Heritability and variance components of seed size in wild species: influences of breeding design and the number of genotypes tested.

Seed size affects individual fitness in wild plant populations, but its ability to evolve may be limited by low narrow-sense heritability (h 2 ). h 2 is estimated as the proportion of total phenotypic variance (σ 2 P ) attributable to additive genetic variance (σ 2 A ), so low values of h 2 may be due to low σ 2 A (potentially eroded by natural selection) or to high values of the other factors that contribute to σ 2 P , such as extranuclear maternal effects (m 2 ) and environmental variance effects (e 2 ). Here, we reviewed the published literature and performed a meta-analysis to determine whether h 2 of seed size is routinely low in wild populations and, if so, which components of σ 2 P contribute most strongly to total phenotypic variance. We analyzed available estimates of narrow-sense heritability (h 2 ) of seed size, as well as the variance components contributing to these parameters. Maternal and environmental components of σ 2 P were significantly greater than σ 2 A , dominance, paternal, and epistatic components. These results suggest that low h 2 of seed size in wild populations (the mean value observed in this study was 0.13) is due to both high values of maternally derived and environmental (residual) σ 2 , and low values of σ 2 A in seed size. The type of breeding design used to estimate h 2 and m 2 also influenced their values, with studies using diallel designs generating lower variance ratios than nested and other designs. e 2 was not influenced by breeding design. For some breeding designs, the number of genotypes included in a study also influenced the resulting h 2 and e 2 estimates, but not m 2 . Our data support the view that a diallel design is better suited than the alternatives for the accurate estimation of σ 2 A in seed size due to its factorial design and the inclusion of reciprocal crosses, which allows the independent estimation of both additive and non-additive components of variance.