marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 93possible genes and the use of other potentiallyuseful parents that do not share theDNA polymorphism that is used in MAS.On the other hand, phenotypic selectionwould permit recognizing different geneticoptions for a desired phenotype. Thus,MAS is most useful when it is applied totruly unique genes.Cassava: importance andgeneticsCassava is a perennial shrub but it isgenerally harvested as an annual crop at 10–11 months of age. Basically every part ofthe plant can be utilized. The starchy rootsare a valuable source of energy and can beboiled or processed in different ways forhuman consumption and different industrialpurposes such as starches, animal feedor alcohol (Ceballos et al., 2006). Cassavastorage roots are not tubers and thereforecannot be used for reproductive purposes;stems are the common planting materials.Cassava foliage is not widely exploited inspite of its high nutritive value (Buitrago,1990; Babu and Chatterjee, 1999). Foliageconsumption by humans is relativelycommon in certain countries of Africa,Asia and Latin America. The use of foliagefor animal feeding is generating increasedinterest in Asia.Cassava can be propagated by eitherstem cuttings or botanical seed. However,the former is the practice most widely usedby farmers for multiplication and plantingpurposes. Propagation from true seed occursunder natural conditions and is commonin breeding programmes. Occasionallybotanical seed is also used in commercialpropagation schemes (Rajendran et al.,2000).Cassava is monoecious and allogamous,with female flowers opening 10–14 daysbefore the male ones on the same branch.Pollination can be done manually in acontrolled way to produce full-sib familiesor else in polycross nurseries whereopen pollination takes place and, therefore,half-sib families are produced. Self-pollinationis feasible when using male andfemale flowers on different branches oron different plants of the same genotypes(Jennings and Iglesias, 2002). Someclones flower relatively early at four or fivemonths after planting whereas others onlydo so at eight to ten months after planting.As a result, the time required for the seedto mature, the growing cycle of the cropand the need to plant with the arrival of therains take about two years between a givencross being planned and the respective seedbecoming available. On average, betweenone and two seeds (out of the three possiblein the trilocular fruit) per pollinationare obtained (Kawano, 1980; Jennings andIglesias, 2002).Breeding objectivesProductivity plays a major role in industrialuses of cassava, whereas stability of productionis fundamental in the many regionswhere cassava is the main subsistence crop.Industrial uses of cassava require high drymatter content as the main quality trait forthe roots, whereas for human consumptionthe emphasis is on cooking quality,frequently even over productivity, as thedetermining trait. Stability of productionis associated with resistance or toleranceto major biotic and abiotic stresses, withthe emphasis varying with the target environment.Genetic resistance to the mostimportant diseases and pests and the prevalentabiotic stresses can be found in cassavagermplasm (Hillocks and Wydra, 2002;Bellotti et al., 2002; Belloti, 2002; Ceballoset al., 2004). Although cyanogenic glucosidesare found in every tissue except