Catling P.M

Craig I Peter

Dressler R.L

Hall A.V

Kullenberg B

Steven D Johnson

PubMed

Mating success in plants depends largely on the efficiency of pollen dispersal. For hermaphrodite plants, self-pollination, either within or among flowers, can reduce mating opportunities because of pollen and ovule discounting and inbreeding depression. Self-pollination may be particularly detrimental in plants such as orchids and asclepiads that package each flower’s pollen into one or more pollinia which, together with accessory structures, comprise a pollinarium. Darwin proposed that physical reconfiguration of pollinaria serves as a mechanism for reducing the likelihood of self-pollination. To be effective, the time taken for pollinarium reconfiguration would need to exceed that spent by a pollinator on a plant. We investigated pollinarium reconfiguration (including pollinarium bending, pollinium shrinking and anther cap retention) in 19 species and found a strong positive relationship between reconfiguration time and the duration of pollinator visits. Reconfiguration times were also consistently longer than pollinator visit times. These results provide strong support for Darwin’s idea that this mechanism promotes cross-pollination.Elsevie

Peter, Craig I

Johnson, Steven D

South East Academic Libraries System (SEALS)

Biol. Lett. (2006) 2, 65–68doi:10.1098/rsbl.2005.0385Published online 6 September 2005Doing the twist: a test ofDarwin’s cross-pollinationhypothesis for pollinariumreconfigurationCraig I. Peter* and Steven D. JohnsonSchool of Biological and Conservation Sciences, University ofKwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa*Author and present address for correspondence: Department of Botany,Rhodes University, PO Box 94, Grahamstown 6140, South Africa(c.peter@ru.ac.za).Mating success in plants depends largely on theefficiency of pollen dispersal. For hermaphro-dite plants, self-pollination, either within oramong flowers, can reduce mating opportunitiesbecause of pollen and ovule discounting andinbreeding depression. Self-pollination may beparticularly detrimental in plants such as orch-ids and asclepiads that package each flower’spollen into one or more pollinia which, togetherwith accessory structures, comprise a pollinar-ium. Darwin proposed that physical reconfi-guration of pollinaria serves as a mechanismfor reducing the likelihood of self-pollination.To be effective, the time taken for pollinariumreconfiguration would need to exceed that spentby a pollinator on a plant. We investigatedpollinarium reconfiguration (including pollinar-ium bending, pollinium shrinking and anthercap retention) in 19 species and found a strongpositive relationship between reconfigurationtime and the duration of pollinator visits.Reconfiguration times were also consistentlylonger than pollinator visit times. These resultsprovide strong support for Darwin’s idea thatthis mechanism promotes cross-pollination.Keywords: self-pollination; out-crossing; pollinia;pollination1. INTRODUCTIONReceivAccept.the movement of depression in the pollinia doesnot commence (as I know by trial) until the polliniaare fairly withdrawn out of their cells [anthers]; norwill the movement be completed, and the pollinia befitted to strike the stigmatic surfaces, until about halfa minute has elapsed, which will give ample time forthe moth to fly to another plant, and thus effect aunion between two distinct individuals(Darwin 1862, p. 31, commenting on the commonEuropean orchid Orchis [Anacamptis] pyramidalis.)Pollinator-mediated self-pollination can stronglydepress fitness in plants. Female fitness is mostobviously affected if it leads to inbreeding depression inprogeny (Charlesworth & Charlesworth 1987; Darwin1878; Keller & Waller 2002), but self-pollination canalso reduce the pool of pollen available for export toother plants and can thus also reduce male fitnessed 3 June 2005ed 12 August 200565through pollen discounting (Barrett 2002a; Herlihy &Eckert 2002). Many plants, regardless of their degreeof genetic self-incompatibility, possess physical mech-anisms for