marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 107action, while cassava expresses importantcomponents of dominance and epistaticaction. Finally, cassava as a clonal cropcan fix heterotic combinations, while alack of genetic male sterility or apomixissystems in common bean have curtailedthe development of a hybrid industry forthis seed crop even though heterosis isobserved.In spite of their biological and otherdifferences, the results of several yearsexperience with MAS in beans and cassavaare surprisingly similar. In both crops,MAS is being employed principally tobolster phenotypic selection for diseaseresistance genes. Disease resistance is oftengoverned by relatively few genes, and phenotypicdata are obtained more easily. Onthe other hand, MAS for more complextraits has yet to find ready application.While there are candidates for such traitsin both crops (root bulking in cassava;low phosphorus or drought tolerance inbeans), the complexity of these traits hasmade the identification of reliable markersmore difficult and has delayed application.Obtaining reliable phenotypic datafor complex traits is especially difficult andis often the biggest bottleneck to eventualapplication of MAS. In the case of cassava,no inbred parents have been used to datefor the development of molecular markers,making the genetic analysis more difficult.However, some differences in the applicationof MAS for the two crops may benoted, arising from the form of reproductionof each crop. The time frame toselect cassava clones through multilocationaltrials is about six to seven years.During this period and with each step thenumber of genotypes is reduced as a resultof the selection exerted, but the genotype ofeach individual clone remains stable. In thecase of beans from the F 1 until stabilizationof pure lines there is an intense segregationprocess in the early generations whichtapers off in later generations. In both cropsMAS can be used in the early stages of theselection process but with different objectives.In cassava, MAS can help to selectearly on the clone that will ultimately bereleased, whereas in beans MAS is usedto “direct” the segregation process in themore desirable direction. Although mapswith significant saturation are available forboth crops, these have been constructedover several years, employing genotypes (inthe case of beans) from different gene poolswith wide polymorphism. A small proportionof these markers (often 20–30 percent)is polymorphic in other hybrid combinationsamong the genotypes within the samegene pool or race that have been created totag a specific trait. Thus, genome coverageis often still not optimal for the high qualityQTL analysis that is usually needed forcomplex traits.RecommendationsCareful prioritization of traits, markersystem and genetic stocks for MASThe limited resources available for cassava orbean research require a judicious allocationof efforts. In the past 10–20 years there hasbeen increased investment in molecularmarker research in both crops. However, aconsiderable proportion of that research wasdirected at demonstrating the usefulness ofdifferent techniques, e.g. RAPD, restrictionfragment length polymorphism (RFLP),amplified fragment length polymorphism(AFLP), etc. Over this period there hasbeen an ever-changing set of technologiesbut relatively little actual benefit derivedfrom their application. There is a tradeoffbetween being on the cutting edgewith the newest technologies and “stickingit out” with an “outdated” technology