Table 1.

Frequency of Kernels and Pollen Resulting from Nondisjunction of 9Bic-1

Kernel Counts for c1/c1 Tester × 2B + 9Bic-1/9 Reciprocal Testcrosses
	Endosperm Color
	Purple	722
	Yellow	81
	Mosaic	76
	Total	879
Total with aberrant phenotype		157
Total kernels (%)		17.86%
Adjusted total with nondisjunction phenotype		314
Estimated total that contain 9Bic-1a		439.5
Estimated frequency of nondisjunction for 9Bic-1b		71.44%

FISH Data for Pollen from 2B + 9Bic-1(C1)/C1 Plants
Number of ZmBs Signals in the Nucleusc
Vegetative	Sperm 1	Sperm 2	No. of Pollen	Total (%)	9Bic-1 Containing Pollen (%)
1	1	0	142	51.64%
2	1	1	52	18.91%	39.10%
2	2	0	50	18.18%	37.59%
2	2	1	31	11.27%	23.31%
		Total	275
Total with 9Bic-1 chromosome			133
Percentage of 9Bic-1 pollen showing nondisjunctiona					76.69%

The endosperm color of kernels from a c1/c1 × 9(C1)/9Bic-1(C1) + 2B testcross was scored as either purple (colored), yellow, or mosaic. Yellow and mosaic endosperm result from failure of the inactive B centromere sister chromatids to separate at the second pollen mitosis. All kernels from the reciprocal crosses were fully colored. Pollen grains with two clearly separated sperm were scored for the number of the B chromosome–specific ZmBs signals in the vegetative and sperm nuclei. Although sperm with a stretched appearance and connected sperm were observed, they were not recorded as such but were assigned to one of the listed categories. Because there are two copies of the B chromosome, every pollen is assured of receiving a single copy. FISH examination of pollen containing only a B chromosome indicates that the B centromere rarely, if ever, separates. Proper disjunction places a ZmBs cluster from 9Bic-1 in both sperm. Failure of the inactive B centromere to separate leads to sperm pairs with two and zero or one and one ZmBs signals. The former results from cases in which the normal B and 9Bic-1 move to the same sperm, while the latter results from their distribution to different poles during the second microspore division. Calculation of ZmBs separation failure using kernel phenotype or pollen FISH produces a similar percentage (pink box).

^aOne-half of the total pollen and seeds should contain 9Bic-1.

^bEmbryo phenotypes were not determined, but assuming no preferential fertilization, they should occur at approximately the same frequency as for the endosperm because each sperm can fertilize either the egg or the central cell. Nonrandom fertilization of the embryo by the sperm containing the B chromosome at rates of ∼70% has been reported in specific genotypes (Gonzalez-Sanchez et al., 2003). It has not been determined if the inactive centromere on 9Bic-1 influences fertilization. If it does, the number of kernels with nonconcordant embryo phenotypes would be less than the endosperm phenotypes. This difference would be minimal because the B chromosome can move independently of 9Bic-1, and only sperm containing both the two B chromosomes and two 9Bic-1 chromosomes would fertilize the embryo at the higher frequency. In any case, the comparison between frequencies of nondisjunction phenotypes in the kernels and pollen is only approximate.

^cSmall ZmBs signals present on the tip of the B chromosome were not counted.