ORIGINAL PAPER Flower colour variation across a hybrid zone in Antirrhinum as perceived by bumblebee pollinators Emmanuelle Tastard ® Christophe Andalo ® Martin Giurfa ® Monique Burrus ® Christophe The ´baud Received: 20 May 2008 / Accepted: 8 September 2008 Ó Springer Science+Business Media B.V. 2008 Abstract To assess if pollinators’ behaviour could explain the maintenance of hybrid zones between different flower colour morphs, we analyzed flower colour variation in an Antirrhinum hybrid zone using spectrometry and a model of bee perception. Some colours generated by hybridization were not observed in any Antirrhinum spe- cies and even appeared to be rare among angiosperms. Variation in flower colours within the hybrid zone was continuous; the most similar colours were predicted not to be discriminated from one another in natural foraging sit- uations. However, when compared at a scale corresponding to bees’ foraging range, some flower colours could be discriminated from all colours displayed by neighbouring plants. This could affect pollinator behaviour and explain lower visitation rates within the centre of the hybrid zone. Behavioural studies involving bumblebees and plant mix- tures of parental and hybrid flower colours carefully characterized with appropriate visual models will be nec- essary to test this hypothesis. Keywords Hybrid zone Á Flower colour Á Bee visual model Á Pollination ecology Introduction Hybridization is a critical issue in evolution. It generates a wide variety of phenotypes which may promote adaptation (Stebbins 1959; Arnold 1992). In most cases, however, related populations, when they come into contact after having evolved in separate geographical areas, co-occur in narrow and stable hybrid zones (Mayr 1963; Endler 1977). Several factors can contribute to the stability of such hybrid zones, including selection against hybrids, positive assor- tative mating within populations, and species segregation by habitat (Barton 1979; Barton and Hewitt 1985). In flowering plants, where many lineages are dependent on biotic vectors for pollen export and reception, pollinator behaviour is thought to contribute to maintaining hybrid zones through effect on mating patterns and individual reproductive success (e.g. Campbell et al. 1997; Emms and Arnold 2000). However, the mechanisms underlying this contribution remain poorly known. A number of authors (e.g. Mele ´ndez-Ackerman et al. 1997; Wesselingh and Arnold 2000; Campbell et al. 2002; Ippolito et al. 2004) have examined the behaviour of pollinators when confronted to both parental and hybrid phenotypes and found little evidence for behavioural mechanisms that could maintain the genetic differences between populations. However, their conclusions may have little bearing on the role played by pollinators in nature since they rely upon experiments in which a limited number of hybrid phenotypes were used. This contrasts with hybrid populations which usually display very diverse phenotypes and often contain phenotypes that exceed the Handling editor: Lars Chittka Electronic supplementary material The online version of this article (doi:10.1007/s11829-008-9046-3) contains supplementary material, which is available to authorized users. E. Tastard Á C. Andalo Á M. Burrus Á C. The ´baud (&) Laboratoire Evolution et Diversite ´ Biologique, UMR 5174, CNRS-Universite ´ Paul Sabatier, 31062 Toulouse cedex 9, France e-mail: thebaud@cict.fr M. Giurfa Centre de Recherches sur la Cognition Animale, UMR 5169, CNRS-Universite ´ Paul Sabatier, 31062 Toulouse cedex 9, France 123 Arthropod-Plant Interactions DOI 10.1007/s11829-008-9046-3