marker-assisted selection in wheat

78Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishc16, c23bot, c25 and A03. Furthermore,it is worth noting that the BC 4 plant No.104 presented in Figure 3, which had introgressedall these five regions, also displayedthe highest fibre strength value of its generation(39.1 g/tex, compared with 33.1 g/texfor the Guazuncho 2 parent). The concomitantintrogression of G. barbadense allelesdisplaying positive marker-trait associationsfor other fibre properties such as length orfineness was also observed. This translatedinto the development of different highlyvaluable BC progenies. These preliminaryresults suggest that the improvement ofG. hirsutum fibre properties through theintrogression of G. barbadense fibre QTLappears feasible.DiscussionIn an attempt to overcome the limitationsof conventional breeding for improvingcotton fibre quality through the use ofinterspecific hybridization, molecularmarkers were used in a MAS scheme toimprove the efficiency of introgressingfibre quality traits. The advanced backcross-QTL(AB-QTL) strategy (Tanksleyand Nelson, 1996) was used as this allowedconcomitant development of a genetic mapof the cotton genome and analysis of fibrequality QTL, and attempts to introgressfavourable alleles in an adequate recipientgenetic background (Figure 1).In contrast to monogenic characteristicssuch as disease and insect resistance,many important traits including yield andquality show continuous phenotypic variationand are governed by a number ofQTL. Cotton fibre quality is a complexconcept that involves a number of traits orcharacteristics. Each of these is under theinfluence of numerous QTL, indicatinga complex genetic determinism. Indeed,from the present results, at least six QTLgovern fibre uniformity and up to 21 QTLinfluence fibre fineness. When consideringsix traits that can account for fibrequality, a total of 80 QTL were detected(Table 2). This figure falls within the samerange as that found by Paterson et al.(2003). As some of these QTL co-localizedwithin the same chromosome region, bychoosing those QTL whose positive allelederived from the donor parent and had thestrongest effect on economically importantfibre characteristics, the number of targetregions to be introgressed was reduced to19 (Table 3). Nevertheless, this number ofQTL remains too high to identify a singleplant that would carry them all. Indeed, inthe authors’ experience, at the BC 3 stage,single plants carried a maximum of fiveregions of interest (eight if consideringregions only partially introgressed), whileat the BC 4 stage, this number was reducedto four (seven if considering regions onlypartially introgressed).At this stage of the MAS process, tworoutes are under way (Figure 1). The firstinvolves identifying the best BC 4 plants,i.e. those showing the highest amount offavourable QTL introgression, and thenfixing the favourable allele by self-pollination.Such BC 4 S 1 plants have been crossedwith other BC 4 S 1 plants of different ascentin order to pyramid as many QTL as possible(each contributing to different traits)within the same genome. Similarly, BC 4 S 1plants were used to pyramid various QTLresponsible for a given trait (“selectivepyramiding”). This latter strategy couldespecially apply to traits of commercialimportance, such as fibre strength or fineness.The second avenue involves repeatingthe backcrossing process until near isogeniclines differing only at a given QTL (QTL-NILs) are developed. Such plant materialcould prove useful not only to study the