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Plants have evolved diverse reproductive allocation strategies and seed traits to aid in dispersal, persistence in the seed bank, and establishment. In particular, seed size, dormancy, and early seedling vigor are thought to be key functional traits with important recruitment and fitness consequences across abiotic stress gradients. Selection for favored seed-trait combinations, or against maladaptive combinations, is likely an important driver shaping recruitment strategies. Here, we test for seed-trait plasticity and local adaptation in contrasting upland and lowland ecotypes of Panicum hallii with field experiments in native versus foreign habitats. Furthermore, we test whether seed traits have been under directional selection in P. hallii using the v-test (Fraser 2020) based on trait variance in a genetic cross. Finally, we evaluate the genetic architecture of ecotypic divergence for these traits with Quantitative Trait Locus (QTL) mapping. Field experiments reveal little plasticity but support a hypothesis of local adaptation among ecotypes based on recruitment. Patterns of segregation within recombinant hybrids provides strong support for directional selection driving ecotypic divergence in seeds traits. Genetic mapping revealed a polygenic architecture with evidence of genetic correlation between seed mass, dormancy, and seedling vigor. Our results suggest that the evolution of these traits may involve constraints that affect the direction of adaptive divergence. For example, seed size and germination percentage shared two colocalized QTL with antagonistic additive effects. This supports the hypothesis of a functional genetic relationship between these traits, resulting in either large seed/strong dormancy or small seed/weak dormancy trait combinations. Overall, our study provides insights into the factors facilitating and potentially constraining ecotypic differentiation in seed traits.Impact SummarySeed size and dormancy are key functional traits with important recruitment and fitness consequences. Theory suggests tradeoffs and plasticity in offspring quantity and quality are important in seed evolution. The genetics of seed size and dormancy traits have been studied extensively, but these studies are mostly limited to model system or domesticated crops. We also know very little about the genetic architecture and evolution of seed-based life history traits especially considering adaptation to xeric and mesic habitats. Here, we explored the genetic basis of trade-offs between seed size and dormancy in a C4 perennial grass, Panicum hallii, endemic to North America. We planted seeds of recombinant inbred lines from a cross between a xeric and mesic ecotype of P.hallii and mapped quantitative trait (QTL) loci for seed size, dormancy and seedling vigor traits. We detected a genetic basis of trade-offs between seed size and dormancy, suggesting that natural selection strongly favored specific trait combinations in ecotype formation. We further explored the role of seed size variation on seedling and adult recruitment in contrasting habitats. Our data showed that seed size was under strong selection through recruitment. Overall, our results demonstrate that adaptive differentiation for seed size and early life stages are important factors in adaptation to contrasting habitats.

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