PLOIDY REDUCTION IN BLACKBERRY

Polyploidy in blackberries and ploidy differences between Rubus specie s are obstacles to the efficient introduction of valuable germplasm, b oth intra-and interspecific, into blackberry breeding programs. Expans ion of the germplasm base would be facilitated by reducing the ploidy level of blackberry cultivars to the diploid level. In this report sel ection of twin seed, interspecific hybridization, and pollen irradiati on were compared as methods in the recovery of dihaploids from tetrapl oid blackberry cultivars. One dihaploid was obtained through selection of twin seed and several were obtained following interspecific hybrid ization. The infrequency of twinning and difficulty in detecting twin seed in Rubus reduced the efficiency of this method. The efficiency wi th which dihaploids could be obtained following interspecific hybridiz ation varied with the pollen parent. Reduced seed set and seed quality following pollinations with respectively R. parvifolius and R. hirsut us could be used to advantage in the recovery of dihaploids from black berries. Ploidy reduction in several tetraploid blackberry cultivars w as obtained following pollinations with 100 and 150 kR gamma irradiate d pollen. Most of the seedlings obtained at the 50 kR dosage were aneu ploid. Pollen irradiation at 150 kR was the most efficient method of o btaining dihaploids from tetraploid blackberries. Twenty percent of th e seedlings obtained following this treatment were dihaploid.