promoting cross-pollination (Barrett2002b). Well-documented mechanisms include dicho-gamy (differences in maturity of male and femaleorgans; Bertin & Newman 1993), herkogamy (thespatial separation of male and female organs (Barrett2002b), which includes stylar polymorphisms such asdi- and tristyly (Cesaro & Thompson 2004; Barrett &Harder 2005)); ‘flexistyly’ (Li et al. 2001) andenantiomorphy ( Jesson & Barrett 2002); rewardless-ness (Dressler 1981; Johnson & Nilsson 1999;Johnson et al. 2004); and unisexuality (Barrett 2002b).For orchids and asclepiads, which package theirpollen into pollinia, self-pollination is potentiallydisastrous for three reasons. First, self-deposition of anentire pollinium may eliminate most or all of theopportunity for a flower’s pollen to be exported( Johnson & Edwards 2000). Second, for self-compa-tible species, such as most orchids, ovules self-fertilized en masse are rendered unavailable forcross-fertilization (Barrett 2002a). Third, self-fertiliza-tion in orchids typically leads to rates of embryoabortion that are double those in seeds arising fromcross-fertilization (Tremblay et al. 2005).Pollinaria (comprising the pollen packets—the pol-linia—as well associated accessory structures) oftenreorient gradually after withdrawal from the anther(figure 1a,b). This is typically due to bending ortwisting of an accessory structure (such as a stipe orcaudicle) that connects the pollinium to a sticky pad(the viscidium) in orchids ( Johnson & Edwards 2000)or mechanical clamp (corpusculum) in asclepiads(Bookman 1981). These structures, in turn, attachthe pollinium to the body of the pollinator. In orchids,the pollinium is rotated through an arc of 30–1208depending on the particular species. This movement isnecessary for the pollinium to become orientatedcorrectly for insertion into a stigma (figure 1c). Inasclepiads the paired pollinia are initially flared at rightangles, but reconfigure to be closely appressed to oneanother in the correct position to be inserted into thestigmatic chamber (Bookman 1981). Darwin wasintrigued by this phenomenon and referred to it as a‘beautiful contrivance’ that would function to reduceself-pollination if the time taken for its completionexceeds the duration of a pollinator’s visit to a plant(Darwin 1862, p. 16). In the only previously pub-lished test of this idea, Johnson et al. (2004) con-firmed that self-pollination in the European orchidAnacamptis morio does not take place unless pollinatorvisits exceed the time taken for pollinaria to undergotheir bending movement. Other mechanisms of polli-narium reconfiguration may serve a similar function,including pollinia that shrink gradually to the correctsize to be inserted into the stigmatic cavity (Borba &Semir 1999), and anther-caps that cover the polli-naria for a period following the pollinarium’s removal(Catling & Catling 1991).The timing of pollinarium reconfiguration variesextensively in orchids and asclepiads. If this durationis adaptive, then two logical predictions fromDarwin’s hypothesis are: (i) that variation in theq 2005 The Royal SocietyFigure 1. Pollinarium bending—the most common form of pollinarium reconfiguration. (a) A pollinarium of the orchid Eulophiaparviflora freshly affixed to a cetoniid beetle will bend in the direction of the yellow arrow as indicated in (b) with the pollinariumhalf bent. (c) After ca 100 s the pollinarium has reconfigured and the paired pollinia can be inserted into the stigma (white arrow)as the beetle backs out of a flower in the direction of the white arrow (shown in cross-section). Scale bar, 5 mm.Table 1. Characteristics and phylogenetic relationships of the study species.66 C. I. Peter & S. D. Johnson Promotion of cross-pollination in orchidstiming of pollinarium reconfiguration reflects theduration of visits by a plant’s pollinators, and (ii) thatreconfiguration times of any particular species exceedthe average duration of pollinator visits to plants ofBiol. Lett. (2006)that species. Using data from 17 orchid and twoasclepiad species, we explore the strength of supportfor Darwin’s hypothesis