Different rates of chromosome elimination in symmetric and asymmetric somatic hybridization between <Emphasis Type="Italic">Festuca arundinacea</Emphasis> and <Emphasis Type="Italic">Bupleurum scorzonerifolium</Emphasis>

The protoplasts of tall fescue (Festuca arundinacea Schreb.) were fused with those of Bupleurum scorzonerifolium Willd. The latter were irradiated with UV at an intensity of 380 μW/cm² for 0 s (combination I), 30 s (combination II), and 60 s (combination III) before fusion. Putative hybrid calli, leaves, and shoots were generated from the fusion products. They were recognized as somatic hybrids by a combined analysis of chromosome numbers, isozyme, RAPD, and 5S rDNA spacer sequence. The hybrid calli with morphogenetic ability and leaves/shoots differentiation had the B. scorzonerifolium phenotype, whether they were derived from symmetric fusion (UV 0 s) or asymmetric fusion (UV 30 s/60 s). Cytological tests revealed that these hybrids contained the complete set (12) of B. scorzonerifolium chromosomes and 0–4 partner tall fescue chromosomes. The tall fescue chromosomes were rapidly eliminated in combinations II and III, but gradually lost in combination I. It was noted that the green leaves and shoots were produced earlier, and the differentiation frequency was higher in combinations II and III than in combination I, which corresponded to the speed of elimination of the tall fescue chromosomes in the hybrids. Therefore, UV irradiation can indirectly promote elimination of tall fescue chromosomes and hybrid differentiation. B. scorzonerifolium can repel partner chromosomes with mechanism that differs from UV.