marker-assisted selection in wheat - ictsd

312Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishis to develop fish that could reproduceand grow in areas of higher salinity andlower temperature, i.e. to increase theproduction area for tilapias. Althoughthese traits could be measured early inthe life of the fish and therefore couldbe selected, sexually mature fish mayrespond differently to the temperatureand salinity conditions.Gjedrem and Olesen (2005) provide amore complete list of aquaculturally relevanttraits and their heritabilities andcorrelations.Structure of breeding schemesMost aquaculture species are currently bredin mono- or polyculture systems (i.e. withone or several species reared together) usingmass selection for growth rate. Only about30 family-based breeding programmesworldwide utilize sib information in theestimation of breeding values (B. Gjerde,personal communication). The main partof a family-based breeding programme isa closed breeding nucleus, with trait informationfrom sibs coming from test stations.Breeding programmes for species with limitedreproductive ability (e.g. salmonids asopposed to several highly fecund marinespecies such as Atlantic cod) have a multiplierunit, where genetic material fromthe nucleus is used to produce eggs or fryfor the grow-out producers. The limitingfactor for the breeding nucleus is often thenumber of tanks, where the fry of a certainfull-sib family are kept until individuals arelarge enough to be physically tagged. Thenumber of offspring per full-sib family islarge, such that a very high selection intensitycan be achieved. Generally, each male ismated to two females in order that the tankeffect can be estimated separately from theadditive genetic effects.The high intensity of selection withinthe nucleus can easily result in high rates ofinbreeding. Introduction of unrelated wildstock is often practised to reduce the rates ofinbreeding. However, introduction of wildstock also leads to reduced genetic gain,and should generally be avoided in ongoingbreeding schemes. Optimum contributionis an approach that maximizes genetic gainwhile restricting the rates of inbreeding forschemes with discrete (Meuwissen, 1997;Grundy, Villanueva and Woolliams, 1998)or overlapping (Meuwissen and Sonesson,1998; Grundy, Villanueva and Woolliams,2000) generation structures or for traitswith a polygenic effect and the effect ofa known single gene (Meuwissen andSonesson, 2004). The key determinationis the number of matings (full-sib families)per selected individual. One practicalconstraint in some marine species is thatmatings are volitional (natural mating in,for example, a tank) and thus depend on theavailability of individuals ready to spawn ata certain moment. Hence, for these species,the number of matings per male or femaleis restricted for each spawning. The use offrozen milt makes the use of males moreflexible. Milt from many species includingsalmonids, carp and shrimps can be frozen(Stoss, 1983), but the practical use of cryopreservedsperm in aquaculture breedingprogrammes has not been fully utilized.Genetic marker mapsA genetic marker map is an ordered listingof the genes or molecular markers occurringalong the length of the chromosomesin the genome. Distances between genesor markers are estimated in terms of howfrequently recombination occurs betweenthem. Genetic marker maps are available forsome aquaculture species (Table 1). Most ofthese genetic maps are constructed usingamplified fragment length polymorphism