marker-assisted selection in wheat - ictsd

Chapter 17 – Marker-assisted selection in fish and shellfish breeding schemes 345Figure 4Deterministic power calculation (following Martinez, Hill and Knott, 2002) for a QTL explaining10 percent of the phenotypic variance for a variable total population size (x axis) andfamily size (25, 50 and 100)10.90.8Power of detecting QTL0.70.60.50.40.30.20.125 50 1000100 200 300 400 500 600 700 800 900 1 000Population sizerobust for detecting QTL and there waslittle loss of information for detecting QTLwhen dominance is present but not used inthe QTL mapping analysis.QTL mapping in practiceTo date, QTL mapping in fish using outbredpopulations has been carried out mostlywith single marker analysis (microsatellitesand AFLP markers), and using relativelysparse linkage maps when interval mappingis used. In tilapia, the F 2 design and afour-way cross between different species ofOreochromis have been used for detectingQTL affecting cold tolerance and bodyweight (Cnaani et al., 2003; Moen et al.,2004c). In outbred populations of salmonids,QTL that influence body weight havebeen mapped (Reid et al., 2005 and referencestherein).Studies seeking linkage of markers totraits amenable to MAS, such as diseaseresistance, have begun to appear in the literatureover the past few years. For example,QTL for resistance have been mappedfor infectious pancreatic necrosis virus(Ozaki et al., 2001), infectious salmonidanemia (Moen et al., 2004c), infectioushaematopoietic necrosis (Rodriguez et al.,2004; Khoo et al., 2004), and stress andimmune response (Cnaani et al., 2004).Also, Somorjai, Danzmann and Ferguson(2003 and references therein) reported evidenceof QTL for upper thermal tolerancein salmonids with differing effects in differentspecies and genetic backgrounds.From fine mapping to finding genesinfluencing complex traitsWhen the number of meioses in thegenotyped pedigree is not sufficient for thelinkage analysis to obtain a precise positionfor the QTL, there is a wide confidenceinterval around an estimated QTL position.