marker-assisted selection in wheat - ictsd

384Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishScientific advances have been instrumentalin increasing the power and accuracyof computational parameters as well asdesigning ways of combining the informationgenerated from molecular geneticswith traditional crop improvement efforts.Numerous simulation studies have beenundertaken to evaluate the effectiveness ofMAS, taking into account the influence ofheritability, population size, linkage distance,etc. (Xie and Xu, 1998; Moreau etal., 1998; Ribaut, Jiang and Hoisington,2002), and MAS procedures have beenused to incorporate traits of interest fromexotic species including wild relatives intoelite cultivars through advanced backcrossQTL analysis (Tanksley and Nelson, 1996;Fulton et al., 2000).Manipulation of qualitative traitsMolecular markers that are tightly linkedto genes having a strong effect on theexpression of a trait can be used to introgressthe genes (and thus the trait) intodifferent backgrounds through backcrossbreeding schemes that rapidly and efficientlyimprove the recurrent parent forthe target trait. In conventional backcrossbreeding schemes and line conversionactivities, the donor parent containing thetrait of interest is crossed with the recurrentparent, normally a well-adapted varietylacking the trait of interest. The resultingprogeny are screened to identify the traitof interest, and individuals exhibiting thetrait are crossed to the recurrent parent.The entire process is repeated several times.For traits that are conditioned by recessivegene action, a cycle of selfing is alsorequired after each crossing cycle. Afterseveral cycles of backcrossing and a finalself-pollination, plant breeders are oftenable to recover lines that are nearly identicalto the recipient parent but also contain thetrait of interest. Compared with traditionalbackcrossing, the use of DNA markers enablesfaster recovery of the recurrent parentgenotype along with the introgressed targettrait in line conversion activities. Ribautand Hoisington (1998) reported that MASshould enable the recovery of the targetgenotype after three cycles of backcrossing,compared with a minimum of six cycleswith traditional approaches (Tanksley etal., 1989).CIMMYT has a long history of usingmolecular markers for certain traits inmaize improvement. Although maize iswidely used for both food and feed, maizekernels do not provide sufficient quantitiesof two essential amino acids, lysine andtryptophan. The opaque2 mutation, identifiedat Purdue University (United States ofAmerica) in the mid-1950s, confers elevatedlevels of these two amino acids. Althoughinitial efforts to introduce the opaque2mutation into breeding materials were notsuccessful (Villegas, 1994), researchers eventuallysucceeded in producing nutritionallyenhanced maize lines. These came to beknown as quality protein maize (QPM).CIMMYT breeders have used traditionalbackcrossing to transfer the opaque2 mutationand associated modifiers into elitelines. To perform phenotypic selection insegregating progenies for lines carrying theopaque2 mutation, it is necessary either towait until the plants produce mature ears,or to do random pollination on a largenumber of plants. Although reliable laboratoryscreening techniques are available,co-dominant microsatellite markers presentwithin the opaque2 mutation can be usedearlier in the growing season. Using thesemarkers in backcross progenies, plants heterozygousfor the opaque2 mutation can beselectively identified as a qualitative traitfor use in the next crossing cycle. Markers