Studies by Lin (Genetics, 1979) and Carlson (Ann. Rev. Genetics, 1978) suggest that the B chromosome is not telocentric. They found that rearrangements at the B centromere which should delete an adjacent short arm have a genetic effect: they reduce the rate of nondisjunction at the second pollen mitosis. The findings provide some evidence for the existence of a B short arm. However, another explanation is possible. Perhaps part of the B centromere itself can be removed without disrupting the basic functioning of the centromere. This component of the centromere affects the rate of mitotic nondisjunction. To accommodate both ideas, the B centromere plus the hypothetical short arm are referred to as region 4. Within this region, there is a site(s) which affects the rate of nondisjunction.
One explanation for the effect of region 4 is to assume that the active site is heterochromatin in the B short arm. The heterochromatin is similar to that in region 3, adjacent to the centromere in the long arm. Region 3 heterochromatin controls nondisjunction through non-division at the second pollen mitosis (Rhoades and Dempsey, Genetics, 1972; Carlson and Chou, Genetics, 1981). Perhaps maximal nondisjunctional rates require that the B centromere be flanked on both sides by non-dividing heterochromatin.
A test can be made for equivalency of region 3 and region 4. In the accompanying diagram, misdivision products of the B9 chromosome from TB-9Sb are shown. The following are depicted: a) standard B9, b) pseudoisochromosome derivative of B9, c) type 1 and type 2 derivatives of the pseudoisochromosome, d) type 2 isochromosome derivative of the type 2 telocentric.
The type 2 isochromosome contains two doses of region 3, flanking the centromere. It should give nondisjunctional rates similar to the standard B9, if region 3 is equivalent to region 4. An ear was constructed which segregated two types of progeny: 9B (Wx) 9B (Wx) type 2-iso B9 (Yg2 Bz) and 9 (wx) 9B (Wx) standard B9 (Yg2 Bz). Seven plants of each type were crossed as male to a tester of B9 nondisjunction: yg2/+ bz bz wx wx (only Wx kernels were used in the analysis, because they contain TB-9Sb).
Nondisjunctional rates were calculated as bz Yg plus 2 X Bz
yg2 phenotypes (the yg2 totals must be doubled due to heterozygosity
of the tester). Rates of nondisjunction for two ears per cross are given
|9B 9B type 2-iso B|
|Plant No.||Rate Nondisjunction|
|9 9B std B9|
There is a large, nonoverlapping difference in rates of nondisjunction
between the two groups. It is concluded that region 4 contains a unique
site of nondisjunctional control which cannot be replaced by nondividing
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