Dominance hypothesis is far from sufficient to explain heterosis

Heterosis in plants refers to the superiority of F1 hybrid over its inbred parents. Although heterosis has contributed to reducing hunger and malnutrition over the past 100 years, its genetic basis remains a matter of conjecture. The dominance hypothesis has mostly held sway. However, it is unclear whether the dominance hypothesis is sufficient to explain heterosis because the complexities of quantitative traits cause the comparison between heterosis and the contribution from dominance to be difficult or impossible in practice. Here we perform a combined theoretical and experimental study enabling the comparison. With the consideration that dominance alone contributes to heterosis, we derive the expression for calculating the ratio of heterosis to midparent value, and determine the maximum ratio. Using the maximum ratio as a standard of comparison, we demonstrate that the contribution from dominance is much lower than heterosis for yield exhibited by the elite maize hybrids and their parents. We conclude that the dominance hypothesis is far from sufficient to explain heterosis. Our results imply that genomic prediction for yields of hybrids would be a considerable challenge facing crop scientists, and the advantage of elite hybrids over concurrently developed elite inbreds would always be maintained by breeding efforts. Our study provides new insights into the role of dominance in heterosis, and is anticipated to pave the way for expanding our understanding of heterosis.