Abstract
Bilateral symmetry has evolved from radial symmetry in several floral lineages, and multiple hypotheses have been proposed to account for the success of this floral plan. One of these hypotheses posits that bilateral symmetry (or, more generally, a reduced number of planes of floral symmetry) allows for more precise pollen placement on pollinators. Greater precision would maximize the efficacy of pollen transfer to conspecifics, while minimizing reproductive interference amongst plant species. Despite the intuitiveness of this hypothesis, it has little experimental support. Here, we tested whether a reduction in the number of floral planes of symmetry (as in the transition from radial to bilateral symmetry) increases the potential precision of pollen placement. We analyzed video recordings of bumblebees (Bombus impatiens) visiting artificial flowers to determine whether consistency in flower entry angle differed between radial (round) and disymmetric (rectangular) “flowers”. We observed more consistent entry angles for disymmetric flowers than for radial flowers, with entry angles to radial flowers 43% more variable on average (standard deviations of 30° vs. 21°). Bees trained on flowers with an intermediate (square) morphology exhibited a slight, non-significant preference for radial symmetry over disymmetry. Our results show that disymmetry—an evolutionarily intermediate form of floral symmetry—has the potential to increase pollen transfer to conspecific stigmas, relative to radial symmetry. Thus, evolutionary reduction in the number of planes of floral symmetry likely provides benefits in terms of pollen delivery, as suggested by the pollen-placement-accuracy hypothesis. These findings offer insight into the evolution of floral symmetry.
Highlights
Bilateral floral symmetry has evolved from the ancestral state of radial symmetry multiple times across the angiosperms (Donoghue et al 1998; Damerval & Nadot 2007; Hileman 2014; reviewed by Citerne et al 2010)
Observed flower-entry angles ranged from −140° to 159° for radially symmetric flowers and from −60° to 169° for disymmetric flowers
Entry angles were more consistent for disymmetric flowers than radial flowers for 15 of the 16 bees tested (Fig. 3)
Summary
Bilateral floral symmetry (zygomorphy) has evolved from the ancestral state of radial symmetry (actinomorphy) multiple times across the angiosperms (Donoghue et al 1998; Damerval & Nadot 2007; Hileman 2014; reviewed by Citerne et al 2010). While greater pollinator fidelity towards bilateral flowers has been observed (Fenster et al 2004), evidence for floral-symmetry preference is mixed (e.g., West & Laverty 1998), with previous studies finding pollinator preferences for both radial (Free 1970; Wignall et al 2006) and bilateral (Leppik 1953; Rodríguez et al 2004; Lázaro et al 2008) floral symmetry Another hypothesis, the pollen-placement-accuracy (or pollen position) hypothesis, states that flowers with fewer planes of symmetry promote more consistent entry by pollinators into the flower, such that the plant’s reproductive parts—both male and female—make more consistent contact with certain parts of pollinators’ bodies (Leppik 1972; Macior 1974). We tested for a symmetry preference in our bees, as a preference for radial symmetry
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
