marker-assisted selection in wheat

Chapter 4 – Marker-assisted selection in wheat: evolution, not revolution 55FHBThe importance of FHB is less in its effecton yield reduction, but rather on the potentiallydamaging reduction in grain qualityassociated with infected grain, which canbe heavily contaminated by the fungal tricothecintoxins. An important source ofFHB resistance originates from the Chinesevariety Sumai 3, and a major component ofthis resistance (up to 50 percent) has beenassociated with a single QTL (Waldron etal., 1999; Anderson et al., 2001; Buerstmayret al., 2002). While this QTL is largelyeffective in preventing the spread of thepathogen following infection, a further QTLthat gives a significant degree of protectionagainst initial infection has been mapped toa different chromosome (Buerstmayr et al.,2003). Selection for FHB resistance by conventionalmeans is complicated both by thequantitative nature of the Sumai 3 resistanceand by difficulties in ensuring evenand reliable artificial infections in breedingnurseries. However, SSR-based MAS protocolshave been developed for both QTL(see http://maswheat.ucdavis.edu/ andBuerstmayr et al., 2003), and the urgencyof breeding for resistance has ensured thatincreasing use is being made of such assays.Both these QTL in concert do not explainall the genetic resistance of Sumai 3 to FHB,but the remainder appears to be determinedby QTL of minor effects and/or pleiotropiceffects associated with an ear morphology,which is inconsistent with a northwestEuropean winter wheat ideotype.STBSTB of wheat is caused by the fungusMycosphaerella graminicola (syn. Septoriatritici), and in recent years has become themajor leaf disease of wheat in many regionsof the world. In past years, good levels ofcontrol were achieved by the application ofstrobilurin fungicides, but their heavy usehas led to the emergence of pathogen strainsthat cannot be so easily controlled by chemicalmeans. A number both of major genesgiving near-complete resistance to specificraces of the pathogen and of quantitativerace non-specific resistances with polygenicinheritance have been defined, andone of the former, Stb6, which maps closeto the SSR locus Xgwm369 on chromosome3A (Chartrain, Brading and Brown,2004), is common in many gene pools. Thisensures that the gene has been retained inelite materials, and its known map positionhas made it relatively straightforward touse a marker assay to track its presence inbreeding populations.BYDVSignificant grain yield losses are attributableto natural infections of BYDV, and no majorsource of resistance has been identified todate in wheat. Control is achieved in theabsence of genetic resistance by insecticidalspray, which is associated with both aneconomic and an environmental cost.However, a potent resistance is present in therelated species Thinopyrum intermedium.It is possible to generate sexual hybridsbetween wheat and this grass, but the F 1plants are self-sterile and either have tobe rescued by chromosome doubling orback-crossed to wheat. By this route, adistal segment of the grass chromosomethat carries the BYDV resistance geneBdv2 has been introduced into wheat. Asthis introgression comprises a significantlength of non-wheat chromosome, it hasbeen relatively straightforward to generatemarkers suitable for MAS use (Ayala et al.,2001a; Zhang et al., 2004). A MAS approachfor screening is attractive because artificialinoculation involves the propagation ofvirus-bearing aphids, while natural infections