marker-assisted selection in wheat

Chapter 21 – Marker-assisted selection as a potential tool for genetic improvement: debating the issues 435progress had been made with multiple genetraits. Makkar (52) stated that many MASstudies had adopted a single trait approach,pointing out that with a multitrait breedingobjective, response for one trait often goesat the expense of another. He also suggestedthe utility of MAS when heritabilityfor the trait was low. Singh (41) indicatedthat “breeders are not much thrilled aboutMAS for simply inherited traits, and notmany QTL (especially the productivityrelated ones) with tightly linked markersare available”.Several other participants mentionedtraits that would be amenable to MAS,including Priyadarshan (11) working withrubber trees, Williams (37) who providedthe case of root nematodes and William(70) who mentioned work being done onbarley yellow dwarf virus resistance incereals, rust diseases, nematode resistanceand root health. Rakotonjanahary (78) alsosuggested that MAS be used when conventionalapproaches to selection were difficultor impossible. For example, Reddy (62)proposed MAS be used for traits where it isdifficult to get phenotypic data, like qualitytraits, and William (70) indicated that proteinassays to develop quality protein maizewere expensive compared with markerassays. Slaughter traits in livestock werealso considered to be amenable to MAS asthe desired traits are otherwise difficult tomeasure without killing the animal (Makkar,52). Muchugi (49) suggested the potentialusefulness of MAS in selecting for medicinaltraits and growth rate in tropical trees.Introgression of genes from wild intocultivated germplasm was proposed to bea good use of MAS (Bhagwat, 46). Notter(25) also commented on the opportunitiesmolecular markers provide for screeningpopulations of animals with favourableor unfavourable genotypes, giving as anexample scrapie in sheep. Krause (75) mentionedother genetic examples, such as asperm defect in pigs and the halothane geneimplicated in low pork quality, that couldbe screened out using MAS. Sex-linkedtraits were also mentioned as being suitablefor MAS (Makkar, 52).Galvez (10) suggested that molecularmarkers could be also useful for work withtransgenic crops, for characterizing GMplants and tracking movement of the transgenein the gene pool. William (70) alsomentioned the use of MAS for transferringa desirable transgene, such as a gene fromBacillus thuringiensis, from one cultivar toanother.In addition to discussing traits consideredamenable to MAS, brief mention wasmade of traits not considered amenable toMAS. It was realized that more progresshad been made with single genes that wererelatively easily transferred, but that therewas potential for facilitating QTL transfer,although this was still relatively undeveloped.Traits that are highly influenced bythe environment or production system,including crop yield (Priyadarshan, 11),were not considered easily amenable toMAS. Williams (37) pointed out that amajor problem associated with MAS waslack of polymorphism at the DNA level,which would render a trait not amenable toMAS. Inadequate coverage of the geneticmap with molecular markers was viewedby Dulieu (23) as an obstacle to applyingMAS. He also detailed other conditionsrelating to the nature of the trait that shouldbe considered for MAS to be efficient:single versus multigene, additive versusdominant, expressivity and penetrance.Practical applications of MASSome participants considered the actualimpact of MAS on genetic products deliv-