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Molecular basis of heterosis and related breeding strategies reveal its importance in vegetable breeding - PubMed

  • ️Fri Jan 01 2021

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Molecular basis of heterosis and related breeding strategies reveal its importance in vegetable breeding

Daoliang Yu et al. Hortic Res. 2021.

Abstract

Heterosis has historically been exploited in plants; however, its underlying genetic mechanisms and molecular basis remain elusive. In recent years, due to advances in molecular biotechnology at the genome, transcriptome, proteome, and epigenome levels, the study of heterosis in vegetables has made significant progress. Here, we present an extensive literature review on the genetic and epigenetic regulation of heterosis in vegetables. We summarize six hypotheses to explain the mechanism by which genes regulate heterosis, improve upon a possible model of heterosis that is triggered by epigenetics, and analyze previous studies on quantitative trait locus effects and gene actions related to heterosis based on analyses of differential gene expression in vegetables. We also discuss the contributions of yield-related traits, including flower, fruit, and plant architecture traits, during heterosis development in vegetables (e.g., cabbage, cucumber, and tomato). More importantly, we propose a comprehensive breeding strategy based on heterosis studies in vegetables and crop plants. The description of the strategy details how to obtain F1 hybrids that exhibit heterosis based on heterosis prediction, how to obtain elite lines based on molecular biotechnology, and how to maintain heterosis by diploid seed breeding and the selection of hybrid simulation lines that are suitable for heterosis research and utilization in vegetables. Finally, we briefly provide suggestions and perspectives on the role of heterosis in the future of vegetable breeding.

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Conflict of interest statement

The author declares no competing interests.

Figures

Fig. 1
Fig. 1. There are five hypotheses to explain the mechanism of heterosis based on gene effects.

Suppose that the biomass is the sum of the genetic effects (A, B, C) and that the biomass of an organism is represented by the circular area. A Dominance effect: the dominant allele (A) inhibits the recessive allele (a); (B) overdominance effect: a single heterozygous allele (B/B−) promotes the development of heterosis; (C) Epistasis effect: nonallelic (A1/B1) interactions in the parents promote the development of heterosis; (D) active gene effect: genes from parents (C) promote heterosis when heterozygous and produce genome imprinting when homozygous, which inhibits the occurrence of heterosis; (E) gene network system: genes from parents (A, B, C) are combined into a coordinated gene network system that enables F1 to develop heterosis; (F) single-cross hybrids P1 (AB) and P2 (CD) produced from four homozygous inbred tetraploids (with genotypes A, B, C, and D) are crossed to produce F1 (ABCD), a double-cross tetraploid hybrid

Fig. 2
Fig. 2. Putative model of heterosis triggered by epigenetics.

A DNA methylation: De novo methylation was catalyzed by DRM2, a homologous enzyme of DNMT3. In maintenance methylation, CG is catalyzed by MET1, a homologous enzyme of DNMT1; CHG is catalyzed by CMT3; and CHH is still catalyzed by DRM2. B Small RNA: Includes the miRNA produced by premiRNA and the siRNA produced by dsRNA. In general, 24 nt-siRNA mediates de novo DNA methylation catalyzed by the AGO4 protein. C Histone modifications: The modifications of histone amino acid residue includes acetylation, phosphorylation, methylation, and ubiquitination processes. Epigenetic modifications are produced by the parents. New epigenetic modifications may occur in F1 hybrids. D Epigenetic modification status of the parents and F1 hybrid: the increase and decrease in or recombination of epigenetic modifications induces the F1 hybrid to exhibit heterosis

Fig. 3
Fig. 3

Statistical analysis of the effect of quantitative trait loci on crop heterosis. A In the statistical analysis of the effect of quantitative trait loci on crop heterosis, the species and frequency of each species were studied; (B) in the statistical analysis of the effect of quantitative trait loci on crop heterosis, the quantitative trait locus effect on each species and the proportion of each type of effect were analyzed

Fig. 4
Fig. 4. By comparing the gene expression of the F1 hybrid and its parents, the gene expression patterns of F1 were divided into additive gene expression patterns and nonadditive gene expression patterns.

Midparent value [MPV = (HPV + LPV)/2]; High-parent value (HPV); low-parent value (LPV)

Fig. 5
Fig. 5

Contributing traits of yield heterosis in cucumber, cabbage and tomato. A Traits contributing to yield heterosis in cucumber, cabbage, and tomato: cucumber yield contributing traits include the number of fruits, days to first female flowering, days to first harvest, first nodal position of female flower, sex ratio (M/F), fruit length, fruit diameter, and fruit weight; cabbage yield contributing traits include fruit length, fruit diameter, and fruit weight; tomato yield contributing traits include number of fruits, days to first female flowering, days to first harvest, number of flowers/fruits per cluster, fruit length, fruit diameter, and fruit weight. B Cucumber: cucumber model in production, gynoecious line with a small number of branches. C Cabbage: an aerial and cross-sectional model of cabbage consisting of leaves and heads. D Tomato: a tomato with single inflorescences and indeterminate growth is crossbred with a tomato with compound inflorescences and determinate growth to produce the hybrid F1 with earlier fruiting, more compound inflorescences, and determinate growth

Fig. 6
Fig. 6. There are two key factors involved in applying heterosis breeding strategies: obtaining heterotic lines and maintaining heterosis in the elite lines in the offspring.

There are two strategies for obtaining heterotic lines in crop breeding. The first is the use of crossbreeding or molecular biotechnology. Genealogical analysis, molecular markers, combining ability, and genetic distance can usually predict heterosis development, so they are often used to classify heterotic groups. The inbred lines from different heterotic groups can be crossed with each other to obtain elite lines that exhibit heterosis. The second strategy is to use modern molecular biotechnology. Elite lines were obtained based on GWAS and linkage analysis, mapping and cloning genes related to heterosis, gene editing, and gene transformation

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