What causes heterosis, the extra vigour of hybrid crops?

Hybrids of two inbred parents often grow larger and yield more than either parent, a phenomenon called heterosis that underpins hybrid maize and rice. Breeders exploit it widely, yet why it happens is still debated, and that limits how well it can be predicted or captured.

Question

What is the mechanism by which heterosis (hybrid vigour) forms in crops? Which genetic models (dominance, overdominance, epistasis) and molecular processes explain why F1 hybrids outperform their parents, how does this differ among crops and traits, and how could the answer help breed better hybrids or capture heterosis in self-pollinating crops?

Background

The question is in the theme "exploring resources and technologies for breeding" of the 100 essential questions for the future of agriculture, compiled from 419 questions submitted worldwide in 2022, among questions on mining genes, germplasm innovation and molecular design breeding [1].

What is known

• Sequencing and phenotyping 10,074 F2 lines from 17 hybrid rice crosses found no heterosis-associated loci shared by all lines; within each group of hybrids, a few loci from the female parents explained a large part of the yield advantage, partly through partial dominance [2].
• Elite maize inbreds carry more than 350,000 putatively deleterious variants; modelling supported incomplete dominance of these alleles as an important source of heterosis, and using them improved genomic prediction of yield and its heterosis [3].
• In determinate tomato, heterozygosity for a single mutation in the florigen gene SFT increased yield dramatically, a case of single-gene overdominance acting through dose-dependent fine-tuning of flowering and shoot architecture [4].
• In wheat and barley, the yield advantage of hybrids does not currently outweigh the cost of producing hybrid seed; one proposal is to engineer "supergenes" combining several heterotic alleles into inbred lines [5].
• Editing four meiosis and fertilisation genes in hybrid rice produced plants that propagate clonally through seed, a possible way to fix heterosis [6].

What would count as progress

No single result settles a question like this; contributions like these would move it forward:

• Large open datasets of hybrids and their parents, with genomes, transcriptomes and multi-environment yields, enabling direct tests of competing genetic models.
• Models that predict the performance of untested parent combinations, scored on new field trials.
• Experiments that recreate a measured share of heterosis in an inbred background by combining identified alleles, testing whether the proposed explanation is sufficient.

References

1. Hu, Y. et al. (2023). 100 essential questions for the future of agriculture. Modern Agriculture 1(1), 4–12.
2. Huang, X. et al. (2016). Genomic architecture of heterosis for yield traits in rice. Nature 537(7622), 629–633.
3. Yang, J. et al. (2017). Incomplete dominance of deleterious alleles contributes substantially to trait variation and heterosis in maize. PLOS Genetics 13(9), e1007019.
4. Jiang, K. et al. (2013). Tomato yield heterosis is triggered by a dosage sensitivity of the florigen pathway that fine-tunes shoot architecture. PLOS Genetics 9(12), e1004043.
5. Paril, J. et al. (2024). Heterosis in crop improvement. The Plant Journal 117(1), 23–32.
6. Wang, C. et al. (2019). Clonal seeds from hybrid rice by simultaneous genome engineering of meiosis and fertilization genes. Nature Biotechnology 37(3), 283–286.

Source

Imported from 100 essential questions for the future of agriculture, question 23, curated by Hu et al. Listed as open in the source (2023) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.