Inter-parent distance affects reproductive success in two clonal dwarf shrubs, Vaccinium myrtillus and Vaccinium vitis-idaea (Ericaceae)

Clonal propagation and limited dispersal of seeds lead to genetic population structures in which most potential mates are relatives. If the species suffers from inbreeding depression or is self-incompatible, the number of seeds matured per fruit may be limited by the lack of outcrossing by unrelated pollen. We tested for distance-related genetic structure by hand-pollinating plants at increasing distances and measuring fruit set (berries/pollinated flowers) and seed number per fruit in natural populations of bilberry (Vaccinium myrtillus L.) and lingonberry (Vaccinium vitis-idaea L.). Bagging of the flower buds without natural (pollination by insects) or hand-pollination yielded almost no fruit or seed, suggesting that the species are obligately insect pollinated. Self-pollination led to a significant reduction in seed number per fruit and increased seed abortion compared with cross-pollination. Moreover, seed number increased with increasing distance between the pollen donor and pollen recipient. Flight distances of bumblebees were estimated for bilberry by monitoring between-flower and, for lingonberry, between-inflorescence flight distances. About 90% of all flights were at distances of less than 1 m. At this distance, we observed less than the maximum seed number per fruit in hand-pollinations in both study species. Consequently, clonal growth is likely to be an important factor that constrains fruit and seed number in these species.