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Review
. 2018 Jun 6;19(6):1681.
doi: 10.3390/ijms19061681.

Morphological Complexity as a Floral Signal: From Perception by Insect Pollinators to Co-Evolutionary Implications

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Free PMC article
Review

Morphological Complexity as a Floral Signal: From Perception by Insect Pollinators to Co-Evolutionary Implications

Shivani Krishna et al. Int J Mol Sci. .
Free PMC article

Abstract

Morphologically complex flowers are characterized by bilateral symmetry, tube-like shapes, deep corolla tubes, fused petals, and/or poricidal anthers, all of which constrain the access of insect visitors to floral nectar and pollen rewards. Only a subset of potential pollinators, mainly large bees, learn to successfully forage on such flowers. Thus, complexity may comprise a morphological filter that restricts the range of visitors and thereby increases food intake for successful foragers. Such pollinator specialization, in turn, promotes flower constancy and reduces cross-species pollen transfer, providing fitness benefits to plants with complex flowers. Since visual signals associated with floral morphological complexity are generally honest (i.e., indicate food rewards), pollinators need to perceive and process them. Physiological studies show that bees detect distant flowers through long-wavelength sensitive photoreceptors. Bees effectively perceive complex shapes and learn the positions of contours based on their spatial frequencies. Complex flowers require long handling times by naive visitors, and become highly profitable only for experienced foragers. To explore possible pathways towards the evolution of floral complexity, we discuss cognitive mechanisms that potentially allow insects to persist on complex flowers despite low initial foraging gains, suggest experiments to test these mechanisms, and speculate on their adaptive value.

Keywords: associative learning; floral tube; perception; pollinator specialization; reward signal; symmetry.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Examples of complex morphologies of flowers. (a) Tecoma stans (Bignoniaceae); (b) Angaecum sesquipedale (Orchidaceae); (c) Impatiens balsamina (Balsaminaceae); (d) Lupinus pilosus (Fabaceae); (e) Ophrys alasiatica (Orchidaceae); (f) Salvia hierosolymitana (Lamiaceae); (g) Iris atropurpurea (Iridaceae); (h) Zingiber officinale (Zingiberaceae); (i) Calceolaria crenatiflora (Calceolariaceae); (j) Antirrhinum majus (Plantaginaceae). Photographers: (a) Calvin Finch; (b) Karole Schon; (d,i) Judith Marcus; (e) Michael Pettemerides; (f) Gideon Pisanty; (g) Ada Knossow; (h) Alastair Culham; (j) George Konstantinu.

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