marker-assisted selection in wheat

188Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 1Multiplication pyramid for a four-strain commercial hybrid layerGenerationMale linesFemale linesTimePure lineselectionA1 ♂x 10 ♀B10 ♂x 100 ♀C10 ♂x 100♀D100 ♂x 1 000♀Year 0GrandparentsA200 ♂XB2 000 ♀C2 000 ♂XD20 000 ♀Year 1ParentsAB40 000 ♂XCD400 000 ♀Year 2CommercialhybridsABCD32.2 millionYears3 to 4The boxes in bold are the multiplication (crossing) phase. The top line of the boxes indicates the lowest level of the pure-line(nucleus) selection stage. The numbers in the boxes refer to the minimal numbers of birds in line A, and corresponding numbersin lines B, C and D, that are required to generate hybrid laying hens at the commercial level and the numbers that result fromthis process. The time scale is indicated on the right hand side of the figure assuming a generation interval of one year.Adapted from Bowman, 1974.box. The chicken genome consists of 39pairs of chromosomes: eight cytologicallydistinct macrochromosomes, the sexchromosomes Z and W and 30 pairs ofcytologically indistinguishable microchromosomes.Linkage maps were developedinitially using three separate mappingpopulations (Bumstead and Palyga, 1992;Crittenden et al., 1993; Groenen et al., 1998)that were later merged to provide a consensusmap with 1 889 markers (Groenenet al., 2000). A good overview of the consensuslinkage map and the cytogeneticmap can be found in the First Report ofChicken Genes and Chromosomes 2000 andits successor in 2005 (Schmid et al., 2000,2005). All chicken maps can be viewed atwww.thearkdb.org.More recently, the chicken genomebecame the first livestock genome to besequenced with a six-fold coverage (sixfull genome equivalents) (Hillier et al.,2004). The chicken genome sequence can bebrowsed via a number of Web sites, whichare summarized at www.chicken-genome.org/resources/databases.html. The genomesequence effort was accompanied by partialsequencing of three distinct poultry breeds(a broiler, a layer and a Chinese Silky), toidentify SNPs between and among theseand the reference sequence of the RedJungle Fowl. This resulted in an SNP mapconsisting of about 2.8 million SNPs (Wonget al., 2004). The chicken polymorphismdatabase (ChickVD) can be browsed at:http://chicken.genomics.org.cn/index.jsp