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 variety have been evaluated for their ability to form callus cultures with a morphogenetic response. Lines were homozygous for heterochromatic knobs at 6L(2), 6L(3), 7L and 8L(1) but differed for the presence or absence of K2L, K3L, K7S and K9S. Clear differences in embryogenic response were observed between the families of inbreds. Only one family formed friable, highly embryogenic type II calli; the other families formed slow growing non-embryogenic or poorly embryogenic cultures. All the genotypes screened showed a similar response to the two culture media tested, suggesting that genetic factors are responsible for the major differences among the families. Mitotic abnormalities were investigated in Feulgen preparations of most cultures. Anaphase bridges resulting from delayed chromatid separation, typical bridges and fragments were observed. In a previous study, delayed chromatids were shown to be held together at heterochromatic knob sites, while typical bridges would be formed by dicentric chromatids arising from breakage-fusion-bridge cycles initiated by chromosome arms broken during the primary event. In the present study, the frequency of both types of bridges was not strictly correlated with the knob content of the genotypes analysed. This suggests that knobs may undergo alterations in culture leading to mitotic disturbance, and that this response may be genotype dependent. (C) 1998 Annals of Botany Company.