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Flower orientation enhances pollen transfer in bilaterally symmetrical flowers


Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.

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  1. Cruden RW (1977) Pollen–ovule ratio: a conservative indicator of breeding system in flowering plants. Evolution 31:32–46

  2. Darwin C (1862) On the various contrivances by which British and foreign orchids are fertilized. Murray, London

  3. Faden RB (1992) Floral attraction and floral hairs in the Commelinaceae. Ann Mo Bot Gard 79:46–52

  4. Fenster CB, Armbruster WS, Wilson P, Dudash MR, Thomson JD (2004) Pollination syndromes and floral specialization. Annu Rev Ecol Evol Syst 35:375–403

  5. Fulton M, Hodges SA (1999) Floral isolation between Aquilegia formosa and A. pubescens. Proc R Soc Lond B 266:2247–2252

  6. Grant V, Grant KA (1965) Flower pollination in the Phlox family. Columbia University Press, New York

  7. Harder LD, Barrett SCH (1993) Pollen removal from tristylous Pontederia cordata: effects of anther position and pollinator specialization. Ecology 74:1059–1072

  8. Harder LD, Wilson WG (1994) Floral evolution and male reproductive success: optimal dispensing schedules for pollen dispersal by animal-pollinated plants. Evol Ecol 8:542–559

  9. Harder LD, Wilson WG (1998) Theoretical consequence of heterogeneous transport conditions for pollen dispersal by animals. Ecology 79:2789–2807

  10. Hrycan WC, Davis AR (2005) Comparative structure and pollen production of the stamens and pollinator-deceptive staminodes of Commelina coelestis and C. dianthifolia (Commelinaceae). Ann Bot 95:1113–1130

  11. Huang S-Q, Takahashi Y, Dafni A (2002) Why does the flower stalk of Pulsatilla cernua (Ranunculaceae) bend during anthesis? Am J Bot 89:1599–1603

  12. Ishii HS (2004) Increase of male reproductive components with size in an animal-pollinated hermaphrodite, Narthecium asiaticum (Liliaceae). Funct Ecol 18:130–137

  13. Ishii HS, Sakai S (2002) Temporal variation in floral display size and individual floral sex allocation in racemes of Narthecium asiaticum (Liliaceae). Am J Bot 89:441–446

  14. Johnson SD, Steiner KE (1995) Long-proboscid fly pollination of two orchids in the Cape Drankensberg mountains, South Arica. Plant Syst Evol 195:169–175

  15. Johnson SD, Steiner KE (2000) Generalization and specialization in plant pollination systems. Trends Ecol Evol 15:140–143

  16. Kevan PG (1975) Sun-trucking solar furnaces in high arctic flowers: significance for pollination and insects. Science 189:723–726

  17. Kudo G (1995) Ecological significance of flower heliotropism in the spring ephemeral Adonis ramosa (Ranunculaceae). Oikos 72:14–20

  18. Morita T, Nigorikawa T (1999) Phenotypic plasticity of floral sex. In: Ohara M (ed) Natural history of flowers. Hokkaido University Press, Sapporo, pp 227–242 (in Japanese)

  19. Neal PR, Dafni A, Giurfa M (1998) Floral symmetry and its role in plant-pollinator systems: terminology, distribution, and hypotheses. Annu Rev Ecol Evol Syst 29:345–373

  20. Nilsson LA (1988) The evolution of flowers with deep corolla tubes. Nature 334:147–149

  21. Patino S, Jeffree C, Grace J (2002) The ecological role of orientation in tropical convolvulaceous flowers. Oecologia 130:373–379

  22. Robertson C (1928) Flowers and insects. Lists of visitors of four hundred and fifty-three flowers. Charles Robertson, Carlinville

  23. Sargent RD (2004) Floral symmetry affects speciation rates in angiosperms. Proc R Soc Lond B 271:603–608

  24. Stebbins GL (1970) Adaptive radiation of reproductive characteristics in angiosperms. Annu Rev Ecol Evol Syst 1:307–326

  25. Tadey M, Aizen MA (2001) Why do flowers of a hummingbird-pollinated mistletoe face down? Funct Ecol 15:782–790

  26. R Development Core Team (2005) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. ISBN 3-900051-07-0.

  27. Ushimaru A, Hyodo F (2005) Why do bilateral symmetrical flowers orient vertically? Flower orientation influences pollinator landing behavior. Evol Ecol Res 7:151–160

  28. Ushimaru A, Nakata K (2001) Evolution of flower allometry and its significance on pollination success in the deceptive orchid, Pogonia japonica. Int J Plant Sci 162:1307–1311

  29. Ushimaru A, Itagaki T, Ishii HS (2003a) Variation in floral organ size depends on function: a test with Commelina communis, an andromonoecious species. Evol Ecol Res 5:615–622

  30. Ushimaru A, Itagaki T, Ishii HS (2003b) Floral correlations in an andromonoecious species, Commelina communis (Commelinaceae). Plant Species Biol 18:103–106

  31. Ushimaru A, Kawase D, Imamura A (2006) Flowers adaptively face down-slope in ten forest-floor herbs. Funct Ecol 20:585–591

  32. Ushimaru A, Watanabe T, Nakata K (2007) Colored floral organs influence pollinator behavior and pollen transfer in Commelina communis. Am J Bot 94:249–258

  33. Vogel S (1978) Evolutionary shifts from reward to deception in pollen flowers. In: Richards AJ (ed) The pollination of flowers by insects. Academic Press, New York, pp 89–96

  34. West EL, Laverty TM (1998) Effects of floral symmetry on flower choice and foraging behavior of bumble bees. Can J Zool 76:730–739

  35. Wilson P (1995) Selection for pollination success and the mechanical fit of Impatiens flowers around bumble bee bodies. Biol J Linn Soc 55:355–383

  36. Wolfe LM, Krstolic JL (1999) Floral symmetry and Its Influence on variance in flower size. Am Nat 154:484–488

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We thank Christina Caruso, Paul Wilson and an anonymous reviewer for giving us many valuable comments and suggestions during the review process and Daiju Kawase for his assistance in pollen counting. This research was partly supported by a grant-in-aid for young scientists (No. 17770023) from the Japan society for the promotion of science. Our experiment complies with the laws of Japan, the country in which it was performed.

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Correspondence to Atushi Ushimaru.

Additional information

Communicated by Christina Caruso.

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Ushimaru, A., Dohzono, I., Takami, Y. et al. Flower orientation enhances pollen transfer in bilaterally symmetrical flowers. Oecologia 160, 667–674 (2009).

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  • Commelina communis
  • Pollinator specialization
  • Horizontal
  • Flower orientation
  • Zygomorphic flower