Direct and correlated responses to upward and downward selection for outcrossing in Vicia faba

Upward and downward artificial selection for outcrossing were performed in faba bean open-pollinated populations: (1) to characterize the mating systems of the populations resulting from upward/downward selection of allogamy; (2) to assess the magnitude and direction of differentiation in floral phenotypes among selected and control populations; and (3) to determine its relationship to experimentally imposed selection.Field experiments were conducted using two original synthetic populations derived from different genetic pools; the control populations (C) and two upward (S+), and two downward (S−) selected populations. In these populations, design and display of floral traits grouped into functional categories was studied. Outcrossing rates were estimated using codominant isozyme markers, and the mixed mating and the multilocus methods. The simultaneous responses of functional floral traits to selection for allogamy/autogamy were analyzed by using multivariate Discriminant Analysis.Whether selection was for increased or decreased outcrossing, the four selected groups shifted in the opposite direction of the selected type. Discriminant Analysis revealed how the patterns of increased or decreased outcrossing in the selected populations were due to a concordant multidimensional change in floral traits, thus limiting or contra-balancing the effects of artificial selection. Indirect selection does not act on traits individually, but on combinations of traits that are functionally integrated. Our results provide evidence that evolution towards self-fertilization has occurred by different genetic pathways after upward selection for outcrossing. This occurrence was by autogamous and by geitonogamous selfing.For the improvement of faba bean populations in the context of sustainable agriculture, direct selection on outcrossing cannot be a selection criterion. However, floral traits, in combination with pollinator behaviour, should be used as indirect selection criteria to increase allogamy. This approach, in turn, will allow the development of both pollinator-friendly varieties and enhance the environmental services of faba beans.