El impacto de las variaciones en la morfología floral sobre las visitas de polinizadores y el éxito reproductivo del arbusto Cytisus scoparius

Abstract

To know the effect of floral morphology on the visitation frequency of pollinators and their consequences on reproduction is key to understand the ecology and evolution of plant-pollinator interactions. We determined if variations in the area and symmetry of the floral banner affect the frequency of pollinator visits and seed production in the shrub Cytisus scoparius (L.) Link (broom). During the flowering period, we followed a large number of flowers that presented natural variation in the area and symmetry of their banners, and we carried out manipulative experiments in the field modifying these characteristics. Finally, we determined the visitation frequency and the seed production/fruit of each flower. While the symmetry of the banner showed a frequency distribution with a vast majority of cases centered in almost perfect symmetry, the banner area presented a flatter normal curve, with a high frequency of cases around the mean. The visited flowers along the sampled period showed more symmetrical banners, but with similar area than the unvisited flowers; the probability of visits were associated with the symmetry of the banner, but not with its area. The field experiment showed that flowers with a banner reduced to 50%, but maintaining their symmetry, were much more visited and produced more seeds/fruit than those with the banner reduced to 50% without maintaining its symmetry. These results suggest that, in Cytisus scoparius, the symmetry of the banner is a key aspect of floral morphology to attract pollinators. The low rate of visited flowers with highly asymmetric banners could be due to a reduction in floral recognition or because it is perceived by visitors as a flower damage, which may be interpreted as a sign of a low reward level. This work highlights the importance of symmetry as an attractive signal for pollinators, and illustrates how the variation of these traits within the same species can affect reproductive success, and therefore be subject to natural selection.