Variation in outcrossing levels in faba bean cultivars: role of ecological factors

Abstract

Understanding the role of pollinators in determining the outcrossing rate is very important for the development of synthetic varieties of faba bean. Isozyme markers have been used to study the multilocus outcrossing rate in five cultivars under field conditions and in two locations. The study was carried out at Córdoba (Spain) and at Rennes (France). These two locations were chosen because previously studies showed strong differences in cross-pollination. Multilocus outcrossing rates at both locations, Córdoba and Rennes, were 0·65 and 0·33 respectively. Local differences in pollinator abundance and foraging activity were 26 and 32 times higher, respectively, at Córdoba than at Rennes which could explain an outcrossing rate twice at Córdoba compared with Rennes. So, pollinators may play a prominent role in determining the differences in outcrossing between locations. However, at a local level, there is a highly significant positive correlation between foraging activity and outcrossing only at Rennes (where the number of pollinators is scarce) but not at Córdoba (where pollinators are abundant). We suggest that although pollinators have a major impact on the level of outcrossing, in circumstances where there are a lot of pollinators the differences in outcrossing between cultivars depends on other factors among which is the ability of the genotype itself to outcross. No type of floral display seems to be clearly important in the determination of the outcrossing level in such a case. The implication of our results for faba bean breeding strategies is discussed.