UC Santa Barbara

I. Seed size is a critical factor in determining various aspects of a plant’s life cycle, such as germination rate, seedling size, and survival. The sources of variation in the trait are essential to understand its adaptive potential to changing environments. This study evaluates the genetic and environmental influences on seed size variation within and among four wild populations of N. menziesii, a widespread California annual herb. By leveraging a greenhouse breeding experiment with a nested half-sibling mating design, this study examines the magnitude of additive genetic variance and heritability of seed size within populations, the concordance between different heritability estimation methods, and the contribution of the maternal and paternal parents to seed size. Mean seed size differed significantly among populations, regardless of growing location, while the magnitude of plasticity in response to environmental conditions was similar among populations. The study revealed a disparity between the two methods used to estimate heritability – parent-offspring regression and the nested half-sibling analysis. We found evidence for maternal variance in seed size in all four populations, but significant additive genetic variance was only present in two populations.

II. Seed size is also a critical trait that affects life-history attributes, reproductive output, and overall fitness of adult plants. Despite the competitive advantages of larger seeds, seed size variation persists within and among plant populations due to additive genetic effects, non-additive genetic effects, environmental influences, and differential seed provisioning by the maternal sporophyte. This study investigates the factors contributing to seed size variation and its fitness consequences in the California-native annual species, Nemophila menziesii, across multiple conspecific populations that were reintroduced to their home environments following a one-generation greenhouse breeding experiment. Seeds produced in greenhouse conditions were generally larger than those produced in the field, and environmental factors were significant determinants of offspring seed size across populations. Parental genotype and phenotype effects on seed size were population-specific, with maternal influences tending to exceed paternal effects. The impact of maternal seed size on reproductive traits and fitness was also population-specific, with larger maternal seed size associated with increased fruit production in three of the four populations, and increased fecundity and reproductive yield in two of the four populations. Tradeoffs between seed mass and the number of seeds produced per fruit by a reproductive individual were detected in two of the four populations.