Does the morphological fit between flowers and pollinators affect pollen deposition? An experimental test in a buzz-pollinated species with anther dimorphism.

Lislie Solís‐Montero,Mario Vallejo‐Marín

doi:10.1002/ece3.2897

Abstract

Some pollination systems, such as buzz‐pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the “fit” between flower and pollinator influences pollen transfer, particularly among buzz‐pollinated species. Here we use Solanum rostratum, a buzz‐pollinated plant with dimorphic anthers and mirror‐image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz‐pollinated species. We speculate that in buzz‐pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).

Highlights

In plants with hermaphroditic flowers, the relative position of male and female sexual organs within a flower can mediate patterns of pollen export and receipt (Barrett, 2002b)
We addressed two specific questions: (1) Is more pollen deposited on stigmas when the difference between the size of the pollinator and the separation of the floral sexual organs is at a minimum? (2) Is fruit and seed production greater when the pollinator size closely matches the separation of the sexual organs? We expected that pollinator which fit closely with the floral sexual organs will deposit more pollen to stigmas and, increase fruit and seed production
Instead of finding that pollen deposition was maximum when the flower and pollinator body matched best as we initially hypothesized, we found that pollen deposition in S. rostratum increased linearly with lower Size-matching index (SMI) values, i.e., when the visiting bee was larger than the separation of the sexual organs of the flower being visited

Summary

| INTRODUCTION

In plants with hermaphroditic flowers, the relative position of male and female sexual organs within a flower can mediate patterns of pollen export and receipt (Barrett, 2002b). Most previous studies in this area have focused on species providing nectar, oils, or scents as rewards (Armbruster & Muchhala, 2009; Kuriya et al, 2015; Nagano et al, 2014; Stang et al, 2009), and only a handful of studies have investigated size matching between pollinators and floral traits in pollen-only reward flowers (Bowers, 1975; Duncan, Nicotra, & Cunningham, 2004; Gao, Ren, Yang, & Li, 2006; Kawai & Kudo, 2009; Liu & Pemberton, 2009) Most of these studies mainly focus in describing whether floral visitors of different size make contact with the floral sexual organs when foraging for pollen (Bowers, 1975; Duncan et al, 2004; Gao et al, 2006; Liu & Pemberton, 2009; Solis-Montero, Vergara, & Vallejo-Marín, 2015). We addressed two specific questions: (1) Is more pollen deposited on stigmas when the difference between the size of the pollinator (i.e., the width of the part of the pollinator’s body that comes into contact with the sexual organs, in this case abdomen width) and the separation of the floral sexual organs is at a minimum? (2) Is fruit and seed production greater when the pollinator size closely matches the separation of the sexual organs? We expected that pollinator which fit closely with the floral sexual organs will deposit more pollen to stigmas and, increase fruit and seed production

| MATERIALS AND METHODS

Findings

| DISCUSSION

| CONCLUSIONS