Ploidy reduction in blackberry

Tetraploidy in blackberries and ploidy differences between Rubus L. species are obstacles to the efficient introduction of valuable germplasm from wild Rubus species. Breeding blackberries at the diploid level would permit rapid reconstruction of the polyploids at the cultivated level following introduction of desirable traits from wild species.;Selection of twin seed, interspecific hybridization, colchicine and toluidine blue (TB) pollen treatments (at 0, 0.025, 0.25 and 2.5 mM and 0, 0.3, 0.6, and 1.8 mM respectively) and pollen irradiation (at 0, 50, 100 and 150 kR) were compared as methods in the recovery of dihaploids from tetraploid blackberry cultivars. In addition, tetraploid raspberry (R. idaeus L.) blackberry (R. spp 'Shawnee') interspecific hybrids were screened for the production of viable super-reduced gametes to explore the possibility that genome fractionation in interspecific hybrids could be used to obtain diploids.;Ploidy reduction in several tetraploid blackberry cultivars was obtained following pollinations with 100 and 150 kR gamma irradiated pollen. Most of the seedlings obtained at the 50 kR dosage were aneuploid. One dihaploid was obtained from twin seed and several were obtained following interspecific hybridizations. No dihaploids were obtained following pollen treatments with colchicine or TB and no diploids were obtained from crosses with tetraploid interspecific hybrids.;Pollen irradiation at 150 kR was the most efficient method of obtaining dihaploids from tetraploid blackberries of the methods examined. Twenty percent of the seedlings obtained following this treatment were dihaploid.