A procedure for localizing genetic factors controlling mitotic nondisjunction in the B chromosome of maize

Abstract

The B chromosome of maize undergoes nondisjunction at the second pollen mitosis at rates as high as 98% (Roman, 1948; Carlson, 1969a). Nondis-junction is controlled by at least two separable regions on the B chromosome (Roman, 1949; Longley, 1956; Carlson, 1969b; Ward, 1972). A procedure for identifying and localizing the chromosomal sites required for nondisjunction is reported here. A translocation between the B and chromosome 9 (TB-9b) was utilized. Plants carrying TB-9b were screened for mutants of nondisjunction, i.e. translocations in which nondisjunction does not occur. Two such translocations were identified in a small screening. While the mutant translocations have not been analyzed in pachytene, they are most likely deletions or rearrangements of regions on the B chromosome vital to nondisjunction. Diminutive and rearranged B chromosomes are known to arise spontaneously in small populations (Randolph, 1941; Longley, 1956). — Also reported here are the nondisjunctional properties of the B⁹ isochromosome (Carlson, 1970) and several telocentric (or subtelocentric) derivatives of this chromosome. Some derivatives of the isochromosome are virtually incapable of nondisjunction, and should provide information on the role of the centromere in nondisjunction.