that gradual reconfigurationof pollen after its removal from flowers serves as an4(a)Promotion of cross-pollination in orchids C. I. Peter & S. D. Johnson 67anti-selfing mechanism that is finely tuned to theduration of pollinator visits to plants.3log reconfiguration time (s)2slope = 0.97r2 = 0.79p <  0.001slope = 1.10r2 = 0.69p <  0.001101.51.00.5–0.5–0.5 0 0.5 1.0contrasts of visit (s)contrasts of reconfiguration (s)–1.0–1.0–1.5–1.500 1 2 3 4log visit time (s)(b)2. METHODSData on the timing of pollinarium reconfiguration and pollinatorvisits to plants of 19 species were obtained from the literature(9 species) and our own observations in Sweden and South Africa(10 species; details and references given in table 1). Bending timesof orchid pollinaria were recorded by withdrawing them from theanthers on the head of a pin and measuring their movement atsuitable intervals with a protractor. Angles were plotted againsttime to determine when the pollinaria had stopped moving, whichis unclear when pollinaria move slowly. The changing orientation ofasclepiad pollinia was recorded by removing them from antherswith a hooked tip of an insect pin and photographing them attimed intervals using a dissecting microscope. Angles weremeasured from the subsequent photographs. In orchids in whichpollinium insertion into stigmas is prevented by anther capretention, the time taken for the anther cap to dry and fall off apollinarium after it had been withdrawn was recorded. Theduration of pollinator visits to inflorescences was recorded in thefield with a portable voice recorder.The relationship between observed values of pollinator visittime and pollinarium reconfiguration time was analysed usingstandard major axis regression (Legendre 2001), while the relation-ship between standardized linear phylogenetically independentcontrasts (PICs) of these values was analyzed in the programPDTREE (Garland et al. 1992). The latter were based on aphylogeny of the 19 study species (table 1) obtained from existingphylogenetic trees (Cameron et al. 1999; Bateman et al. 2003)trimmed to include just the 19 species of interest. Branch lengthswere assigned according to Pagel’s arbitrary method (Pagel 1992).Relationships within Eulophia have not been fully resolved and arebased on an existing classification (Hall 1965). Changing theposition of the three Eulophia species in the phylogeny had noinfluence on the results using PICs. The variance homogeneity ofcontrasts was verified by examining the linear relationship betweenthe absolute value of the standardized contrasts and the sum of thesquares of branch lengths (Garland et al. 1992).Figure 2. (a) There is a positive relationship betweenpollinarium reconfiguration and pollinator visit times. Thedata points are above the dashed line of unity, indicatingthat pollinarium reconfiguration tends to take place afterthe end of a pollinator visit. (b) The positive relationshipbetween phylogentically independent contrasts of pollinar-ium reconfiguration and pollinator visit times.3. RESULTSThe average time taken for a species’ pollinaria toreconfigure varied positively with the average timethat pollinators spent visiting individual plants, bothwhen actual values and phylogenetically independentcontrasts are considered (figure 2a,b). Importantly,the duration of pollinarium reconfiguration almostalways (in 18/19 cases, two-tailed sign test, p!0.001)exceeded pollinator residency times (figure 2b). Polli-narium reconfiguration times varied from 22 to8100 s (meanZ952 s, medianZ88 s). On average,reconfiguration times were 1.58 times longer thanpollinator residency times.4. DISCUSSIONThe findings of this study provide strong empiricalsupport for the idea that promotion of cross-pollina-tion underlies the evolution of pollinarium reconfi-guration. As predicted by Darwin’s hypothesis,pollinarium reconfiguration times are both positivelyrelated to, and invariably exceed, pollinator visit times(figure 2a,b).Self-pollination could also be prevented effectivelyby very long pollinarium reconfiguration times; how-ever, reconfiguration times that