MOLECULAR-GENETICS OF GROWTH AND DEVELOPMENT IN POPULUS .1. TRIPLOIDYIN HYBRID POPLARS

While constructing a genetic linkage map of a hybrid poplar genome (Po pulus trichocarpa x P. deltoides), we identified several restriction f ragment length polymorphismus (RFLPs) for which the parental trees are heterozygous. Although 8 of the 11 F1 hybrid offspring inherited, as expected, single RFLP alleles from each parent, 3 F1 trees in the mapp ing pedigree inherited both maternal alleles along with a single pater nal allele at some loci. Aneuploidy or polyploidy in these 3 F1 trees due to partial or complete nondisjunction during female gametogenesis is the simplest explanation for this finding. Of the 3 F1 offspring wi th supernumerary RFLP alleles 2 have triploid nuclear DNA contents as measured by fluorescence flow cytometry; the 3rd F1 with supernumerary alleles has a sub-triploid nuclear DNA content and is probably aneupl oid. Among the tri/aneuploid hybrids, leaf quantitative traits either are skewed toward those values characteristic of the P. trichocarpa fe male parent (adaxial stomate density, petiole length: blade length rat io; abaxial color) or show transgressive variation (epidermal cell siz e). Abaxial leaf color was use to screen a large population of P. tric hocarpa x P. deltoides hybrids for further evidence of tri/aneuploidy. In each case where a ''white'' abaxial leaf surface was observed and the nuclear DNA content measured, the hybrid proved to be tri/aneuploi d. All sexually mature female triploids examined were sterile, althoug h the inflorescences completed their development in the absence of emb ryo formation. The (probably) aneuploid F1 hybrid is a fertile female. Of 15 female P. trichocarpa parents used in crosses to P. deltoides, 10 produced one or more tri/aneuploid hybrid offspring. In an intraspe cific cross using a P. trichocarpa female that had produced triploid h ybrids with five different P. deltoides males, no tri/aneuploid offpsr ing were found.