marker-assisted selection in wheat - ictsd

106Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishproblem have been proposed, such as theuse of candidate genes involved in the traitsdirectly as selectable markers without theneed for laborious gene tagging experiments.However, unravelling the geneticsand the development of markers for suchtraits is still many years down the road.New methods of association mapping andlinkage disequilibrium mapping that relyupon non-random association of candidategenes or markers on a high resolution mapwith a phenotype of interest in a non-structuredcollection of genotypes have beenused extensively in human medicine toidentify genes involved in disease (Cardonand Bell, 2001). Given the enormous difficultiesof quantitative mapping in humansand the success of association mapping,these methods have also been proposed asways around the problems in developingmarkers for low heritability traits in plants(Gaut and Long, 2003). The developmentof (partially) inbred cassava genetic stockswill certainly accelerate the application ofMAS for the genetic improvement of thecrop.ConclusionsGiven limited resources, further prioritizationof traits is needed for the developmentof markers if they do not already exist. Toppriority should be given to MAS for themost important pests and diseases prevalentin the region for which durable sources ofresistance genes exist. Priority should alsobe given to DMC as this is another traitthat, although having a high narrow senseheritability at the time of evaluation (usuallyafter the onset of the rains to permitplanting immediately thereafter), is significantlyaffected by non-genetic factors andis not as highly heritable. There are severalinitiatives to assist national programmesacquire new molecular tools to increase thecost-effectiveness of breeding. Prominentamong these are the “molecular breedingcommunities of practice” project of theGeneration Challenge Programme (GCP,www.generationcp.org) and the RockefellerFoundation-funded African MolecularMarker Network (AMMANET, www.africancrops.net/ammanet).Both have trainingprogrammes on molecular breeding that areopen to national programme scientists. TheCIAT cassava project has also developeda Web-based database resource includingprotocols, populations, and markers forMAS in cassava that can easily be accessedby national programmes (www.ciat.cgiar.org/mascas).Cassava and common beans: contrastsCassava and beans are similar with respectto the modest level of research input theyhave enjoyed over the past three to five decades.Both have been part of the researchagenda of CIAT and of the CGIAR fornearly thirty years, and especially beanshave benefited from inputs from laboratoriesand programmes in the UnitedStates of America and, to a lesser degree,Europe. However, research investments forhigh-scale genomics through marker developmentin these crops has been far less thanfor the “super crops” like maize, rice orsoybean that enjoy participation by the privatesector, but are more than minor orphancrops with local usage in the tropics.Yet biologically, these two crops arewidely contrasting. Cassava is a perennialversus beans, which are short-seasonannuals, although climbing beans at highaltitudes can be similar to cassava in growthcycle. Beans are an autogamous seed cropwhile cassava is an allogamous crop withvegetative propagation. Accompanying thislatter dichotomy are differences in geneaction. Beans present largely additive gene