greatly exceedpollinator residency times could be detrimental.In particular, cross-pollination opportunities coulddiminish if pollinia are lost quickly from pollinatorsand/or if pollinators leave a patch of conspecific plantsbefore reconfiguration is complete. Thus matingBiol. Lett. (2006)opportunities should be maximized if pollinariareconfigure shortly after pollinators have departedfrom the source plant, so that pollinia are ready forinsertion into stigmas of the next plant visited. Thepositive relationship between reconfiguration andpollinator visit times (figure 2a,b) is consistent withthis theoretical prediction.The evolutionary lability in the timing of pollinar-ium reconfiguration is evidenced from the variationobserved in this trait among two subspecies ofEulophia parviflora. One subspecies pollinated byslow-moving beetles has pollinaria that take anaverage of 100 seconds to reconfigure (figure 1),while the other subspecies pollinated by rapidlymoving bees has pollinaria that reconfigure in just28 s (t111Z17.53, p!0.0001). The mechanism(s)behind the bending movements of both orchid andasclepiad pollinaria remain largely undocumented,but preliminary work by the authors supports thesuggestion by Darwin (1862) that reconfiguration insome orchids involves differential drying of layers oftissue of the accessory structures attaching the polli-nia to the pollinators. Darwin noted that tissue of theviscidium and base of the stipe may be important in68 C. I. Peter & S. D. Johnson Promotion of cross-pollination in orchidscausing reconfiguration. In other orchids, tissue inthe middle of the stipe appears to be responsible forreconfiguration. In some asclepiads, differential dry-ing of tissue at the base of the translator arms maychange the orientation of the pollinia (C. I. Peter,unpublished data).Mechanisms that reduce the likelihood of self-pollination appear to be particularly prevalent inplant families in which pollen is aggregated as pollinia(cf. Johnson & Nilsson 1999; Harder & Johnson inpress). Reconfiguration mechanisms have evolved atleast four times ( pollinarium bending in orchids andasclepiads, anther cap retention and pollinium shrink-ing in orchids) and perhaps many more times in thesefamilies. In this study, for example, the mechanism ofbending in Eulophia and Acrolophia does not appearto be homologous with the mechanism found in theorchidoid species examined. While pollinarium recon-figuration is a mechanism confined to orchids andasclepiads, the remarkable evolutionary fine-tuning ofthis trait in response to pollinator visit times, asdemonstrated in this study, conveys a broader mess-age about the central role that pollinator behaviourplays in the evolution of plant traits that promotecross-pollination.We thank Barry Lovegrove for assistance with the softwareto calculate contrasts and Lawrence Harder and BruceAnderson for commenting on the manuscript. The workwas support by grants from the National Research Foun-dation (South Africa) and Rhodes University.Ayasse, M., Schiestl, F. P., Paulus, H. F., Lofstedt, C.,Hansson, B., Ibarra, F. & Francke, W. 2000 Evolution ofreproductive strategies in the sexually deceptive orchidOphrys sphegodes: how does flower-specific variation ofodor signals influence reproductive success? Evolution 54,1995–2006.Barrett, S. C. H. 2002a Sexual interference of the floralkind. Heredity 88, 154–159. (doi:10.1038/sj.hdy.6800020)Barrett, S. C. H. 2002b The evolution of plant sexualdiversity. Nat. Rev. Genet. 3, 274–284. (doi:10.1038/nrg776)Barrett, S. C. H. & Harder, L. D. 2005 The evolution ofpolymorphic sexual systems in daffodils (Narcissus). NewPhytol. 165, 45–53. (doi:10.1111/j.1469-8137.2004.01183.x)Bateman, R. M., Hollingsworth, P. M., Preston, J., Yi-Bo, L., Pridgeon, A. M. & Chase, M. W. 2003Molecular phylogenetics and evolution of Orchidinaeand selected Habenariinae (Orchidaceae). Bot.J. Linnean Soc. 142, 1–40. (doi:10.1046/j.1095-8339.2003.00157.x)Bertin, R. I. & Newman, C. M. 1993 Dichogamy inangiosperms. Bot. Rev. 59, 112–152.Bookman, S. S. 1981 The floral morphology of Asclepiasspeciosa (Asclepiadaceae) in relation to pollination and aclarification in terminology for the genus. Am. J. Bot. 68,675–679.Borba, E. L. & Semir, J. 1999 Temporal variation inpollinarium size after its removal in species of Bulbo-phyllum: a different mechanism preventing self-pollina-tion in Orchidaceae. Plant Syst. Evol. 217, 197–204.