Effects of floral symmetry on pollination in Bidens aristosa.
In flowering plants, floral symmetry is a reliable cue for high phenotypic and genotypic fitness: flowers that exhibit greater symmetry tend to be larger and generally produce more nectar than asymmetrical neighbors (Mqller and Eriksson, 1994; Wolfe and Krstolic, 1999; Frey et al., 2005). This is because plants must expend a large amount of energy to produce not only abundant nectar and large flowers but also symmetrical petals (Mqller and Eriksson, 1995), and only supremely "fit" plants can afford this cost (Mqller and Swaddle, 1997). In addition, studies have shown that some insects that pollinate flowers are capable of perceiving symmetry (Giurfa et al., 1996; Horridge, 1996). What is unclear, and thus the focus of my study, is whether or not pollinators show a preference toward flowers with greater symmetry.
[FIGURE 1 OMITTED]
Previous studies have tested pollinator preference for symmetrical flowers (Moller, 1995; Moller and Sorci, 1998; West and Laverty, 1998; Frey and Bukoski, 2014), but most take advantage of asymmetrical flowers found in nature. This situation poses an issue of control, as unmonitored asymmetrical flowers could have been damaged before the study, rendering their fitness lower than symmetrical flowers because of external factors. A pollinator preference for symmetry has been found in some studies but not in others, with the latter citing a lack of innate preference for symmetry in bumblebees (West and Laverty, 1998). In this study, I use experimental manipulations of natural flowers to investigate the effects of floral symmetry on pollination in Bidens aristosa (Asterales: Asteraceae), the western tickseed. This species is abundant throughout the central and southern United States (McGregor and Barkley, 1986; Turner et al., 2003). A typical tickseed flower has eight uniform petals (Britton, 1893) and is radially symmetrical (Fig. 1a), which makes it well suited for studies of symmetry. I examined whether symmetrical flowers attracted more pollinators than their counterparts that were manipulated to be asymmetric.
The study was conducted at Oxley Nature Center in Tulsa County, Oklahoma (36[degrees]13'24.9"N, 95[degrees]54'10.4"W) 20-27 September 2013 in a managed prairie containing a mixture of big bluestem (Andropogon gerardi), Indian grass (Sorghastrum nutans), goldenrod (Solidago species), and other species. I recorded visits to flowers of primarily beetles, bees, and moths, with the principal pollinator species being Chauliognathus pensylvanicus (Pennsylvania leatherwing beetle; Coleoptera: Cantharidae) and Bombus impatiens (common eastern bumble bee; Hymenoptera: Apidae).
Before-treatment observations paired two tickseed flowers from the same plant that were similar in size, height above the ground, and location on the plant. I arbitrarily designated one flower of each pair as a control and one as experimental. I utilized a matched-pairs design to control for natural shifts in temperature, weather, or pollinator activity (Lachin, 2011), so that any differences in the number of pollinators after manipulation would be due to the manipulation itself. I observed five flower pairs at a time. I initially observed each flower for 30 min and tabulated all pollinators that visited each flower.
For after-treatment observations, I manipulated the experimental flower in each pair so that it was asymmetrical: for some trials, I removed three or four petals from every experimental flower (petal removal; Fig. 1b); for others, I shortened several petals on each experimental flower (petal shortening; Fig. 1c); unmanipulated control flowers were left naturally symmetrical (Fig. 1a). Both manipulations yielded five symmetrical control flowers and five asymmetrical experimental flowers. I repeated the 30-min watch period and recorded all pollinators for each flower.
Because floral manipulation could cause possible interference with the chemical and optical properties as well as a reduced petal area (Neal et al., 1998), I addressed these possibilities with a control manipulation. I cut V-shaped notches into flower petals of the experimental flowers (symmetrical manipulation; Fig. 1d).
I performed analysis of variance tests comparing the number of pollinator visits with the control and experimental flowers before and after the manipulations were performed, and for postmanipulation I assessed pair-wise differences with the Scheffe test corrected for multiple comparisons. Proc GLM and Proc ANOVA in SAS were used for statistical tests (SAS Institute Inc., 2008).
Before manipulation, control and experimental flowers did not differ significantly in pollinator visits ([F.sub.3,143] = 0.13, P = 0.94, two-way analysis of variance) after controlling for date ([F.sub.3,143] = 7.09, P < 0.001). This indicates that controls did not differ from the experimental flowers in their attractiveness to pollinators before manipulation. After manipulation, the flowers that had petals removed or shortened were visited by significantly fewer pollinators than their unaltered counterparts (Fig. 2). Flowers that had been treated with the symmetrical manipulation showed no significant difference in pollinators relative to the unmanipulated control flowers (Fig. 2). These results indicate that insect pollinators showed a preference toward symmetrical tickseed flowers relative to more asymmetrical ones. Symmetric flowers are thus more efficient at attracting pollinators, suggesting that they are pollinated more often in a natural setting and may experience enhanced reproductive success over asymmetrical flowers (Moller, 1995) as a result of likely higher developmental stability and fitness (Moller and Pomiankowski, 1993).
[FIGURE 2 OMITTED]
Bumblebees utilize past learned associations when choosing flowers from which to collect nectar (Dawson et al., 2013), and the ability to apply learned information may be extended to many other species besides bees, including the other pollinator species in the experiment. In a similar experiment (Moller and Eriksson, 1995), pollinator symmetry preference was attributed to simple positive reinforcement: bumblebees may have learned through experience that symmetrical flowers yield reward, and the nectar continually reinforced the behavior of visiting symmetrical flowers. However, there is not a clear consensus on whether insect preference is due to positive reinforcement or inherent pollinator tendency toward symmetrical structures (Moller and Sorci, 1998), as pollinators show preference for symmetrical flowers of species that do not produce nectar (Moller and Eriksson, 1995). In either case, bees apparently actively choose the flowers they visit rather than simply visiting flowers randomly. Bee species, which alone comprised nearly 60% of all the insects in my experiment, clearly seemed to choose the symmetrical flowers over the inflorescences that had been made asymmetrical.
In similar analyses of symmetry and plant-pollinator systems (Neal et al., 1998), it has been suggested that the results might be affected by confounding variables associated with the manipulation: since the flower petals were cut or pulled off, the flowers may have exuded different chemical signals or optical properties due to being damaged. However, if this were the case, significantly fewer pollinators should have also been seen in the symmetrical manipulation. The lack of difference in pollinator visits in this manipulation showed that these confounding variables did not affect the results of the experiment.
I thank C. R. Brown for advice and comments on the manuscript. I also thank D. G. Ivanoff, T. E. Rogers, and A. M. Crouch for helping record pollinators. Mohawk Park allowed me access to the field site.
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Submitted 5 April 2015.
Acceptance recommended by Associate Editor, James E. Moore, 29 May 2015.
JOLEE G. POTTS
Department of Biological Sciences, University of Tulsa, Tulsa, OK 74104
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|Author:||Potts, Jolee G.|
|Date:||Dec 1, 2015|
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