marker-assisted selection in wheat - ictsd

Chapter 11 – Marker-assisted selection in poultry 187Current status of chickenbreeding programmesPoultry production has been the fastestgrowing livestock industry over the lastdecades especially in middle- and lowincomecountries (Taha, 2003). In 2001,poultry production accounted for 70 milliontonnes of poultry meat and 47 milliontonnes of eggs (Arthur and Albers, 2003).Among poultry, chicken account for 85 percentof meat production and 96 percent ofegg production (Bilgili, 2001; Arthur andAlbers, 2003; Taha, 2003). While chickenshave been domesticated and selectedfor thousands of years, modern poultrybreeding started during the 1950s. One ofthe most notable features is the diversificationbetween chickens bred for meatproduction (broilers) and those bred fortable egg production (layers). This is aresult of the negative genetic correlation inchicken between growth and reproductivetraits. Within breeds, there is a separationinto male and female lines that are crossedto produce commercial hybrids. In broilers,male lines are selected for growth and carcassquality whereas in female lines lessemphasis is placed on growth and moreon reproductive traits such as egg productionand hatchability. In table egg-layingchickens, male lines are selected for high eggproduction and high egg weight whereas infemale lines selection may emphasize rateof lay with less attention to egg size. Inboth broiler and layer lines the primaryselection goal is the improvement of feedefficiency and economic gain.Significant heterosis for fitness traits inpoultry is well established and all commercialpoultry (chickens, turkeys and ducks) arehybrids that are produced in a selection andmultiplication pyramid that is illustrated inFigure 1. Crossing male and female linesmaximizes heterosis at the grandparentand parent levels of the hierarchy, andallows traits that have been geneticallyimproved in different lines to be combinedin the commercial birds. The power of thisstructure to deliver large economic gains inchickens is a result of their high reproductiverate and short generation interval and isclearly illustrated by this example of anegg-laying improvement programme. Evengreater numerical efficiency is possiblein broilers: a single pen containing tenfemales and one male at the nucleus levelmight produce 150 great-grandparentsafter selection (line D of Figure 1); thesewill produce 50 female offspring each or7 500 grandparents in a year and thesegrandparents will generate 375 000 femaleparent stock during the succeeding year.These hybrid parent females will eachproduce over 130 male and female offspringand generate nearly 50 million commercialbroilers or 70 000 tonnes of meat. Thefigure illustrates the rapidity with whichgenetic improvement at the nucleus levelcan be disseminated to commercial flocksand the fact that relatively few pure-linebirds are needed to produce very largenumbers of commercial layers.The existence of this breeding structureresults in rapid transmission of geneticchange to commercial flocks (about fouryears), including traits that might beimproved by MAS. Conversely, undesirablegenetic change can also be disseminatedvery quickly to a very large number ofbirds. In practice, far more birds are keptat the nucleus level than shown in Figure 1where the numbers presented are purely forillustrative purposes.Status of functional genomics inchickenAmong the various livestock species, chickenhas the most comprehensive genomic tool-