marker-assisted selection in wheat - ictsd

332Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fish10 percent for body weight in a breedingprogramme for coho salmon (CMG-IFOP)initially funded by FAO (Martinez andHidalgo, unpublished data), and a similarresponse was obtained for this species inthe United States of America (Hershbergeret al., 1990). Estimates for tilapia followlargely the same trend, with a response ofabout 10 percent (Ponzoni et al., 2005). Incommon carp, responses to selection forbody weight were inconsistent between upselectedand down-selected lines, althoughexhaustion of additive genetic variationfor increased growth rate, genotype-byenvironmentinteraction, or competitioneffects could not be ruled out (Moav andWohlfarth, 1976). In oysters, asymmetricalresponse to selection for body weight wasfound (Toro et al., 1995; Ward, English andMcGoldrick, 2000).Although responses to selection havenot been well documented, significant estimatesof genetic parameters have beenobtained for carcass traits (Gjerde andSchaeffer, 1989; Kause et al., 2002; Quinton,McMillan and Glebe, 2005) and diseaseresistance (Gjøen et al., 1997; Henryon etal., 2002, 2005). Rates of genetic gain areexpected to be lower for these traits thanfor body weight because breeding valuepredictions rely solely on measurementsfrom relatives.Several breeding programmes have beeninitiated recently for new aquaculture species,such as mussels, scallops, Artemia andshrimp. The biology of these species posesinteresting avenues for the design of conventionalbreeding programmes, taking intoaccount factors such as self-fertilization,intrafamily competition, cannibalism, lackof methods for physical tagging, and matingpreferences. For example, competition canaffect the expression of quantitative traitsdue to co-variances among members of agroup managed together in a pond or tankand, if not considered properly, this effectcan seriously affect the rates of response toselection (Muir, 2005). However, this effectcan be included explicitly in the model ofanalysis using the co-variance among membersof a group, the so-called “associativeeffects” from other genotypes in the group.The theory of Griffing (1967) for BLUPevaluation was developed in the context oftree breeding, but deserves further investigationin the analysis of fish and shellfishbreeding. This may be especially true forspecies taken recently from the wild orthose that show cannibalistic behaviour.Another recent example is the developmentof scallop breeding programmes.Argopecten purpuratus is a simultaneouslyhermaphroditic species. In the first breedingphase, the scallop liberates sperm, afterwhich the eggs are expelled. To decreasethe level of self-fertilization, it is customaryto use only the last pulses of eggs. Thissystem reduces rates of self-fertilizationto 20 percent (A. Vergara, personal communication),but a residual proportion ofeggs are still already fertilized with spermfrom the same individual. As this processoccurs within the reproductive tract, itis not possible to detect which individualsare selfed or outcrossed, althoughthe rate of residual self-fertilization varieswidely among families and produces biasedestimates of heritability (Martinez and diGiovanni, 2006). Information from molecularmarkers can be of benefit under thesecircumstances (see below).DNA markers used in aquacultureMutations in the genome create geneticvariability (or polymorphism), whichis reflected as allelic diversity of molecularmarkers. While genomic sequencingwould greatly facilitate the development