marker-assisted selection in wheat

292Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishand haplotype analyses in 290 trees froma natural population, they observed twohaplotypes that were significantly associatedwith microfibril angle, a major determinantof timber strength. These results wereconfirmed in a full-sib family in E. nitensand in the related species E. globulus. In apowerful demonstration of the resolutionof association genetics, Thumma et al.(2005) detected an alternatively-splicedvariant of the CCR gene from the region ofthe significant haplotype, thereby revealingthe probable molecular basis of the traitvariation.Association mapping is a particularlyuseful approach when genes are availablethat are likely to be functionally relevantto the trait of interest. Homologues ofgenes characterized in model species suchas Arabidopsis, maize or rice, and poplarare excellent targets for association studiesin forest species. In most cases, putativeorthologues can be identified by comparingESTs to gene sequences in public databases.In some cases, gene function may bedetermined by modulating the expressionof selected genes using sense and antisensemodification to up- and down-regulategene expression, or intron RNA hairpinconstructs to silence genes (Smith et al.,2000). However, one of the advantages ofassociation studies is the capacity to studya large number of genes simultaneouslywithout the need for transformation (Peterand Neale, 2004).There is considerable interest in understandingthe genes controlling wood fibrecell wall development in forest trees asfibre microstructure is a major determinantof the commercial value of wood. Forexample, the angle of orientation of cellulosemicrofibrils (MFA) in fibre cell wallsis known to affect timber strength andstiffness as well as fibre collapsibility, animportant determinant of tensile strength inpaper. Knowledge of cell wall biosynthesiswould also assist in understanding andmanipulating the development of abnormalwood, e.g. tension/compression wood (seePaux et al., 2005; Pavy et al., 2005), whichis known to have an impact on woodstability, sawing patterns and pulpability.Wood is primarily composed of secondaryxylem, and its properties are the product ofsequential developmental processes fromcambial cell division and expansion, to secondarywall formation and lignification.Genes expressed during xylogenesis determinethe physical and chemical propertiesof wood. Important genes are now beingidentified that control the synthesis of themajor constituents of the cell wall: cellulose,hemicellulose and lignin. Genes forcellulose synthesis (CesA) have been clonedin aspen (Joshi, Wu and Chiang, 1999; Wu,Joshi and Chiang, 2000), poplar (Djerbiet al., 2005) and loblolly pine (Nairn andHaselkorn, 2005). Characterizing the CesAgene in aspen revealed strong similaritywith a secondary cell wall protein in cotton,indicating that they serve similar functionsin the two evolutionarily divergent genera.Transformation of cellulose synthase genesin aspen (Joshi, 2004) should further elucidategene function. Each of the threeloblolly CesA genes is orthologous to oneof the three angiosperm secondary cell wallCesAs, suggesting functional conservationbetween angiosperms and gymnosperms.A search of the poplar genome revealed18 distinct CesA gene sequences in Populustrichocarpa (Djerbi et al., 2005). The CesAgenes belong to a superfamily of CesA-like(Csl) genes, which includes a very largenumber of glycosyltransferases that arelikely to be involved in the synthesis ofthe numerous non-cellulosic polysaccharidesin plants (Liepman, Wilkerson and