Foi padronizada a metodologia de cultivo in vitro de calos de milho, induzidos de embriões imaturos de genótipos adaptados ao clima tropical. Foram estudadas as variedades Piranão de Inverno e Jac-Duro e linhagens com diferentes composições de knobs heterocromáticos, pertencentes a quatro famílias obtidas desta ultima. Genótipos da família 300-1-3 produziram calos friáveis e embriogênicos. A maioria das linhagens das famílias 301-2-1, 303-4-1 e 303-4-4 formaram calos não embriogênicos. Alguns genótipos da família 303-4-4 induziram calos com setores altamente friáveis e com rápido crescimento. Todos os tipos de calos foram utilizados em cultivos de suspensões celulares. Foi avaliada a frequência de formação de pontes cromossômicas em anáfases mitóticas de calos de quinze linhagens das famílias descritas. Foi comprovado, por bandamento-C, que as pontes têm origem em braços cromossômicos contendo knobs. Genótipos com maiores conteúdos de knobs não apresentaram maiores frequências de pontes. Figuras mitóticas sugerem a ocorrência de ciclos de quebra-fusão-ponte nos calos, a partir de um evento de atraso na separação das cromátides irmãs, ao nível dos knobs, durante anáfases mitóticas em calos

Maize embryogenic callus cultures initiated from ímmature embryos of several genotypes adapted to the tropics, were established and showed to be able to regenerate plants. The Piranão de Inverno (dent) and Jac-Duro (flint) varieties were used in this study. RI, R2 and R3 progenies from regenerated plants of these cultivars showed variation ta the following caracters: plant height, panicle morphology, seed color and texture. The culture response of calli derived from four families of inbred lines with different knob compositions, originated from the Jac-Duro variety, was evaluated as well. Highly embryogenic callus cultures were obtained from inbred lines of the 300-1-3 family, confirmed by three different experiments. Some inbred lines beIonging to this family were tested for their callus totipotency and showed high capacity of plant regeneratian and seed production. Immature embryos from inbred lines of the 301-2-1, 303-4-1 and 303-4-4 families induced non-embryogenic callus cultures. However, non- embryogenic calli formed from the 44114/1 and 44114/2 inbred lines, belonging to the 303-4-4 family, produced friable and embryogenic sectors with high growth rate. Cell suspension cultures were induced using different materials: Type II calli (from 300-1-3 family), non-embryogenic calli (from 301-2-1 and 303-4-1 families) and the cell lines mentioned above (from 303-4-4 family). All of them produced well-established cell suspension in a relatively short period of time (1 to 4 months). A cytological analysis of the calIus cultures of 15 inbred lines belonging to the four families studied was carried out. The occurrence and frequency of mitotic anaphase bridges was evaluated. ln order to investigate correlations between knobs and anaphase bridges, according to the hypothesis of PHILLIPS et alii (1988), that the chromosome aberrations observed in regenerants, usually involving knobbed chromosomes, are the result of chromosome breakage occurring in culture. The second point analysed was the possible relation between the morphogenetic potential of the genotypes and their degree of mitotic stability in cultures. In C-banded preparations, it was observed that the anaphase bridges are originated at the knob level. However, genotypes with higher heterochromatic knob contents did not present higher frequency of bridge formation. lt has been identified a preliminary event, characterized by a deIay in the sister-chromatids eparation, originated at the knob leveI. This delay causes a strong tension on chromatids and might result In chromosome breakage. Several mitotic figures suggest the occurrence of breakage-fusion-bridge cycIes initiated by broken chromosomes. Correlations between callus type and higher frequency of chromosome aberration were not observed. Cell lines selected from callus cultures from the 44114/2 inbred line showed lower frequency of mitotic aberrations, although its knob content is the highest among the genotypes analysed