PLOIDY LEVEL EVALUATION OF ROOTSTOCK SELECTIONS AND PYRUS GENUS ACCESSIONS

The INRA pear rootstock breeding programme makes use of interspecific hybridization between the new INRA pear rootstock ‘Pyriam’ as female and Pyrus species as male parents to develop improved rootstocks. Pyrus species germplasm contains a large range of phenotypes and the INRA pear rootstock breeding programmes have led to a wide range of dwarfing to vigorous selections. As the ploidy level of this material has not yet been tested, cytogenetic characterization was used to optimize hybridization and see whether some of the differences in growth habit or vigour are due to ploidy level. Cytogenetic characterization was done by flow cytometry, using DAPI fluorochrome to evaluate A-T DNA content. ‘Pyriam’ rootstock was used as diploid control. Results show that the Pyrus genus is very homogeneous for ploidy level: a great majority of species are diploids and the intraspecific variation in A-T DNA bases is very low. Pyrus species have a small content of A-T DNA bases compared to Pisum sativum; P. communis content is smaller than P. calleryana. Diploid, triploid and aneuploid genotypes were found in P. nivalis. The vigorous, easy-rooting commercial rootstocks used as controls were found to be diploids whereas the difficult to root INRA dwarfing rootstock selections were found to be aneuploids. This concurs with the hypothesis that vigour reduction by P. nivalis rootstock selections might be linked to aneuploidy.