--Zheng, Y

A crossover between the B-9 chromosome from TB-9Sb and the duplication 9 chromosome from McClintock (Genetics, 26:234-282, 1941) has been used to establish a chromosome breakage-fusion-bridge cycle, as described in an accompanying article. Briefly, a B-9-Dp9 chromosome is capable of self-pairing and crossing-over to produce a chromatid breakage-fusion-bridge cycle. Subsequently, nondisjunction at the second pollen mitosis converts the chromatid cycle to a chromosome cycle. Crossing 9-B 9-B B-9-Dp9 plants as male parents to a bz1 bz1 yg2 yg2 tester produces some progeny with the bz endosperm phenotype. Among these kernels, many have a B-9 chromosome dicentric in the embryo. Since the dicentric undergoes a chromosome-type breakage-fusion-bridge cycle, the phenotype of the correct plants is variegation for green and yellow stripes.

In order to study the fate of B-9 dicentric chromosomes during development, tassel samples (sporocytes) were collected from 41 variegated plants and examined in meiosis. Most of the plants had been checked previously for double bridges in primary root tip cells and 34 showed double bridge configurations. Therefore, at least 34 of the plants initially contained a B-9 dicentric.

When the chromosomes were checked at pachytene, a lot of diversity was found in chromosome structure between plants and even within the same plant. Breakage occurred at various positions between the two centromeres, producing both long and short chromosomes, as well as sizes in between. "Mini-chromosomes" were identified in 13 of the 41 variegated plants at pachytene. The term "mini-chromosomes" is used for a collection of very small chromosomes that are heterogeneous in size. They all probably arose from bridge breakage at or near the B centromere.

In metaphase-I cells , the mini-chromosomes orient on the plate along with the other chromosomes (Figure 1). In anaphase-I, the mini-chromosomes usually lag and do not migrate early to one pole, unlike complete B chromosomes (Carlson and Roseman, Genetics, 131:211, 1992). They often split after lagging in anaphase I. They can also be observed in metaphase-II and anaphase-II cells. These are preliminary observations, without quantitation at this point. The main finding is that extremely small chromosomes frequently arise among the variegated plants.

Figure 1. A mini-chromosome in a metaphase-I cell. 

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