Maternal provisioning, sibling rivalry and seed mass variability in the dioecious shrub Rhamnus alpinus

In this paper, we study the sources of variability in seed provisioning in Rhamnus alpinus, a dioecious shrub producing three-seeded fleshy fruits (range 1-4). Discarding position effects in access to resources as a source of variation, since pyrenes are radially oriented inside the fruit, we conducted a pollination experiment, in which different pollen donors were used to make single and mixed pollinations on different female plants. We found strong maternal effects on brood size and seed mass. Although we found no effects of pollen load or pollen source on fruit set, brood size or seed mass, all variables showed enhanced values after multiple pollination treatments. Furthermore, we found an increase of intra-brood variance in seed mass after multiple pollination, suggesting that multiple siring really occurs when multiple donors were involved in pollination, and that sibling rivalry could be a mechanism implicated in seed provisioning, discarding maternal choice as the alternative scenario. Our results on resource provisioning fits with a hierarchical model, since disparities in shares increased down the dominance rank, being the middle sized seed the least sensitive to resource partitioning, and the disparities were reduced when more resources were available in terms of fruit mass. Maternal effects, number of pollen donors, and sibling competition are therefore accounting for seed mass variability in R. alpinus.