Recurrent Mass Selection for Routine Improvement of Common Wheat: A Review

Abstract

The pursuit of sustainable wheat production has significant economic, social, and environmental relevance. Yield levels and stability thereof are determined by continuously changing and fluctuating biotic and abiotic stresses. Achieving higher and more stable yields requires constant genetic improvement of numerous aspects of new wheat cultivars. Targeted traits may include wide adaptation, abiotic stress such as drought and salinity, tolerance, polygenic non-specific disease resistance, pyramided disease resistance, etc. Broadly defined breeding objectives such as these involve complex, polygenic, genetic mechanisms that pose formidable challenges to breeders. Fortunately, a diverse array of increasingly more sophisticated biotechnological tools is becoming available. Advances in understanding the mechanisms that determine sustainability traits, coupled with versatile and unambiguous genetic markers and generation acceleration methodologies, foster new opportunities for selection of such traits, yet breeding methodologies need to be adapted in parallel to fully capitalize on the new technology. Recurrent selection applied to a self-pollinator provides for a powerful breeding tool. Continuous cross-hybridization maximizes heterogeneity and forges new linkage associations in genes. Subsequent inbreeding helps to weed out deleterious recessive genes, fixes desirable genes in the homozygous state, and allows for accurate progeny testing. In the past, the difficulty of randomly intercrossing large numbers of selected wheat plants has frustrated the application of the technique, however this problem has been solved through the use of genetic male sterility in conjunction with the hydroponic culture of tillers that are cut and pollinated at anthesis. Thus, it is possible to randomly intercross hundreds of selected genotypes to produce large F1 populations (upwards of 50,000 seeds). A recurrent wheat mass selection program is being conducted at Stellenbosch University with the purpose of developing and testing the methodology. A highly heterogeneous base population was established and is being managed as a medium-sized breeding program. The experience we gained allowed us to streamline its execution, and herewith we review current methodology and progress made. Recurrent mass selection proved a simple, yet highly effective, technique that has major advantages compared to conventional wheat breeding methods; among these are reduced operational costs, accelerated selection progress, maximization of crossover and genetic recombination potential, and its suitability for broad breeding strategies.