Apoptotic cell death observed during the expression of hybrid lethality in interspecific hybrids between Nicotiana tabacum and N. suaveolens

Hybrid seedlings from the cross Nicotiana tabacum x N. suaveolens, obtained by test-tube pollination and ovule culture, expressed lethality at 28degreesC. Characteristic lethal symptoms in these hybrid seedlings consisted of browning of hypocotyls and roots. One hundred and seventeen hybrid seedlings were eventually obtained by the use of test-tube pollination and ovule culture. Hybrid seedlings maintained at 36degreesC did not express any lethal symptoms. Hybrid seedlings used for further experiments were transferred to 36degreesC immediately after germination at 28degreesC. When hybrid seedlings cultured at 36degreesC were transferred to 28degreesC, their growth stopped and lethal symptoms were expressed. During the progressive expression of lethality, apoptotic features such as chromatin condensation, nuclear fragmentation and DNA fragmentation were detected. On the other hand, there was no sign of apoptotic cell death in the hybrid seedlings at 36degreesC. Based on the observation that the same lethal symptoms and the same apoptotic features were observed in the reciprocal cross, N. suaveolens x X tabacum, we suggest that not only the underlying causes of hvbrid lethality but also the underlying causes of apoptotic cell death are due to the interaction of coexisting heterogeneous genomes, rather than to the effect of cytoplasmic genes. Furthermore, the progression of apoptotic cell death in the cross N. tabacum x N. suaveolens began in stems and roots, followed by leaves.