Pollen viability reduction as a potential cost of ant association for Acacia constricta (Fabaceae)

Field studies investigating the impact of ants on the reproduction of plants bearing extrafloral nectaries have traditionally focused on seed production, a component of female fitness. The purpose of this study was to test whether ants can affect the pollen viability, a component of male fitness, when they visit flowers of the shrub Acacia constricta. Acacia constricta inflorescences hand-pollinated with flowers over which Formica perpilosa ants had crawled set significantly fewer seed pods than inflorescences hand-pollinated by control flowers that had no contact with ants. Many ant species secrete antibiotic substances onto the integument that render pollen inviable, and these secretions are probably the mechanism for reduced pollen viability in this study. The ratio of seed pods produced by self-pollinated inflorescences to those produced by cross-pollinated inflorescences was 0.16, indicating that A. constricta is largely self-incompatible. Because F. perpilosa workers forage primarily on the acacia tree under which they nest, they are unlikely to serve as efficient vectors of outcrossing. Previous work showed that A. constricta shrubs with F. perpilosa ants produce approximately twice as many seeds as similarly sized plants not so associated. The results indicate that association with F. perpilosa could cause a reproductive trade-off for A. constricta: benefits to female function may be accompanied by costs to male function. Selection to discourage ant visitation to flowers may have affected the pollination biology of this and other ant-associated plant species.