Somatic instability of telocentric chromosomes in wheat and the nature of the centromere.

HE centromere, a necessary part of all chromosomes, is the region responsible Tfor congression and poleward movement. It is called “kinetochore” by those who wish to emphasize that it is the center of kinetic chromosome activity. This name is also more suitable because the centromere often is far from central in its location. In the following, the name centromere is retained because of its relation to the term “telocentric,” which denotes a one-armed chromosome with a terminal centromere. Although in some materials with favorably large chromosomes, some investigators have seen a complex structure (LIMA-DE-FARIA 194.9; MATTSON 1963), and this had been suggested even earlier (NEBEL 1939), the centromere of most mitotic chromosomes is seen only as a constriction, while in meiotic chromosomes it appears as a simple, homogeneous, translucent body (RHOADES 1940). The lack of staining with the usual chromatin stains shows that there is less DNA than in the rest of the chromosome at this time, which could be related to the fact that it is active at this stage by analogy with the “puffs” of giant chromosomes (BEERMAN 1963) or compared to the converse situation of the Barr bodies (THOMPSON 1965). Because of their universal occurrence, and their usual optical emptiness, it has been considered that “ . . . the centromeres of different chromosomes are all alike in their form and behavior” ( DARLINGTON 1939). This behavior is highly organized and sensitive to environmental and chemical influences and is likely to have as a basis a complex structure, even though this structure is usually microscopically invisible. LIMA-DE-FARIA ( 1958) in pachytene chromosomes of maize could distinguish two large and one small pair of chromomeres within the centromere region. Ordinarily centromeres divide quite regularly at mitosis and meiosis, but when a chromosome is unpaired, misdivision may occur at meiosis and give rise to telocentric chromosomes. If the centromere has a complex structure, the transverse split of misdivision might occw in different portions, resulting in telocentrics that differ in the completeness of their centromeric regions (Figure 1 ) . It is known that acentric fragments are lost; lacking a kinetic center, they fail to reach the poles and are excluded from the telophase nuclei. Since it has been observed that telocentrics are also lost somatically or may change to isochromo’ This work was supported by National Science Foundation grant GB-3596. Jonrnal Paper No. 3068 of the Missoun Agricultural Expenment Station