Nondisjunction and isochromosome formation in the B chromosome of maize

Bianchi et al. (1961) found that sectored losses of B-translocation chromosomes occur at a significant rate during early development of the endosperm and sporophyte. The losses were attributed to nondisjunction of the chromomosome, since B type chromosomes are known to undergo nondisjunction at the second pollen mitosis. Sector formation was further analyzed in the present paper, using the translocation, TB-9b. It was found that losses of the B⁹ chromosome during early endosperm mitoses occur only if the 9^B chromosome is present. In addition, sectors are produced in the sporophyte only if the 9^B and B⁹ chromosomes are inherited from the male parent. Both of these findings suggest that nondisjunction is indeed responsible for the B⁹ losses (see text). However, cytological observation of sectored plants demonstrates that isochromosome formation, rather than nondisjunction, produces most B⁹ losses in the sporophyte. The conflicting results can be reconciled by assuming that the same basic event, perhaps stickiness of the B⁹ chromosome, produces nondisjunction at the second pollen mitosis and isochromosome formation in the developing sporophyte. Observation of the isochromosome in pachytene reveals that a heterochromatic region corresponding to the short arm of the normal B⁹ is missing. The normal B⁹ chromosome is, therefore, an acrocentric chromosome.