A common origin for the B chromosome and the extra chromatin of Abnormal 10 was once proposed. Subsequent studies, however, generally did not support the hypothesis (Reviewed in Ann. Rev. Gen., p. 15, 1978). In one experiment, the ability of Abnormal 10 to carry out a B chromosome function was tested. The B9 chromosome of TB-9Sb was isolated as a supernumerary in 9 9 B9 plants. In this situation, the B9 cannot normally undergo nondisjunction due to absence of a required B chromosome region carried on 9B . The missing chromatin is the distal tip of the B and is referred to as region 1 (Carlson and Chou, Genetics, 1981). Plants containing 9 9 B9 plus Abnormal 10 were tested for B9 nondisjunction. Results were negative. It was concluded that Abnormal 10 lacks the nondisjunctional function found in region 1 of the B and, to that extent, lacks homology with the B chromosome.

Subsequently, several modified (deletion) derivatives of TB-9Sb have been recovered, allowing further tests of homology between the B and Abnormal 10 (Carlson, Maize Breeding and Genetics, 1978). For example, one modified TB-9Sb (#1866) has a B9 which is missing most of region 2 (proximal euchromatin) and consequently lacks a function that is essential for nondisjunction. An experiment was designed to test nondisjunction of the deletion B9 in the presence and absence of Abnormal 10. The Wx locus was used to mark TB-9Sb-1866. The R locus was used to mark the extra chromatin on Abnormal 10. Fortunately, both linkages of gene to chromosome type are very strong (Robertson, Genetics, 1967; Rhoades, Genetics, 1942). As a result, cytological studies to correct for crossing over were unnecessary.

Plants were constructed with the following constitution: 9(wx) 9B(Wx) 1866-B9(Sh1); N10(R) Abn10(r). Six plants of this type were crossed as male parents to an r sh wx tester. Among the progeny, Wx kernels were selected. The Wx phenotype results from transmission of pollen which derived from one microspore type: 9B(Wx) 1866-B9(Sh1). The Wx kernels were classified for nondisjunction using the Sh1 marker. Kernels with the recessive (shrunken) phenotype result from one class of nondisjunction (hypoploid endosperm). The effect of Abnormal 10 on nondisjunction was determined by separating white (r - Abnormal 10) and colored (R -normal 10) seeds. Results are given below (Two ears per male parent):
 

Male Parent	R Sh Wx	R sh Wx	r Sh Wx	r sh Wx
6554-1	213	0	202	0
6554-3	167	1	182	0
6554-4	201	0	182	0
6554-9	201	0	177	1
6554-12	234	1	176	1
6554-19	220	2	168	1
	1236	4	1087	3

The number of shrunken kernels is very low in both the white and colored seed classes. The frequency of shrunken kernels is 0.3% for both groups. Obviously, nondisjunction of the 1866 B9 did not increase in the presence of Abnormal 10. The rate is negligible for both R and r kernels. Abnormal 10 does not, therefore, contain the nondisjunctional function present in region 2 of the B chromosome.

W. R. Carlson
 
 

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