The B⁹ chromosome of TB-9b is relatively stable in mitotic divisions. However, an isochromosome derived from the B⁹ frequently becomes unstable when transmitted through the male parent, producing mosaic kernels. Endosperm instability was correlated with the presence of B⁹ telocentrics in plants derived from mosaic kernels, indicating misdivision of the isochromosome (Carlson, Chromosoma 42:127). Misdivision in the male parent could occur meiotically, due to orientation difficulties of a univalent isochromosome. Alternately, it could result from improper nondisjunction at the second pollen mitosis.

This problem was investigated by utilizing a C C-I B⁹ isochromosome to produce endosperm mosaics. Crosses of the type C C bz bz X 9^B 9^B iso-B⁹ C Bz C-I Bz, and C C bz bz X 9 9^B C Bz C-I Bz were used Co produce the mosaics. By selecting Bz/bz progeny, misdivision involving loss of the C-I arm of the isochromosome was examined. The Bz/bz kernels were germinated and the plants tested for the presence of C-I. If a plant contains C-I whereas the endosperm contained only C, the genotypes are said to be non-correspondent. A difference between endosperm and embryo indicates that the pollen grain which produced the kernel had genetically dissimilar sperm, suggesting misdivision at the second pollen mitosis. If, on the other hand, correspondence occurs between embryo and endosperm, misdivision may have preceded the second pollen mitosis, probably occurring at meiosis.

Sixty-one percent (71/117) of the kernels with the Bz/bz endosperm phenotype contained the C-I marker in the embryo and were classified as non-correspondent. (Chromosome constitution of the C-I containing plants has not been determined.) Correspondence between endosperm and plant for the C marker was found in 39% (46/117) of the cases, indicating that meiotic misdivision may also cause fractional formation. However, crossing over can occur between isochromosome arms, producing a C C isochromosome. This chromosome could undergo centromeric breakdown at the second pollen mitosis and, nevertheless, show correspondence between endosperm and embryo. Thus, 61% is a minimum estimate of the Bz/bz fractionals generated at the second pollen mitosis.

The isochromosome may be lacking a factor which aids nondisjunction. Absence of the factor makes migration of the isochromosome to one pole difficult, and misdivision frequently occurs (Carlson: Ann. Rev. Genet., 1978). In support of this idea, Carlson (MNL, 1978) found reduced rates of nondisjunction (20%) for the B⁹ isochromosome in comparison to standard B⁹'s (50-98%).

Elaine Brannen

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