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 (sigma(2)(P)) attributable to additive genetic variance (sigma(2)(A)), so low values of h(2) may be due to low sigma(2)(A) (potentially eroded by natural selection) or to high values of the other factors that contribute to sigma(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 sigma(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 sigma(2)(P) were significantly greater than sigma(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) sigma(2), and low values of sigma(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 sigma(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.

Automated summary

Seed size affects individual fitness in wild plant populations, and its evolution may be limited by low narrow-sense heritability (h(2)). Low h(2) values may be due to both low additive genetic variance (sigma(2)(A)) eroded by natural selection and high values of other components (m(2) and e(2)) contributing to total phenotypic variance (sigma(2)(P)). This study reviewed literature and conducted a meta-analysis to investigate the h(2) of seed size in wild populations and the contribution of different variance components. Results showed that maternal and environmental components were greater than sigma(2)(A), dominance, paternal, and epistatic components, suggesting that low h(2) of seed size in wild populations is due to high maternal and environmental variance and low sigma(2)(A) values. The breeding design and the number of genotypes included in the study also influenced the estimates of h(2), m(2), and e(2).