NUCLEOLAR ACTIVITY OF LAGGING CHROMOSOMES IN WHEAT

Abstract

ONE THEORY OF the development of the nucleolus holds that it arises directly from the matrices of all the chromosomes (see reviews by Gates, 1942, and Vincent, 1955). There appears to be evidence for the matrix theory from a number of unrelated plant species, among them Zea mays (McClintock, 1934), Narcissus ( Sikka, 1940), and Luzula campestris (Brown, 1954). In other organisms, however, following the same or similar fixation and staining techniques, no scattered nucleolar-like material is noted about the telophase chromosomes, and instead the nucleolus appears to grow at specific points in the telophase nuclei, as in Pisum (Hakansson and Levan, 1942), and amphibian ovarian eggs (Duryee, 1947). In recent years the existence of a matrix as an accessory component of the chromonemata has been questioned by some workers, among them Ris /1947). On the other hand, Serra (1947) and Kaufmann et al. (1948) uphold the existence of the matrix. On the basis of their ribonucleic acid studies utilizing enzymatic degradation of plant and animal chromosomes, Kaufmann et al. (1948) suggest that the nucleolus may serve as a reservoir of ribonucleic acid, which is gradually transferred to the chromosomes during the prophase, providing a temporary state of heterochromatinization. Caspersson and Schultz (1940) are of the opinion that the heterochromatic regions of the chromosomes are responsible for synthesizing the nucleolar material. Rattenbury and Serra (1952) maintain that it is formed from an interaction between the nucleoplasm and the cytoplasm. With few exceptions, such as in certain egg cells, the nucleoli are held at specific sites on specific chromosomes. These chromosomes are called the nucleolar chromosomes, and the region of attachment of the nucleolus, the nucleolus-organizer. In most species the nucleolus-organizer is characterized by heterochromatin (see reviev by Gates, 1942). Whether this region merely serves to collect the dispersed or scattered nucleolar material produced elsewhere in the nucleus, or whether the nucleolus-