marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 101CMD2 mediated resistance to CMD.Polymorphism in pair-wise combinationsof the parental lines was observed with atleast one of the five markers and will beused on the progeny. The phenotype ofthe progeny will be evaluated at three andsix months after planting for resistanceto CMD, CBSD and CGM. Markers arecurrently being tested for CGM resistanceand are being developed for resistance toCBSD; when their utility is confirmed,they will also be used on progenies.Using published broad sense heritabilityof 0.6 for CMD resistance (Hahn,Terry and Leuschner, 1980), it is expectedthat 24 000 symptomless genotypes willbe analysed with markers associated withresistance to CMD. The gain of MAS willbe the elimination of at least 38 400 (4800 x 8 plants) that would have been carriedto the single row trial stage (eightplant-rows per genotype), considering thatbreeders traditionally select 20 percent atthe seedling trial stage. This representsa reduction of about 4 ha at the CET. Ifmarkers can be used to select for resistanceto CGM and CBSD, then an additionalnumber of genotypes can be eliminatedfrom the CET leading to even greater savings.Using MAS for CMD alone wouldreduce the size of field trials by 50 percent.If additional second and third traits wereincluded, reductions could be as high as 75and 87.5 percent, respectively. Perhaps themost important advantage, however, comesfrom the increased genetic gain arising fromhigher heritabilities in these field evaluationswith fewer genotypes.MAS for transferring useful traits from wildrelatives of cassava into the cultivated genepoolWild Manihot germplasm offers a wealth ofuseful genes for the cultivated M. esculentaspecies, but its use in regular breedingprogrammes is restricted by linkage drag andthe long reproductive breeding cycle. Forexample, several accessions of M.esculentasub spp. flabellifolia, M. peruviana andM. tristis have high levels of proteins(Nichols, 1947; Asiedu et al., 1992; CIAT,2004). Low amylose content starch (3–5 percent) or waxy starch of relevance tothe cassava starch industry has also beenidentified in two wild relatives of cassava,namely M. crassisepala and M. chlorostricta.The only source of dramatically delayedpost-harvest physiological deterioration(PPD) has been identified in an interspecifichybrid between cassava and M. walkerae.The M. walkerae parent was collectedin Mexico and held at the WashingtonUniversity, St. Louis, United States ofAmerica (Bertram, 1993). It was broughtto CIAT in 1998 in an attempt to useit in improving PPD. Furthermore, theonly source of resistance to the cassavahornworm and the most widely deployedsource of resistance to CMD wereidentified in fourth backcross generationprogenies of M. glaziovii (Jennings, 1976;Chavarriaga et al., 2004). Moderate to highlevels of resistance to CGM, whitefliesand the cassava mealybug have been foundin interspecific hybrids of M. esculentasub spp. flabellifolia. The delayed PPDtrait and resistance to the pests weresuccessfully transferred to F 1 interspecifichybrids suggesting dominant or additivegene action of the gene(s) involved (CIAT,unpublished data).The long reproductive cycle and lengthytime required to develop new cassavavarieties (10–15 years) often discouragesthe use of wild species in most conventionalcassava breeding programmes. However,the use of molecular markers to introgressa single target region of the genome can