marker-assisted selection in wheat

Chapter 16 – Possibilities for marker-assisted selection in aquaculture breeding schemes 313(AFLP) markers (Vos et al., 1995), whichare generally anchored to a smallercollection of microsatellites. The markerdensity of the maps is currently rather low,and the markers are spread unevenly overthe genome, which may explain why thenumber of linkage groups found in, forexample, channel catfish (Waldbieser et al.,2001) or white shrimp (Pérez et al., 2004)does not correspond to the number ofchromosomes. In rainbow trout, tetraploidyhas been found for 20 chromosome arms(Sakamoto et al., 2000). Recombinationrates can differ between males and females,and hence marker map lengths can differconsiderably between males and females.In salmonids, females have the higherrecombination rate, which implies that mostinformation comes from the females whenconstructing the marker map. The ratiobetween female and male recombinationrates was 3.25:1.00 for rainbow trout(Sakamoto et al., 2000), 1.69:1.00 for Arcticchar (Woram et al., 2004) and 8.25:1.00for Atlantic salmon (Moen et al., 2004a).However, in other aquaculture species,males have the higher recombination rate.For example, the ratio between male andfemale recombination rates was 7.4:1.00 inJapanese flounder (Coimbra et al., 2003).There are also differences in recombinationrate over the length of the chromosomes inmales, i.e. recombination rate was higher intelomeric regions than in proximal regionsin rainbow trout (Sakamoto et al., 2000).Mapping of QTLQTL are loci whose variability underliesvariation in expression of a quantitativecharacter (Geldermann, 1975). Detectionof QTL would help in understanding thegenetic architecture of the trait, i.e. thenumbers and relative effects of genes thatdetermine expression of a trait. A small,Table 1Aquaculture species for which there are geneticmarker mapsSpeciesReferenceScallop Li et al. (2005)Wang et al. (2004)Pacific oyster Hubert and Hedgecock (2004)Eastern oyster Yu and Guo (2003)White shrimp Pérez et al. (2004)Kuruma prawn Li et al. (2003)Black tiger shrimp Wilson et al. (2002)Kuruma prawn Moore et al. (1999)Atlantic salmon Moen et al. (2004a)Gilbey et al. (2004)Arctic char Woram et al. (2004)Rainbow trout Nichols et al. (2003)Sakamoto et al. (2000)Young et al. (1998)Salmonids May and Johnson (1990)Common carp Sun and Liang (2004)European sea bass Chistiakov et al. (2005)Channel catfish Waldbieser et al. (2001)Liu et al. (2003)Tilapia Lee et al. (2005)McConnell et al. (2000)Agresti et al. (2000)Kocher et al. (1998)Japanese flounder Coimbra et al. (2003)but growing, number of QTL for importanttraits have been identified in farmedaquatic species (Table 2). In tilapias, QTLhave been identified for cold tolerance(Cnaani et al., 2003; Moen et al., 2004b).QTL for upper temperature tolerance(Jackson et al., 1998; Danzmann, Jacksonand Ferguson, 1999; Perry, Ferguson andDanzmann, 2003; Somorjai, Danzmannand Ferguson 2003), and for resistance todifferent disease traits have been foundin salmonids, e.g. for infectious hematopoieticnecrosis virus (Rodriguez et al.,2005), infectious pancreatic necrosis virus(Ozaki et al., 2001), Ceratomyxa shasta(Nichols, Bartholomew and Thorgaard,2003) and infectious salmon anemia (Moenet al., 2004c, 2006). QTL for general diseaseresistance and immune response have beenfound in tilapias (Cnaani et al., 2004) and