marker-assisted selection in wheat - ictsd

84Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishto families. This system takes advantage ofthe variability among F 1 plants that is createdbetween segregating parental plants.The choice of breeding method and itsadaptation to specific circumstances, thegrowth cycle of the crop in relation todifferent planting seasons, the access toselection environments and the accompanyingopportunities for phenotypicselection and the ease of implementing thespecific markers to be used will all influencethe decisions about where and howMAS will be most cost effective and usedto best advantage.MAS in bean breeding: experiences ofCIAT and NARSMolecular markers have been sought forboth simple and complex traits in beans,with an eye to eventual application in MAS.Tagging of genes and QTL in common beanand their application to MAS have beenreviewed previously (Kelly et al., 2003;Miklas et al., 2006). In the present chapter,some of the aspects that contribute to thesuccessful use of MAS are considered ingreater detail, referring to examples takenfrom bean breeding in the tropics at CIATand within NARS. Simple and complextraits are discussed separately, as they representtwo contrasting sorts of experience.Simple traitsBean golden yellow mosaic virus resistanceBean golden yellow mosaic virus (BGYMV)is a white fly-transmitted Gemini virus, anda major production limitation of beans inthe mid-to-low altitude areas of CentralAmerica, Mexico and the Caribbean.Host resistance to the virus is the mostpractical means of control, and any newvariety in these production areas must carryresistance. Studies on inheritance of resistancerevealed a major gene denominatedbgm-1 in breeding line A429 (Blair andBeaver, 1993) that originates in the Mexican(Durango race) accession “Garrapato” orG2402. Minor genes (Miklas et al., 2000c)as well as additional recessive and dominantresistance genes exist for the virus (Miklaset al., 2006). In most production areaswhere BGYMV exists, it is necessary topyramid genes for adequate disease control.Although lines developed in CIAT targetthese areas, BGYMV does not exist at levelsthat would permit selection under fieldconditions in Palmira, Colombia, at CIATheadquarters. Therefore, MAS was desirableto assure recovery of at least the mostimportant resistance genes. MAS has alsobeen employed in the Panamerican Schoolin Zamorano, Honduras, as a complementto field screening, to extend selection tosites and seasons with less disease pressure(J.C. Rosas, personal communication).A co-dominant RAPD marker wasidentified for the bgm-1 gene (Urrea etal., 1996) that was subsequently convertedto a SCAR marker named SR2 (CIAT,1997). The DNA fragment associated withbgm-1 gene has only been observed inone genotype other than G2402 and itsderivatives, and thus the polymorphismhas been very useful for recognizing thepresence of the gene in different geneticbackgrounds. This SCAR was evaluated onas many as 7 000 plants in a single sowing(CIAT, 2001; 2003). The uniqueness of themarker’s polymorphism and its reliabilityover laboratories, seasons and geneticbackgrounds have facilitated its wide use.More recently, a second SCAR (SW12.700)was developed from the W12.700 RAPDfor a QTL located on linkage group b04(Miklas et al., 2000c), and this has alsobeen incorporated into the breedingprogramme of CIAT. The combination ofbgm-1 and the QTL is expected to offer an