marker-assisted selection in wheat

Chapter 14 – Marker-assisted selection in Eucalyptus 265detailed mating system study in a E. grandisorchard in Madagascar, the outcrossing ratewas found to be 96.7 percent but a pollinationrate from outside the seed orchardof 39.2 percent was estimated based on sixmicrosatellite markers (Chaix et al., 2003).The ability to determine paternity preciselyusing DNA markers was recentlyproposed as a short-term breeding tacticfor Eucalyptus. The conventional wayto drive modifications in old forest treeseed orchards is to establish progeny trialsinvolving each parent tree and then evaluateits contribution to the performanceof the progeny by estimating its generaland specific combining ability (GCA andSCA). Grattapaglia, Ribeiro and Rezende(2004) successfully applied an alternativeretrospective parent selection tactic basedon paternity testing of superior offspring.After identifying seed mixtures, selfedindividuals and offspring sired by pollenparents outside the orchard, one particularpollen parent was found to have siredsignificantly more high-yielding progenytrees. Based on these results, low reproductivesuccess parents were culled fromthe orchard and management procedureswere adopted to minimize external pollencontamination. A significant difference(p < 0.01) in mean annual increment wasobserved between forest stands producedwith seed from the orchard before andafter selection of parents and revitalizationof the orchard. An average realizedgain of 24.3 percent in volume growth wasobtained from the selection of parents asmeasured in forest stands at age two to fouryears. The marker-assisted tree breedingtactic efficiently identified top parents in aseed orchard and resulted in an improvedseed variety. It should be applicable for rapidlyimproving the quality of output fromseed orchards especially when the breederis faced with an emergency demand forimproved seeds (Figure 4).Molecular breedingMolecular markers and maps forEucalyptusIn the last ten years a number of studieshave reported genetic maps for Eucalyptusbuilt from combinations of several hundredRAPD, AFLP or RFLP markers(Grattapaglia and Sederoff, 1994; Verhaegenand Plomion, 1996; Marques et al., 1998;Myburg et al., 2003), together with RFLP,isozymes, EST, genes and some microsatellites(e.g. Byrne et al., 1995; Gion et al.,2000; Bundock, Hayden and Vaillancourt,2000; Thamarus et al., 2002; Brondani etal., 2002). In contrast to crop species wheremapping populations are designed basedon contrasting inbred lines, map constructionin eucalypts has relied on availablepedigrees drawn from operational breedingprogrammes. These pedigrees generallyinvolve only the highly heterozygous parentsand their F 1 progeny, either full-sibsof half-sibs. Genetic mapping has thereforebeen carried out using a pseudo-testcrossstrategy, analysing dominant markerspresent in one parent and absent in theother (Grattapaglia and Sederoff, 1994).Maps are therefore individual-specific andcannot be aligned or integrated as suchunless other markers common to both mapsare also used. Consequently, although somegenome maps of eucalypts have been constructed,the use of the linkage informationtends to remain restricted to the pedigreeemployed as the mapping population,limiting the interexperimental sharing oflinkage mapping and QTL data generated.It is now well accepted that true advancementsin QTL validation across pedigrees forthe eventual practice of MAS in Eucalyptus,will strongly depend on the availability of