GENETIC-MAPPING OF QTLS CONTROLLING VEGETATIVE PROPAGATION IN EUCALYPTUS-GRANDIS AND E-UROPHYLLA USING A PSEUDO-TESTCROSS STRATEGY AND RAPDMARKERS

We have extended the combined use of the ''pseudo-testcross'' mapping strategy and RAPD markers to map quantitative trait loci (QTLs) contro lling traits related to vegetative propagation in Eucalyptus. QTL anal yses were performed using two different interval mapping approaches, M APMAKER-QTL (maximum likelihood) and QTL-STAT (non-linear least square s). A total of ten QTLs were detected for micropropagation response (m easured as fresh weight of shoots, FWS), six for stump sprouting abili ty (measured as # stump sprout cuttings, #Cutt) and four for rooting a bility (measured as % rooting of cuttings, %Root). With the exception of three QTLs, both interval-mapping methods yielded similar results i n terms of QTL detection. Discrepancies in the most likely QTL locatio n were observed between the two methods. In 75% of the cases the most likely position was in the same, or in an adjacent, interval. Standard ized gene substitution effects for the QTLs detected were typically be tween 0.46 and 2.1 phenotypic standard deviations (sigma p), while dif ferences between the family mean and the favorable QTL genotype were b etween 0.25 and 1.07 sigma p. Multipoint estimates of the total geneti c variation explained by the QTLs (89.0% for FWS, 67.1% for #Cutt, 62. 7% for %Root) indicate that a large proportion of the variation in the se traits is controlled by a relatively small number of major-effect Q TLs. In this cross, E. grandis is responsible for most of the inherite d variation in the ability to form shoots, while E. urophylla contribu tes most of the ability in rooting. QTL mapping in the pseudo-testcros s configuration relies on within-family linkage disequilibrium to esta blish marker/trait associations. With this approach QTL analysis is po ssible in any available full-sib family generated from undomesticated and highly heterozygous organisms such as forest trees. QTL mapping on two-generation pedigrees opens the possibility of using already exist ing families in retrospective QTL analyses to gather the quantitative data necessary for marker-assisted tree breeding.