marker-assisted selection in wheat - ictsd

318Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishTable 3Genetic gain in schemes with heritability (h 2 ) of 0.06 or 0.12Generation number1 2 3 4h 2 = 0.06Conventional 0.176 0.121 0.134 0.117MAS 0.202 0.203 0.135 0.114h 2 = 0.12Conventional 0.337 0.206 0.196 0.191MAS 0.361 0.318 0.206 0.177genetic variance (as long as the frequencyof the positive allele is less than 0.5). Forsituations with a higher h 2 of 0.12, geneticgain was 7 percent higher after selection ingeneration 1 and 54 percent higher afterselection in generation 2, i.e. the superiorityof MAS was somewhat lower for schemeswith the higher heritability.Identification of individualsusing genetic markersIn traditional family-based breeding programmes,individuals from the same full-sibfamilies are reared separately (e.g. in tanks)until they are large enough to be taggedphysically. This mode of rearing is verycostly, and the number of full-sib familiestherefore limits the size of the breedingnucleus. In addition, separate rearing offull-sib families results in common environmental(tank) effects that need to beestimated, which in turn affects other partsof the design and analysis of the breedingprogramme. That is, in the mating design, asire has to be mated to several dams (or viceversa) in order to be able to separate analyticallythese common environmental effectsfrom additive genetic effects. The tank effectis generally higher for newly domesticatedspecies, where feed and other environmentaleffects are not yet standardized.For example, the tank effect was 3–12 percentfor juvenile Atlantic cod (Gjerde etal., 2004), and the nursery pond effect was32 percent for rohu carp (Gjerde et al.,2003) compared with, for example, a tankeffect of 2–6 percent for Atlantic salmonand rainbow trout (Rye and Mao, 1998;Pante et al., 2002). Were it possible to poolprogeny groups into one tank, tank effectswould be eliminated, a smaller number oftanks would be needed per spawner andmore pairs could be spawned.Parentage assignment using molecularmarkers is useful for tracing pedigrees inbreeding programmes, and can be used toidentify parents in breeding schemes whereprogeny groups are pooled. Parentageassignment implies that parents and offspringare all genotyped for a numberof genetic markers that are well spreadover the genome and that the informationon genotypes is used to assign individualprogeny to the correct parental pair. Theparent-offspring relationship is such thateach offspring inherits one allele from eachparent, making it possible to exclude possibleparents when this condition is notfulfilled.Exclusion of parents is the basic methodof assigning parents. The exclusion probabilityper locus (E l ) can be calculatedaccording to the formulae of Hanset (1975)and Dodds et al. (1996). The global exclusionprobability over loci (E g ) is:E g = 1 - Π = (1 -= 1llLE l)