(doi:10.1007/BF00984365)Cameron, K. M., Chase, M. W., Whitten, W. M., Kores,P. J., Jarrell, D. C., Albert, V. A., Yukawa, T., Hills,Biol. Lett. (2006)H. G. & Goldman, D. H. 1999 A phylogenetic analysisof the Orchidaceae: evidence form RBCL nucleotidesequences. Am. J. Bot. 86, 208–224.Catling, P. M. & Catling, V. R. 1991 Anther-cap retentionin Tipularia discolor. Lindleyana 6, 113–116.Cesaro, A. C. & Thompson, J. D. 2004 Darwin’s cross-promotion hypothesis and the evolution of stylar poly-morphism. Ecol. Lett. 7, 1209–1215. (doi:10.1111/j.1461-0248.2004.00683.x)Charlesworth, D. & Charlesworth, B. 1987 Inbreedingdepression and its evolutionary consequences. Ann. Rev.Ecol. Syst. 18, 237–268. (doi:10.1146/annurev.es.18.110187.001321)Cole, F. R. & Firmage, D. H. 1984 The floral ecology ofPlatanthera blephariglottis. Am. J. Bot. 71, 700–710.Darwin, C. 1862 On the various contrivances by which Britishand foreign orchids are fertilised by insects and on the goodeffects of intercrossing, 1st edn. London: John Murray.Darwin, C. 1878 The effects of cross- and self-fertilization inthe vegetable Kingdom 2nd edn. London: John Murray.Dressler, R. L. 1981 The Orchids: natural history andclassification. Cambridge, MA: Harvard University Press.Garland, T., Harvey, P. H. & Ives, A. R. 1992 Proceduresfor anlaysis of comparative data using phylogeneticallyindependent constrasts. Syst. Biol. 41, 18–32.Hall, A. V. 1965 Studies of the South African species ofEulophia. J. S. Afr. Bot. Supplementary volume No. V.Harder, L. D. & Johnson, S. D. In press. Adaptive plasticityof floral display size in animal-pollinated plants. Proc. R.Soc. B. (doi:10.1098/rspb.2005.3268.)Herlihy, C. R. & Eckert, C. G. 2002 Genetic cost ofreproductive assurance in a self-fertilizing plant. Nature416, 320–323. (doi:10.1038/416320a)Jesson, L. K. & Barrett, S. C. H. 2002 Solving the puzzle ofmirror-image flowers. Nature 417, 707. (doi:10.1038/417707a)Johnson, S. D. & Edwards, T. J. 2000 The structure andfunction of orchid pollinia. Plant Syst. Evol. 222,243–269. (doi:10.1007/BF00984105)Johnson, S. D. & Nilsson, L. A. 1999 Pollen carryover,geitonogamy and the evolution of deceptive pollinationsystems in orchids. Ecology 80, 2607–2619.Johnson, S. D., Peter, C. I. & Agren, J. 2004 Theeffects of nectar addition on pollen removal andgeitonogamy in the non-rewarding orchid Anacamptismorio. Proc. R. Soc. B 271, 803–809. (doi:10.1098/rspb.2003.2659)Keller, L. F. & Waller, D. M. 2002 Inbreeding effects inwild populations. Trends Ecol. Evol. 17, 230–241.(doi:10.1016/S0169-5347(02)02489-8)Kullenberg, B. 1961 Studies in Ophrys pollination. Zool.Bidr. Uppsala 34, 57.Legendre, P. 2001 Model II regression—user’s guide.De´partement de sciences biologiques, Universite´ deMontre´al. Available at http://www.fas.umontreal.ca/biol/legendre/Li, Q. J., Xu, Z. F., Kress, W. J., Xia, Y. M., Zhang, L.,Deng, X. B., Gao, J. Y. & Bai, Z. L. 2001 Pollination:flexible style that encourages outcrossing. Nature 410,432. (doi:10.1038/35068635)Pagel, M. D. 1992 A method for the analysis of comparativedata. J. Theor. Biol. 156, 431–442.Queller, D. C. 1985 Proximate and ultimate causes of lowfruit production in Asclepias exalta. Oikos 441, 373–381.Tremblay, R. L., Ackerman, J. D., Zimmerman, J. K. &Calvo, R. N. 2005 Variation in sexual reproduction inorchids and its evolutionary consequences: a spasmodicjourney to diversification. Biol. J. Linnean Soc. 84, 1–54.(doi:10.1111/j.1095-8312.2004.00400.x)

Doing the twist: a test of Darwin's cross-pollination hypothesis for pollinarium reconfiguration

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Doing the twist: a test of Darwin's cross-pollination hypothesis for pollinarium reconfiguration

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