Regional control of nondisjunction of the B chromosome in maize.

Control of nondisjunction in the maize B chromosome was studied using a set of B-10 translocations. The study focused on the possible effect of the proximal region of the B long arm. The experimental procedure utilized a combination of a 10(B) chromosome from one translocation with a B(10) from another translocation. The breakpoints of the two translocations were so located that combination of the two elements created a deletion in the proximal region of the B chromosome, but no deletion in chromosome 10. Two different types of deletions were established; one involved a portion of the euchromatic region and the other the entire heterochromatic portion comprising the distal half of the B long arm, except for the small euchromatic tip. Deletion of the heterochromatic portion did not exert any effect on nondisjunction. Deletions of different portions of the euchromatic region produce different responses. Some deletions resulted in typical B nondisjunctional activity; others resulted in the disappearance of this activity. It is concluded that a region within the euchromatic portion of the chromosome is critical for the nondisjunction of B chromosomes. Among 22 translocations with breakpoints in the euchromatic regions, three were proximal to the critical region, 16 were distal and the position of three others was not determined.