marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 105Laboratories (MBL), Kampala, and CIAT,is aimed at the genetic mapping of CNPin cassava. An S 1 family-AM 320, derivedfrom the bitter variety MTAI 8 is thebasis for the study. This family has beenevaluated for cyanogenic glucoside contentand has been genotyped with morethan 200 diversity array technology (DarT)markers at CAMBIA, Australia, and 150SSR markers at CIAT. The discovery ofmolecular markers for CNP will providea tool to select efficiently for low cyanogenicpotential in cassava. Also ongoing isthe genetic mapping of the two cytochromeP450 genes CYP79D1 and D2 that catalysethe rate-limiting step of the biosynthesisof the cyanogenic glucosides, linamarin inthe S 1 family AM 320. The group is alsolooking for an association with QTL forCNP. It is expected that markers associatedwith CNP will be identified at the end ofthe study.Dry matter contentFew key traits in cassava hold greaterpotential for increasing cost-effectivenessvia MAS than root dry matter content(DMC). This trait is usually measured atthe end of the growth cycle. A number ofgenetic and environmental effects influenceDMC. It is usually highest before the onsetof rains, but drops after the rains beginas the plant mobilizes starch from theroots for re-growth of leaves (Byrne, 1984).Defoliation from pest and disease attackscan lower DMC. Breeding programmeshave been quite successful in improvingDMC, especially for industrial markets.The entry point for developing markersassociated with DMC was recent diallelexperiments (Jaramillo et al., 2005; Calleet al., 2005; Pérez et al., 2005a, b; Cachet al., 2005b). Diallels, in this case madeup of 90 families, are an ideal methodto identify genes controlling DMC thatare useful in many genetic backgrounds.Estimates of general and specific combiningability (SCA and GCA, respectively) formany traits of agronomic interest werecalculated, with emphasis on DMC. Basedon GCA estimates, parents were selected togenerate larger-sized progenies for DMCmapping. Sizes of families in the originaldiallel experiment were about 30 progenies,which is rather small for genetic mapping.Parallel to the development of mappingpopulations was the search for markersassociated with DMC using two F 1 families,GM 312 and GM 313, selected from thediallel experiment having parents with highGCA for DMC.Initial marker analysis using bulked segregantanalysis led to the discovery of twomolecular genetic markers, SSRY160 andSSRY150, which explain about 30 and18 percent, respectively, of phenotypic variancefor DMC. These markers are beinganalysed on approximately 700 genotypesderived from 23 crosses with parents havinghigh GCA for DMC in order to confirmtheir utility across genetic backgrounds.Parallel to this, larger families are beingdeveloped from selected parents for QTLmapping of DMC.Disadvantages of MASPerhaps the greatest disadvantage of MAS isthe time and financial investment requiredto develop markers that are widely applicablefor traits of agronomic importance.Often a marker developed in one or afew related genotypes will not work forother genotypes in a breeding scheme dueto allelic effects. Furthermore, developmentof markers, particularly for QTL, iscomplicated by epistatic interactions andthe critical need for good quality phenotypicdata. Several ways around this