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Heterosis is an important phenomenon for high-yield crop breeding and is utilized for breeding F1 varieties in horticultural crops. However, its molecular mechanism has not been elucidated, and compared to cereals, heterosis is less explored at the molecular level in horticultural crops. In this review, we compiled the new genetic and epigenetic studies on heterosis in horticultural crops. Because of the difficulty of predicting the level of heterosis from the parental genetic distance, molecular approaches are being used to study its molecular basis in horticultural crops. Transcriptome analyses in vegetables have identified photosynthesis-related genes as important in heterosis. Analysis of noncoding RNAs has suggested their involvement in regulating the heterosis of vegetative and fruit tissues. Quantitative trait locus (QTL) analysis has revealed the association of heterozygosity of a specific locus or multiple loci with heterosis of vegetative and fruit tissues. A higher level of DNA methylation was noted in the heterotic F1 of Brassica rapa leafy vegetables, while the roles of other epigenetic modifications such as histone marks have not been explored.

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