GENETIC-LINKAGE MAPS OF EUCALYPTUS-GRANDIS AND EUCALYPTUS-UROPHYLLA USING A PSEUDO-TESTCROSS - MAPPING STRATEGY AND RAPD MARKERS

We have used a ''two-way pseudo-testcross'' mapping strategy in combin ation with the random amplified polymorhic DNA (RAPD) assay to constru ct two moderate density genetic linkage maps for species of Eucalyptus , In the cross between two heterozygous individuals many single-dose R APD markers will be heterozygous in one parent, null in the other and therefore segregate 1:1 in their F-1 progeny following a testcross con figuration. Meiosis and gametic segregation in each individual can be directly and efficiently analyzed using RAPD markers. We screened 305 primers of arbitrary sequence, and selected 151 to amplify a total of 558 markers. These markers were grouped at LOD 5.0, theta = 0.25, resu lting in the maternal Eucalyptus grandis map having a total of 240 mar kers into 14 linkage groups (1552 cM) and the paternal Eucalyptus urop hylla map with 251 markers in 11 linkage groups (1101 cM) (n = 11 in E ucalyptus). Framework maps ordered with a likelihood support greater t han or equal to 1000:1 were assembled covering 95% of the estimated ge nome size in both individuals. Characterization of genome complexity o f a sample of 48 mapped random amplified polymorphic DNA (RAPD) marker s indicate that 53% amplify from low copy regions. These are the first reported high coverage linkage maps for any species of Eucalyptus and among the first for any hardwood tree species. We propose the combine d use of RAPD markers and the pseudo-testcross configuration as a gene ral strategy for the construction of single individual genetic linkage maps in outbred forest trees as well as in any highly heterozygous se xually reproducing living organism. A survey of the occurrence of RAPD markers in different individuals suggests that the pseudo-testcross/R APD mapping strategy should also be efficient at the intraspecific lev el and increasingly so with crosses of genetically divergent individua ls. The ability to quickly construct single-tree genetic linkage maps in any forest species opens the way for a shift from the paradigm of a species index map to the heterodox proposal of constructing several m aps for individual trees of a population, therefore mitigating the pro blem of linkage equilibrium between marker and trait loci for the appl ication of marker assisted strategies in tree breeding.