Rate and extent of spontaneous and induced chromosome loss have been determined at the callus level of somatic hybrids of mutants of Solanum tuberosum and Nicotiana plumbaginifolia. AEC (amino ethyl cystein) resistance in potato and Nitrate-Reductase deficiency in N. plumbaginifolia have been used as genetic markers and chromosome morphology as a cytological marker. In this combination, development of hybrid callus was late and slow. Only a limited number of non-regenerable hybrid calli have become available. Chromosome loss could clearly be established in these hybrids from loss of markers and from chromosome cytology. Loss of markers occurred independently.
The best conditions to induce loss of chromosomal material in donor cells by irradiation were found by cytological investigations. A very drastic reduction in chromosome transfer by fusion could be effected by irradiation of plant tissue and subsequent preparation of protoplasts after a few days. Following fusion, hybrid callus was recovered with the potato genome drastically reduced. The amount of loss was deduced from the presence of a few fragments in metaphase cells or from interphase nuclei after in situ hybridization with a repetitive potato DNA probe.