A B⁹ isochromosome was isolated in 1970 (Chrom. 30:356). It was shown to carry out nondisjunction at low, variable rates and to generate sectored kernels for B⁹ markers at relatively high frequency (Chromosoma 42:127). Germination of sectored kernels produced plants that contained, in several instances, telocentric B⁹'s. Centromeric misdivision of the isochromosome at the second pollen mitosis apparently produced the telocentrics (E. Brannen, unpublished). Four telocentric B⁹'s were recovered from the B⁹ isochromosome. Testcrosses showed negligible rates of nondisjunction for the telocentrics and little sector formation. Telocentric B⁹'s differ markedly from the isochromosome in these properties.

Telocentric B⁹'s occasionally give rise to isochromosomes by centromeric misdivision. Three potential isochromosomes, derived from telocentric 1854, were identified in Table 1 of an accompanying article. Cytological examination of root tip mitoses confirm the presence of an isochromosome in each case. Genetic tests indicate a low, but significant, rate of nondisjunction for each of the isochromosomes (Table 5). Also, they frequently produce sectored kernels and particularly multiple sectored kernels (Table 6). Properties of the new isochromosomes seem very

similar to those of the original isochromosome. The results shown in Tables 5 and 6 may be compared to data for the original isochromosome in an accompanying article (Tables 7 and 8).

Table 5.

Table 6.

With the present information, it seems likely that the new isochromosomes are similar or identical in nondisjunctional properties to the original isochromosome and markedly different from telocentric 1854. If true, telocentric 1854 contains all the genetic factors needed to construct an isochromosome similar to the one from which it was derived. One may speculate, due to the work of Rhoades, Dempsey, and Ghidoni (PNAS 57:1626), that telocentrics fail to undergo nondisjunction because they lack heterochromatin on one side of the centromere. Stickiness of centric heterochromatin in both chromosome arms may play an important role in holding the centromere together for nondisjunction.

The instability of isochromosomes and their relatively low rates of nondisjunction suggest that they lack a factor, probably on the B short arm, which promotes nondisjunction. The B⁹ isochromosome may stick together at sites flanking the centromere but then have difficulty in migrating to one pole.

Wayne R. Carlson

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