MICRURGICAL STUDIES ON CHIRONOMUS SALIVARY GLAND CHROMOSOMES

Abstract

The salivary gland chromosomes of mature Chironomus larvae were studied both within the intact cell and after their isolation into a Ca-free soln. (0.09 [image] KC1, 0.06 [image] NaCl, 0.005 [image] Phosphate, pH 7). Puncture of the cell with a fine microneedle had little or no effect on the structure and physical properties of the chromosomes, whereas tearing the cell or gross puncture caused the chromosomes to shrink markedly and to become highly viscid. In order to maintain the isolated chromosomes in the fresh state it was necessary to remove them at once, without contact with the torn cytoplasm, into the Ca-free medium. The chromosomes, either within the fresh cell or immediately after isolation, were easily deformable, highly extensible and elastic. Maximal stretching of the chromosomes could not be accomplished without rupturing the nuclear membrane. Isolated chromosomes were stretched 12- to 25-fold before breaking and could regain their original length if stretching did not exceed 10-fold. Treatment with acids, osmium tetroxide and formalin caused the chromosomes to become stiff and inelastic; treatment with alkalies rendered them ductile. The following observations appeared to support best the polytene concept of chromosome structure: (1) the occasional appearance of fibrillae in the untreated cell, (2) the presence of fibrillae at the broken ends of stretched chromosomes, (3) the removal of longitudinal fibrillae from the chromosome by micro-dissection, (4) the shredding of chromosomes into numerous fibrillae by expulsion from a micropipette. Further expts. are described which indicate that the salivary gland chromosome possesses a delicate membrane, and that the chromosomes maintain their morphological integrity in nuclei hyalinized by hypotonic or mildly alkaline Ringer''s soln.