Seed size variation impacts local adaptation and life-history strategies in a perennial grass

Seed mass is an ecologically important trait that often differs considerably among ecotypes. Yet, because few studies examine the impacts of seed mass on adult life-history traits, its role in local adaptation is unclear. In this study, using accessions of Panicum hallii that spanned the two major ecotypes, we examined whether covariation between seed mass, seedling and reproductive traits impacts ecotypic divergence and local adaptation. The perennial grass P. hallii has two distinct ecotypes-a large-seeded upland ecotype adapted to xeric environments and a small-seeded lowland ecotype adapted to mesic environments. In the greenhouse, seed mass varied greatly across P. hallii genotypes in a manner consistent with ecotypic divergence. Seed mass covaried significantly with several seedling and reproductive traits. At field sites representing the habitats of the two ecotypes, seed mass had different impacts on seedling and adult recruitment: selection favoured large seeds in upland habitat and small seeds in lowland habitat, which was consistent with local adaptation. By demonstrating the central role of seed mass in ecotypic differences in P. hallii and its importance to seedling and adult recruitment under field conditions, these studies show that early life-history traits can promote local adaptation and potentially explain ecotype formation.

Automated summary

Seed mass is an important ecological trait that varies among different ecotypes. However, the role of seed mass in local adaptation is not clear due to limited studies on its impacts on adult life-history traits. This study examined the relationship between seed mass, seedling and reproductive traits in Panicum hallii, which has two distinct ecotypes adapted to different habitats. The results showed that seed mass differed significantly among P. hallii genotypes and was correlated with various seedling and reproductive traits. Field experiments demonstrated that selection favored large seeds in upland habitat and small seeds in lowland habitat, supporting the concept of local adaptation. These findings highlight the central role of seed mass in ecotypic differences and its importance in seedling and adult recruitment, providing insights into ecotype formation.