Establishment of heterotic groups for hybrid wheat breeding

Hybrids utilized in maize and rice production are excellent examples of enhancing crop yield to meet the demand of the increasing world population and climate change. Similarly, hybrid wheat should provide an avenue to increase wheat yield and stress tolerance. Heterosis of wheat is relatively weak due to its allopolyploidy, narrow genetic base between varieties, and long-term varietal selection. The construction of wheat heterotic groups can improve hybrid parental breeding and selection, increase the genetic difference between parents, and enhance the performance of hybrids. To form heterotic groups, it is necessary to understand how heterotic groups are structured. The genetic mechanism of heterosis is not completely understood at present. Research shows that there is no clear relationship between heterosis-related QTL and crops. Also, there is no fixed mode of heterosis in a species. Because multiple genes control heterosis, selection with molecular markers is difficult. Heterosis can be largely attributed to non-additive effects of genes and specific combining ability (SCA) of hybrid combinations. High general combining ability (GCA) of parents plays an important role in maintaining strong heterosis between heterotic groups. Therefore, wheat heterotic group development needs to increase the GCA of parental lines within each heterotic group and select hybrid combinations with high SCA. Some desirable traits for a male heterotic group include tall or medium plant height, heavy pollen production, and strong anther extrusion. Desirable traits for female parents include dwarf or semi-dwarf plant height and large glume opening angle. Disease resistance is important in both male and female groups. Commercial production of hybrid wheat is possible only through the use of male sterility systems. Male sterility in the female parents must be restored by genes in the male parent. Pedigrees, phenotypes, combining ability, genetic distances, and genomic prediction can be used to develop heterotic groups. Genomic prediction can be used to predict heterosis and identify potential heterotic patterns/groups existing in a large number of materials, and, therefore, guide selection of parent lines for development of excellent hybrids. However, the prediction accuracy of statistical models needs to be improved. Utilization of materials from wheat core collection in genomic prediction will facilitate the formation of the training population, and has the potential to improve the prediction accuracy. Recurrent selection in both male and female groups plus reciprocal recurrent selection based on genome prediction can be used to improve wheat heterotic groups. The process of constructing heterotic groups in wheat includes primary population classification, prediction and improvement of heterotic groups, and identification of the best hybrid combination. Development of heterotic groups adaptable to different wheat production regions may utilize both domestic high performing varieties and foreign elite wheat germplasm. Wild relatives with unique traits may also facilitate hybrid wheat breeding and production in China.

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