marker-assisted selection in wheat - ictsd

128Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishand selecting individuals at a few lociat early generations, usually F 2 or F 3 ,(Eathington, Dudley and Rufener, 1997),using large populations (Ribaut and Betrán,1999). Individuals displaying homozygousfavourable genotypes at the loci of interestare selected and self-pollinated while othersare discarded. Self-pollinated progeny ofthe selected plants then proceed normallythrough subsequent steps of pedigreebreeding. Screening large populations isnecessary to ensure that genetic diversity ismaintained at regions not under genotypicselection, thereby allowing furtherphenotypic selection to be conducted.Loci at which marker selection operatescan be QTL as described by Ribaut andBetrán (1999). SLS-MAS is thus limitedby issues such as the precision of the QTLparameters (position, effect), and relevanceof the QTL across environments or genepools. SLS-MAS can also be conducted forgenes, eliminating many of the limitationspertaining to QTL. Although a powerfulapproach adopted in several species (barley,soybean, sunflower, wheat) to enrichbreeding populations at a few loci (Crosbieet al., 2006), SLS-MAS does not appear tohave been widely implemented in maizebreeding programmes.MARS targets all traits of importancein a breeding programme and for whichgenetic information can be obtained.Genetic information is usually obtainedfrom QTL analyses performed on experimentalpopulations and comes in the formof maps of QTL with their correspondingeffects. If the QTL mapping analysisis conducted based on a bi-parental population,the sign of the effect at each QTLindicates which of the two parents carriedthe favourable allele at that QTL. As bothparents often contribute favourable alleles,the ideal genotype is a mosaic of chromosomalsegments from the two parents. Thisassumes that the goal is to obtain individualswith as many accumulated favourable allelesas possible, a different goal from that ofmarker-assisted population improvement asstudied elsewhere (Lande and Thompson,1990; Gimelfarb and Lande, 1994; Gallais,Dillmann and Hospital, 1997; Hospital,Chevalet and Mulsant, 1997; Knapp, 1998;Moreau et al., 1998; Xie and Xu, 1998).Population improvement schemes aregenerally based on the random matingof selected individuals while the schemeproposed here is based on directed recombinationbetween specific individuals. Asreported by Stam (1995), the ideal genotype,defined as the mosaic of favourablechromosomal segments from two parents,will usually never occur in any F n populationof realistic size. It is, however, possibleto design a breeding scheme to produceor approach this ideal genotype based onindividuals of the experimental population.This breeding scheme could involve severalsuccessive generations of crossing individuals(Stam, 1995; Peleman and van der Voort,2003) and would therefore constitute whatis referred to as MARS or genotype construction.This idea can be extended tosituations where favourable alleles comefrom more than two parents (Stam, 1995;Peleman and van der Voort, 2003).Van Berloo and Stam (1998, 2001) andCharmet et al. (1999) developed computersimulations around this idea and assessedthe relative merits of marker-assistedgenotype construction over phenotypicselec-tion. MARS was simulated in anexperimental population where QTL hadbeen mapped. Index (genetic) values werecomputed for each individual based onits genotypes at QTL-flanking markers(van Berloo and Stam, 1998, 2001). Allsimulation studies of MARS found that