marker-assisted selection in wheat - ictsd

Chapter 19 – Technical, economic and policy considerations on marker-assisted selection in crops 389these favourable alleles are used in heteroticstudies, the predictive power of markers inestimating heterosis for practical applicationsmay not be very high.At CIMMYT, large-scale, rapid characterizationmethods for inbred lines andpopulations have been optimized usingup to 120 microsatellite markers spreadthroughout the maize genome. In the past,characterizing maize populations was costlyand time-consuming, given that as many as22 individuals had to be analysed individuallyto calculate allele frequencies for eachmarker. Currently, a bulking method inwhich 15 individuals from a populationare amplified in the same polymerase chainreaction (PCR) and run on an automaticDNA sequencer, provides a reliable estimateof the allele frequencies within thatparticular population. Between one andtwo bulks can now be used to fingerprintpopulations with considerable savings intime and resources. Other studies of maizegenetic diversity have been conducted forCIMMYT maize breeders as well as outsidecollaborators with objectives that include:determining how maize inbred lines fromdifferent national breeding programmesare related to each other (and to determinethe possibility of sharing among regionsor using lines from one region to expanddiversity in another); establishing heteroticgroups; determining levels of geneticdiversity present in synthetic varieties;determining how landraces and farmers’varieties from different regions are relatedto each other; monitoring homozygositylevels in inbred lines; and tracking changesin lines that have been intensively selectedfor a given trait.A core set of 100 microsatellite markershas been selected for wheat genetic diversitystudies. Recent fingerprinting studiesby CIMMYT and national programmescientists have been conducted to assist inregenerating gene bank accessions withoutlosing genetic diversity, measuring thecontribution of wild ancestors and exoticspecies in advanced backcross progeniesof synthetic bread wheat, and to track thechanges over time in diversity levels ofCIMMYT wheat cultivars from the originalGreen Revolution varieties to modernbreeding lines.Marker implementationTo facilitate the use of MAS activities inwheat and maize improvement efforts,CIMMYT has recently established amarker implementation laboratory. Thisprovides the facilities and technical expertiseto provide CIMMYT wheat and maizebreeders with access to biotechnologytools, including MAS. The laboratory carriesout two main MAS-related activities,marker adoption and research support. Thefirst includes constantly reviewing the literatureto identify markers developed bythird parties and verifying that these can beused to detect traits or genes of interest inCIMMYT germplasm improvement efforts,and developing efficient protocols for theirin-house use. The second consists of a rangeof routine tasks that include growth and/orsampling of plant tissue, DNA extraction,marker detection, data analysis and disseminationof results to breeders.Close cooperation between field andlaboratory staff is important to be able toapply molecular markers in crop improvementefforts. Ideally, laboratory staff shouldhave an understanding of field activities andfield workers should have basic knowledgeof different aspects of MAS-associatedlaboratory procedures. MAS is used whenthere is a high probability that markers willhelp plant breeders achieve genetic gainsfaster and more economically than field