Interspecific pollinator movements and heterospecific incompatibility: comparisons between Phyllodoce caerulea and Phyllodoce aleutica along snowmelt gradients. [Erratum: 2005 Mar., v. 7, no. 3, p. 514.]

Relationships between interspecific pollinator movements and heterospecific incompatibility were investigated in two co-flowering alpine-snowbed shrubs, Phyllodoce caerulea and Phyllodoce aleutica, among plots arranged along snowmelt gradients. Bumblebees, the predominant pollinator of these species, preferred P. caerulea in early- to middle-snowmelt plots but P. aleutica in late-snowmelt plots. This switching of preference reflected the changes in the relative floral densities of these species along the snowmelt gradients. The frequency of bumblebee movements from P. aleutica to P. caerulea flowers increased at late-snowmelt plots, whereas that from P. caerulea to P. aleutica flowers decreased at late-snowmelt plots. Thus, the risk of heterospecific pollen receipt increased in P. caerulea but decreased in P. aleutica from early- to late-snowmelt habitats. Phyllodoce caerulea showed similar seed-set rates among conspecific, heterospecific and species-mixed pollination treatments throughout the plots, indicating that there is no mechanism to avoid hybridization at P. aleutica pollen. On the other hand, P. aleutica pollinated with P. caerulea pollen showed a very low seed set in comparison with that pollinated with conspecific pollen in early- and middle-snowmelt plots. However, the seed set of P. aleutica did not differ among the treatments at late-snowmelt plots. Therefore, P. aleutica showed heterospecific incompatibility with P. caerulea pollen only in the early- to middle-snowmelt populations. Changes in interspecific bumblebee movements might be responsible for the variation in heterospecific incompatibility among the P. aleutica populations.