Maternal quality, paternal effects, and sibling interactions influence seed size in the eelgrass, Zostera marina

Seed size is an essential determinant of germination and survival in angiosperms. Zostera marina, one of few marine angiosperms, is a key foundation species present in temperate coastal marine ecosystems, and edge-of-range population persistence is increasingly reliant on seed production and successful seedling establishment. While environmental conditions have been linked to variability in regional patterns of seed size variation, far less is known about what influences seed size differences within a plant. Here, we genotype and measure individual seeds across parent plants to investigate the relative contributions of maternal, paternal, and offspring traits on seed size. Maternal plants varied in the size and number of seeds produced, with more heterozygous mothers producing heavier seeds. Outcrossed seeds were heavier than selfed seeds, and seeds on inflorescences with low sibling relatedness were likewise heavier. We also provide some of the first empirical evidence that seed size in natural plant populations is strongly affected by paternity. Moreover, fathers with high siring success produced significantly smaller seeds, suggesting that investment in fertilization could be condition- or stage-dependent. As such, life-history and mating system shifts, resulting from environmental stressors, may result in changes in seed size and number, with important implications for population stability and productivity.