marker-assisted selection in wheat - ictsd

264Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 4Retrospective selection strategy of elite parents based on paternity testing of progenyindividuals displaying superior performance using microsatellite markersSeed orchardSeed harvest with maternal controlCommercial plantation by half- sib familyblocks or half-sib progeny trial?????????????ES76EG62EG65???????Identification of top trees for specificphenotypic traits of interestRetrospective selection of parents with highspecific combining ability (SCA) to be usedin controlled crosses and/or to cull fromseed orchard parents of low SCAPaternity testing of selected trees usingmicrosatellite markers to identifyprecisely theirpollen parentshalf-sib progeny testing is still commonpractice in some eucalypt breeding programmes.It is a low cost option thatallows good estimation of the breedingvalue of maternal parents. Nevertheless, alarge amount of genetic variation is usuallyencountered within half-sib families andselection intensity within families is limitedby the number of individuals usuallydeployed in a progeny test. Knowledge ofoutcrossing versus selfing rates is essentialfor maintaining adequate levels of geneticvariability for continuous gains. A numberof studies have shown that eucalypts arepreferentially outcrossed both in naturalpopulations as well as seed orchards.Isozyme markers were originally used(Moran, Bell and Griffin, 1989), but othertypes of markers now provide a muchhigher level of resolution. Outcrossing ratein an open pollinated breeding populationof E. urophylla was estimated at 93 percentusing RAPD markers, indicating predominantoutcrossing and maintenance ofadequate genetic variability within families(Gaiotto, Bramucci and Grattapaglia, 1997).A complex pattern of mating was describedin a E. regnans seed orchard in Australiawhere gene dispersal was influenced bycrop fecundity and orchard position ofmother trees with approximately 50 percentof effective pollen gametes comingfrom males more than 40 metres away frommother trees (Burczyk et al., 2002). In a