marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 103in a CET for evaluation of root proteincontent at ten months. The grand parentallines of the BC 1 population were genotypedwith over 800 simple sequence repeat (SSR)markers available for cassava and about 300polymorphic markers were identified. Thepolymorphic markers are being assayed inthe progenies after which QTL analysis willbe conducted using the phenotypic proteinand molecular marker data. Genotypes thathave QTL for protein and a minimum of thedonor parent genome will be selected andused for producing the BC 2 generation.For introgression of naturally occurringmutant granule-bound starch synthase(GBSSI) for waxy starch in wild relatives,a more targeted approach was taken.Sequencing of the glycosyl transferase regionof the GBSSI gene from the wild relativesand two cassava accessions identified foursingle nucleotide polymorphisms (SNPs)that differentiated the wild accessions fromcassava. Allele-specific molecular markersunique to these SNPs were developed forselection of these alleles in a breedingscheme.Genetic crosses were made betweenM. chlorosticta accession CW14-11 andMTAI8, and the resulting F 1 was backcrossedto MTAI8. The allele specificmarker will be used together with otheragronomic traits, particularly performance,to select for BC 1 that carry the mutantGBSS alleles for self-pollination to recoverthe waxy trait. The identification of naturalmutants in a key gene and development ofmarkers represent an innovative moleculartool to accelerate the introgression offavourable alleles from wild relatives intocassava. Backcross derivatives have alsobeen developed from M. walkerae (MWal001) for delayed post-harvest physiologicaldeterioration; from MNG11 (a BC 4derivative of M. glaziovii) for resistance tohornworm; and from M. esculenta sub spp.flabellifolia (FLA447-1) for resistance towhiteflies. Phenotypic and genetic mappingof these backcross populations are inprogress to be followed by identification ofQTL and selection of progenies to generatethe next generation. MAS will later be usedto combine these genes into progenitors foruse as parents in breeding which, togetherwith low cost marker technologies, willbe distributed extensively to nationalprogrammes in Africa, Asia and LatinAmerica to produce improved varieties.Marker-assisted estimation of averageheterozygosity during inbreeding of cassavaA principal use of molecular markers byprivate sector breeding companies is toaccelerate the development of inbred lines.Cassava genotypes are heterozygous andvery little inbreeding has been practisedto date. However, inbred lines arebetter as parents as they do not have theconfounding effect of dominance and carrylower levels of genetic load (undesirablealleles). Speed of inbreeding depends uponthe average heterozygosity of the originalparental lines, the homozygosity level ofthe selected genotypes at the end of theself-pollinating phase and the process ofselection of progenies to be self-pollinated(Scotti et al., 2000). Basically in theinbreeding process two events go together:phenotypically there is a decrease in vigour,which is correlated with the increased levelsof homozygosity. While the aim is to selectvigorous plants (tolerant to inbreeding), inthe process plants may be selected that areless homozgygous than the expected averagefor their generation. It is expected that thefirst few cycles of self-pollination will resultin a marked reduction of vigour (inbreedingdepression associated with the genetic loadof the parental lines); therefore, selection for