EMBRYOGENIC RESPONSE AND MITOTIC INSTABILITY IN CALLUS-CULTURES DERIVED FROM MAIZE INBRED LINES DIFFERING IN HETEROCHROMATIC KNOB CONTENT OF CHROMOSOMES

Four families of sister inbred lines derived from a tropical maize var iety have been evaluated for their ability to form callus cultures wit h a morphogenetic response. Lines were homozygous for heterochromatic knobs at 6L(2), 6L(3), 7L and 8L(1) but differed for the presence or a bsence of K2L, K3L, K7S and K9S. Clear differences in embryogenic resp onse were observed between the families of inbreds. Only one family fo rmed friable, highly embryogenic type II calli; the other families for med slow growing non-embryogenic or poorly embryogenic cultures. All t he genotypes screened showed a similar response to the two culture med ia tested, suggesting that genetic factors are responsible for the maj or differences among the families. Mitotic abnormalities were investig ated in Feulgen preparations of most cultures. Anaphase bridges result ing from delayed chromatid separation, typical bridges and fragments w ere observed. In a previous study, delayed chromatids were shown to be held together at heterochromatic knob sites, while typical bridges wo uld be formed by dicentric chromatids arising from breakage-fusion-bri dge cycles initiated by chromosome arms broken during the primary even t. In the present study, the frequency of both types of bridges was no t strictly correlated with the knob content of the genotypes analysed. This suggests that knobs may undergo alterations in culture leading t o mitotic disturbance, and that this response may be genotype dependen t. (C) 1998 Annals of Botany Company.