A molecular genetic assessment of mating-system variation in a naturally bird-pollinated shrub: Contributions from birds and introduced honeybees

Introductions of European honeybees have occurred globally, potentially affecting the natural pollination ecology of many plants, Introduced honeybees are now the most frequent visitors to the inflorescences of the self-compatible, bird-pollinated Australian shrub Grevillea macleayana and may therefore be expected to have altered the mating system. To examine the degree to which birds continue to play a role in determining the mating system of G. macleayana in this disturbed system, we compared outcrossing rates in open-pollinated inflorescences with inflorescences from which birds bad been selectively excluded. Outcrossing rates were estimated from the microsatellite genotypes of over 100 seeds per population in three populations, Outcrossing rates (t) in open-pollinated seeds were surprisingly low (0.062-0.225) and did not vary significantly among the three populations, Nevertheless, outcrossing was significantly lower when birds were excluded (data pooled from all populations). Two lines of evidence suggest that there are temporal fluctuations in outcrossing rate and hence that birds usually have a major effect on the mating system of G. macleayana. First at one site, t was substantially lower than estimates from an earlier study (0.06 in 1995 cf. 0.85 in 1990). Second, fixation indices based on seeds were high in all populations (>0.68), whereas values for the established plants (parental generation) were much lower in two of the three populations (0.06-0.32). Our findings suggest that honeybee activity is so high that the contribution of birds to pollination in G. macleayana is sometimes relatively trivial.