The internal mechanics of the chromosomes - IV—Pairing and coiling in salivary gland nuclei of drosophila

Abstract

The most urgent problem in dealing with the salivary gland chromosomes is to find out their relationships with ordinary mitotic and meiotic chromosomes—first in structure and secondly in movement. Others have already dealt with structure in some respects, but movement has been generally neglected. It has been known since the work of Balbiani in 1881 that the chromosomes in the resting nuclei of salivary gland cells in the larvae of Diptera appear as enormously enlarged threads. These chromosomes are cylindrical threads consisting of transverse bands of deeply-staining material separated by non-staining internodes. During the last two years it has been found possible to relate this structure with the genetic properties of the chromosomes. Heitz and Bauer (1933) showed in Bibio hortulanus that these cylinders were the product of the pairing side by side of two cylinders corresponding in structure. The pairing is analogous with that observed in the zygotene stage of meiosis in Tulipa and Stenobothrus , where chromomeres correspond. Painter (1933) showed the even more significant fact that the banded structure was constant and characteristic of individual chromosomes of Drosophila melanogaster . Further Muller and Prokofyeva (1935) have shown by deletions with X-rays in Drosophila that the individual band or part of a band corresponds with a gene. Finally, it seems probable, from the observations of Koltzoff (1934) and Bridges (1935), that in the paired thread each band represents the reduplication of a chromomere or gene by division. Apparently the inert regions of the chromosomes in the neighbourhood of the attachment constriction are not differentiated at this stage, but appear, together with the spindle attachment chromomere, as undifferentiated masses which later become fused into a single body. This body seems to me to be optically homogeneous in the living condition, and I shall therefore refer to it as the magma .