Marker-Assisted Selection in common beans and cassava

Abstract

Marker-assisted selection (MAS) in common beans (Phaseolus vulgaris L.) and cassava (Manihot esculenta) is reviewed in relation to the breeding system of each crop and the breeding goals of International Agricultural Research Centres (IARCs) and National Agricultural Research Systems (NARS). The importance of each crop is highlighted and examples of successful use of molecular markers within selection cycles and breeding programmes are given for each. For common beans, examples are given of gene tagging for several traits that are important for bean breeding for tropical environments and aspects considered that contribute to successful application of MAS. Simple traits that are tagged with easy-to-use markers are discussed first as they were the first traits prioritized for breeding at the International Center for Tropical Agriculture (CIAT) and with NARS partners in Central America, Colombia and eastern Africa. The specific genes for MAS selection were the bgm-1 gene for bean golden yellow mosaic virus (BGYMV) resistance and the bc-3 gene for bean common mosaic virus (BCMV) resistance. MAS was efficient for reducing breeding costs under both circumstances as land and labour savings resulted from eliminating susceptible individuals. The use of markers for other simply inherited traits in marker-assisted backcrossing and introgression across Andean and Mesoamerican gene pools is suggested. The possibility of using MAS for quantitative traits such as low soil phosphorus adaptation is also discussed as are the advantages and disadvantages of MAS in a breeding programme. For cassava, the use of multiple flanking markers for selection of a dominant gene, CMD2 for cassava mosaic virus (CMV) resistance at CIAT and the International Institute of Tropical Agriculture (IITA) as well as with NARS partners in the United Republic of Tanzania using a participatory plant breeding scheme are reviewed. MAS for the same gene is important during introgression of cassava green mite (CGM) and cassava brown streak (CBS) resistance from a wild relative, M. esculenta sub spp. flabellifolia. The use of advanced backcrossing with additional wild relatives is proposed as a way to discover genes for high protein content, waxy starch, delayed post-harvest physiological deterioration, and resistance to whiteflies and hornworm. Other potential targets of MAS such as beta carotene and dry matter content as well as lower cyanogenic potential are given. In addition, suggestions are made for the use of molecular markers to estimate average heterozygosity during inbreeding of cassava and for the delineation of heterotic groups within the species. A final section describes the similarities and differences between the MAS schemes presented for the two crops. Differences between the species can be ascribed partially to the breeding and propagation systems of common beans (seed propagated, selfpollinating) and cassava (clonally propagated, cross-pollinating). In addition, differences in growth cycles, breeding methods, availability of genetic markers, access to selection environments and the accompanying opportunities for phenotypic selection influence the decisions in both crops of when and how to apply MAS. Recommendations are made for applying MAS in breeding of both crops including careful prioritization of traits, marker systems, genetic stocks, scaling up, planning of crosses and the balance between MAS and phenotypic selection