A 2017 genome-wide quantitative-genetic study of 1,604 wheat hybrids and their 135 elite parents concluded that epistatic effects, rather than dominance, dominate grain-yield midparent heterosis in bread wheat — but the analysis rested on biallelic SNP markers in elite Central European germplasm, leaving open whether the detected epistasis is genuine or an artifact of linkage to unobserved variants, and leaving higher-order epistasis, structural variation, and multiallelic variation unassessed. This problem asks for a faithful genetic decomposition of grain-yield heterosis in bread wheat: sequence-resolved disentangling of genuine from apparent epistasis, quantification of higher-order and structural/multiallelic contributions to heterosis, generalization of the dominance–epistasis balance across germplasm, and, on that basis, validated strategies for exploiting epistatic heterosis in hybrid wheat breeding. The question is open and actively contested: later wheat studies disagree about the dominance–epistasis balance; the only sequence-resolved wheat heterosis mapping study to date (Li et al. 2025) leaves the apparent-epistasis confound untested and does not partition heterosis by effect class; and no wheat study has assessed higher-order epistatic or structural/multiallelic contributions to heterosis.

Background

Heterosis (hybrid vigor) is the superior performance of an F1 hybrid relative to its parents; in crop breeding the usual measure is midparent heterosis (MPH), the excess of the hybrid over the average of its two inbred parents. Hybrid wheat could raise yields by roughly 10% — about 15 years of conventional breeding progress — but wheat is self-pollinating, so exploiting heterosis requires redesigning breeding around predicted single crosses. The classical genetic explanations for heterosis are dominance (the hybrid masks deleterious recessive alleles of each parent), overdominance (the heterozygote itself outperforms both homozygotes), and epistasis (interactions between alleles at different loci, e.g. additive-by-additive or dominance-by-dominance effects). A quantitative framework attributes observed heterosis to these effect classes by fitting quantitative trait loci (QTL, genomic regions statistically associated with a quantitative trait) in a population of hybrids and parents genotyped at SNPs (single-nucleotide polymorphisms, the most common biallelic marker type). Jiang, Schmidt, Zhao and Reif (2017) applied such a framework to 1,604 hybrids and 135 elite Central European winter-wheat parents evaluated for grain yield in 11 environments, and found dominance effects less prominent than epistatic effects — with the digenic (pairwise) epistatic components, especially additive-by-additive, carrying about half of the heterosis variance. The study itself flagged what its design could not decide. First, epistatic signals from biallelic SNP scans can be apparent epistasis: Wood et al. (2014) showed that multilocus marker combinations in linkage disequilibrium (LD, the non-random co-inheritance of nearby variants) with unobserved causal variants can mimic genuine gene-gene interactions; ruling this out requires sequence-resolved variant catalogs in the mapping population, which did not exist for the studied germplasm at the time. The parents of a larger integrated Central European hybrid-wheat panel — including exactly the 135 parents and 1,604 hybrids of the 2017 study — have since been whole-genome resequenced (Li et al. 2025), enabling sequence-based heterosis mapping, but that work does not test the genuine-versus-apparent distinction, does not partition heterosis variance by effect class, and does not assess structural or multiallelic variants. Second, the 2017 analysis covered only digenic interactions among biallelic SNPs (with a 3-SNP haplotype — a combination of neighboring SNPs inherited as a unit — scan for dominance), so three-locus and higher-order epistasis, structural variation such as presence/absence variants and copy-number variation, and full multiallelic sequence variation — all abundant in hexaploid bread wheat (whose cells carry six genome copies divided among A, B and D subgenomes), whose alien introgressions (chromosome segments inherited from wild relatives or other species) make it structurally partitioned — were not assessed. Third, the inference was confined to elite Central European material, while sequence-level studies in rice (Huang et al. 2016; Xie et al. 2022) reached different balances (dominance or nonadditive QTL effects as the main contributors), making generalization an empirical question rather than a settled fact.

Problem Statement

Determine the genetic architecture of grain-yield midparent heterosis in bread wheat, resolving what Jiang et al. (2017) could not: (i) disentangle genuine epistasis from apparent epistasis — multilocus marker signals that merely tag unobserved linked causal variants — by performing heterosis mapping with sequence-resolved variant catalogs (whole-genome resequencing or pangenome representations, i.e. the union of sequence variants across many genomes) in the hybrid population — parental whole-genome resequencing now exists for one Central European panel (Li et al. 2025) but has not been used to test this confound — so that the epistatic share of heterosis is established rather than confounded by linkage; (ii) quantify the contributions of higher-order (three-locus and beyond) epistasis, structural variation (presence/absence and copy-number variation, including alien introgressions), and full multiallelic sequence variation to midparent heterosis, which the biallelic SNP analyses left unassessed; (iii) establish whether the dominance–epistasis balance for grain-yield heterosis, found in elite Central European material, generalizes to broader wheat germplasm pools, environments, and later-stage hybrid-breeding populations — noting that current evidence is contradictory rather than confirmatory; and (iv) on the basis of that decomposition, develop and validate prediction and selection strategies that exploit epistatic heterosis efficiently in hybrid wheat breeding. These components form one scientific objective: an effect-class decomposition of wheat heterosis faithful enough to its causal variants to be generalized and exploited.

References

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