Li Wang, Ian K. Greaves, Michael Groszmann, Li Min Wu, Elizabeth S. Dennis, and W. James Peacock

Significance

Hybrids have an important role in many crops used for global food production. The increased production levels of biomass and grain are restricted to the first-generation hybrid. We report stabilization of hybrid vigor traits in pure breeding hybrid mimic lines. Comparison of the patterns of their gene activity with those of the F1 hybrids has identified metabolic pathways associated with the generation of the hybrid vigor phenotype. The hybrid mimics are homozygous for long chromosomal segments showing that heterozygosity is not required for the production of the heterotic phenotype. Interactions between the two parental genomes and epigenomes contribute to the vigor of the Arabidopsis hybrids and are likely to be the basis of hybrid vigor in other plant species.

Abstract

F1 hybrids can outperform their parents in yield and vegetative biomass, features of hybrid vigor that form the basis of the hybrid seed industry. The yield advantage of the F1 is lost in the F2 and subsequent generations. In Arabidopsis, from F2 plants that have a F1-like phenotype, we have by recurrent selection produced pure breeding F5/F6 lines, hybrid mimics, in which the characteristics of the F1 hybrid are stabilized. These hybrid mimic lines, like the F1 hybrid, have larger leaves than the parent plant, and the leaves have increased photosynthetic cell numbers, and in some lines, increased size of cells, suggesting an increased supply of photosynthate. A comparison of the differentially expressed genes in the F1 hybrid with those of eight hybrid mimic lines identified metabolic pathways altered in both; these pathways include down-regulation of defense response pathways and altered abiotic response pathways. F6 hybrid mimic lines are mostly homozygous at each locus in the genome and yet retain the large F1-like phenotype. Many alleles in the F6 plants, when they are homozygous, have expression levels different to the level in the parent. We consider this altered expression to be a consequence of transregulation of genes from one parent by genes from the other parent. Transregulation could also arise from epigenetic modifications in the F1. The pure breeding hybrid mimics have been valuable in probing the mechanisms of hybrid vigor and may also prove to be useful hybrid vigor equivalents in agriculture.

See: http://www.pnas.org/content/112/35/E4959.abstract

PNAS September 1, 2015; vol. 112 no. 35: E4959–E4967

Fig. 1.

The two parents, C24 and Ler, together with the F1 hybrid and F2 plants at 28 d after sowing. F2 plants show increased variance of rosette diameter and flowering time. Reproduced with permission from ref. 6, copyright American Society of Plant Biologists.