DNA Synthesis in the Giant Nuclei of Insects - Control Machinery and Structures Observed in the Silk-Producing Gland of Bombyx Mori

The existence in many insect organs of giant nuclei without visible chromosomes raises the question of possible homologies between the chromatin structures of these nuclei and those of polytene nuclei or common euploid cells. Studies have been made of the nuclei in the silk-producing gland of Bombyx mori. The DNA synthesis is cyclic. During the third stage there are three successive synthesis cycles, which appear to be relatively autonomous in the individual nuclei. For more than 24 hours after moulting, however, synthesis is greatly reduced; moulting factors thus cause synchronization of all the nuclei. This leads to the conclusion that the triggering of a synthesis cycle is controlled by general factors external to the cell. At the end of larval development, DNA synthesis is suspended at the moment when large-scale secretion of silk begins. Evaluation of the pool of endogenic precursors of DNA shows that it is considerably reduced at the end of the DNA synthesis period. The hypothesis proposed is that large-scale synthesis of fibroin requires polarization of the metabolism, hence the depletion of the nucleotide pool and the end of DNA synthesis. DNA synthesis within a single nucleus is to some extent asynchronic. In particular, a well-defined, delayed-synthesis structure visible only in the female seems to be a possible homologue of a sex chromosome. Other asynchronisms are also apparent, though less clearly. Functional studies thus allow the supposition that in the giant nucleus replication units retain an individuality comparable to that of a polytene chromosome. These observations together lead to the conclusion that a nucleus in the silk-producing gland has physiological and structural characteristics similar to those of a polytene nucleus, differing from it essentially in the lesser degree of condensation of its structures. (author)