marker-assisted selection in wheat

Chapter 1 – An overview of the issuesIntroductionThe potential benefits of using markerslinked to genes of interest in breedingprogrammes, thus moving from phenotypebasedtowards genotype-based selection,have been obvious for many decades.However, realization of this potential hasbeen limited by the lack of markers. Withthe advent of DNA-based genetic markersin the late 1970s, the situation changedand researchers could, for the first time,begin to identify large numbers of markersdispersed throughout the genetic material ofany species of interest and use the markersto detect associations with traits of interest,thus allowing MAS finally to become areality. This led to a whole new field ofacademic research, including the milestonepaper by Paterson et al. (1988). This showedthat with the availability of large numbersof genetic markers for their species ofinterest (tomato), the effects and locationof marker-linked genes having an impact ona number of quantitative traits (fruit traitsin their case) could be estimated using anapproach that could be applied to dissectthe genetic make-up of any physiological,morphological and behavioural trait inplants and animals.Most of the traits considered in animaland plant genetic improvement programmesare quantitative, i.e. they are controlled bymany genes together with environmentalfactors, and the underlying genes have smalleffects on the phenotype observed. Milkyield and growth rate in animals or yieldand seed size in plants are typical examplesof quantitative traits. In classical geneticimprovement programmes, selection is carriedout based on observable phenotypesof the candidates for selection and/or theirrelatives but without knowing which genesare actually being selected. The developmentof molecular markers was thereforegreeted with great enthusiasm as it was seenas a major breakthrough promising to overcomethis key limitation. As Young (1999)wrote: “Before the advent of DNA markertechnology, the idea of rapidly uncoveringthe loci controlling complex, multigenictraits seemed like a dream. Suddenly, it wasdifficult to open a plant genetics journalwithout finding dozens of papers seeking topinpoint many, if not most, agriculturallyrelevant genes.” However, despite the considerableresources that have been investedin this field and despite the enormous potentialit still represents, with few exceptions,MAS has not yet delivered its expected benefitsin commercial breeding programmesfor crops, livestock, forest trees or farmedfish in the developed world. In developingcountries, where investments in molecularmarkers have been far smaller, delivery ofbenefits has lagged even further behind.The focus of this chapter is on the use ofmolecular markers for genetic improvementof populations through MAS, includingmarker-assisted introgression. Its aim is toprovide an easily understandable overviewof the techniques, applications and issuesinvolved in the use of DNA markers inMAS for genetic improvement of domesticplant and animal populations in developingcountries. In the next section of the chapter,a brief description of the technical aspectsof molecular markers and MAS is provided.The current status of the application ofMAS in crops, forestry, livestock and fishis then summarized, while the final sectionNote: This chapter is based on the Background Document to Conference 10 (on molecular marker-assistedselection as a potential tool for genetic improvement of crops, forest trees, livestock and fish in developingcountries) of the FAO Biotechnology Forum, 17 November–14 December 2003 (available at www.fao.org/biotech/C10doc.htm).