Prevalence of legitimate pollinators and nectar robbers and the consequences for fruit set in an Antirrhinum majus hybrid zone

ABSTRACT

Pollinators display a remarkable diversity of foraging strategies with flowering plants, from primarily mutualistic interactions to cheating through nectar robbery. Despite numerous studies on the effect of nectar robbing on components of plant fitness, its contribution to reproductive isolation is unclear. We experimentally tested the impact of different pollinator strategies in a natural hybrid zone between two subspecies of Antirrhinum majus with alternate flower colour guides. On either side of a steep cline in flower colour between Antirrhinum majus pseudomajus (magenta) and A. m. striatum (yellow), we quantified the behaviour of all floral visitors at different time points during the flowering season. Using long-run camera surveys, we quantify the impact of nectar robbing on the number of flowers visited per inflorescence and the flower probing time. We further experimentally tested the effect of nectar robbing on female reproductive success by manipulating the intensity of robbing. While robbing increased over time the number of legitimate visitors tended to decrease concomitantly. We found that the number of flowers pollinated on a focal inflorescence decreased with the number of prior robbing events. However, in the manipulative experiment, fruit set and fruit volume did not vary significantly between low robbing and control treatments. Our findings challenge the idea that robbers have a negative impact on plant fitness through female function. This study also adds to our understanding of the components of pollinator-mediated reproductive isolation and the maintenance of Antirrhinum hybrid zones.

KEYWORDS:

• Nectar robbing
• pollination
• bumblebee
• hybrid zone
• Antirrhinum

Acknowledgments

The authors thank Dominique Pelletier for identifying bumblebee species and Cristian Pasquaretta for reviewing an initial version of the manuscript. We are also very grateful to all the students and colleagues that helped us in collecting data: Mabon Elis, Maria Clara Melo Hurtado, Serena Quiros, Pauline Guillaumeau, Bastien Delseny and Auriol Djoumou Djoumou. We thank Maud Comb from the CNRS Research Centre on Animal Cognition (Toulouse) who has adapted her software AntEthoc to our requirements for video analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the French Laboratory of Excellence project  LABEX TULIP (ANR-10-LABX-41) and Toulouse Initiative of Excellence IDEX UNITI (ANR-11-IDEX-0002-02).

Notes on contributors

Christophe Andalo

Christophe Andalo is teacher-researcher at University Paul Sabatier where he conducts research on pollinator-mediated evolution of plant population and pollination network at the community scale. Contribution: conceived the study, performed field work and data analysis, co-wrote the paper.

Monique Burrus

Monique Burrus is teacher-researcher at University Paul Sabatier where she conducts research in plant population genetic and conservation. Contribution: conceived the study, performed field work, co-wrote the paper.

Sandrine Paute

Sandrine Paute is laboratory assistant at University Paul Sabatier where she manages practical work and museological material. Contribution: conceived the study, performed field work.

Christine Lauzeral

Christine Lauzeral is teacher-researcher at University Paul Sabatier where she conducts research on species distribution. More generally, she develops modelling approaches in ecology. Contribution: conceived the study, performed field work.

David L. Field

David L. Field is teacher-researcher at University of Vienna where he conducts research on plant evolutionary ecology, mating system evolution, population genetics and developing methods for analysing population genomic data. Contribution: conceived the study, performed field work, co-wrote the paper.