Gene Dispersal by Bumblebees between Two Lines of Faba Bean

Faba bean (Vicia faba L.) is pollinated by several apoid species which trip the flowers and cause either self or cross fertilization. Great variations in hybridization rate occur under field conditions. A better understanding of gene dispersal among faba bean is required to improve breeding programs. Our objective was to determine the contribution of foreign pollen to the fertilization of recipient flowers according to their position in the visitation sequence. Single workers of Bombus terrestris L. were used with caged faba bean. They foraged 10 flowers of a donor line (D-27) prior to those of a recipient line (D-23). Seventeen bumblebee workers visited 1261 flowers of D-23. The mean number of D-23 flowers visited per individual run was 74. The 17 runs induced 2812 seeds which were harvested and analyzed by starch-gel electrophoresis to estimate the cross-fertilization rates by allozyme markers. These rates were 21.3 and 17.5% for the first five and 10 flowers visited, respectively. The percentage of pods containing one, two, or three hybrid seeds was 78.0, 13.5, and 8.5%, respectively. These proportions were affected neither by the floral node level nor by the ovule position in ovary. After the first visits to the recipient flowers, the insect foraging rapidly loses its efficacy in terms of cross fertilization. Therefore, hybridization between lines requires a high frequency of alternate foraging which can be influenced by the breeding procedures, in particular by the spatial arrangement of parental lines.