marker-assisted selection in wheat

82Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishSummaryMarker-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 thebreeding goals of International Agricultural Research Centres (IARCs) and NationalAgricultural Research Systems (NARS). The importance of each crop is highlighted andexamples of successful use of molecular markers within selection cycles and breeding programmesare given for each. For common beans, examples are given of gene tagging forseveral traits that are important for bean breeding for tropical environments and aspectsconsidered that contribute to successful application of MAS. Simple traits that are taggedwith easy-to-use markers are discussed first as they were the first traits prioritized forbreeding at the International Center for Tropical Agriculture (CIAT) and with NARSpartners in Central America, Colombia and eastern Africa. The specific genes for MASselection were the bgm-1 gene for bean golden yellow mosaic virus (BGYMV) resistanceand the bc-3 gene for bean common mosaic virus (BCMV) resistance. MAS was efficientfor reducing breeding costs under both circumstances as land and labour savings resultedfrom eliminating susceptible individuals. The use of markers for other simply inheritedtraits in marker-assisted backcrossing and introgression across Andean and Mesoamericangene pools is suggested. The possibility of using MAS for quantitative traits such as lowsoil phosphorus adaptation is also discussed as are the advantages and disadvantages ofMAS in a breeding programme. For cassava, the use of multiple flanking markers for selectionof a dominant gene, CMD2 for cassava mosaic virus (CMV) resistance at CIAT andthe International Institute of Tropical Agriculture (IITA) as well as with NARS partners inthe 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) andcassava 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 wayto discover genes for high protein content, waxy starch, delayed post-harvest physiologicaldeterioration, and resistance to whiteflies and hornworm. Other potential targets of MASsuch 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 averageheterozygosity during inbreeding of cassava and for the delineation of heterotic groupswithin the species. A final section describes the similarities and differences between theMAS schemes presented for the two crops. Differences between the species can be ascribedpartially to the breeding and propagation systems of common beans (seed propagated, selfpollinating)and cassava (clonally propagated, cross-pollinating). In addition, differencesin growth cycles, breeding methods, availability of genetic markers, access to selectionenvironments and the accompanying opportunities for phenotypic selection influence thedecisions in both crops of when and how to apply MAS. Recommendations are made forapplying MAS in breeding of both crops including careful prioritization of traits, markersystems, genetic stocks, scaling up, planning of crosses and the balance between MAS andphenotypic selection.