Spiral structure of salivary chromosomes in Lycoria (Sciara) and Drosophila (a preliminary note)

Abstract

Recent reports of Heitz (1933, '34), Painter (1934), Metz and Gay (1934), Geitler (1934), and others on the morphology of chromosomes in salivary glands of Diptera have greatly attracted our attention and have compelled us to take up the same objects and reexamine their results, especially with regard to the structure of chromosomes. According to the authors above mentioned, the chromosomes found in the salivary glands of Drosophila (Painter, Heitz), Sciara (Metz and Gay), Simulium (Geitler), etc.“show very conspicuous bands or disks of deeply staining material alternating with clearer areas.”Painter tried to determine the position of gene loci in such banded chromosomes and put forward a new map of the chromosomes.
The larvae of Lycoria sp. and Drosophila melanogaster (normal; Star Curly; attached-X's (ω/yy)) which are being kept in culture in our laboratory have been dissected in Ringer's solution, tap water or in body fluids and the salivary glands have been treated by the aceto-carmine method and subjected to observation. Sometimes Carnoy's fluid was used before treatment with aceto-carmine. To some preparations Feulgen's nucreal-reaction method was applied. The chromosomes or spiremes seen under the cover-glass which was not so tightly pressed have shown beautifully coloured spiral chromonemata buried in the clearer matrix. The chromosomes in the spireme-nucleus generally show in their matrix chromonemata which we call“secondary chromonemata” (Fig. 2). Such chromosomes as show the secondary chromonemata are found to show coiling again in earlier stages, so that they are seen as larger and thicker chromonemata which we call“primary chromonemata”(Fig. 1). The matrix in which the primary chromonemata were buried seems to have disappeared by becoming continuous with karyolymph. The case that seems to show chromosomes extended artificially as a result of pressure subjected to the cover-glass is shown in Fig. 4 where the spiral condition of secondary chromonemata is fairly clearly recognized. The so-called chromioles are also seen in the secondary chromonema which shows doubleness in places. They may be an initial state of coils of a tertiary chromonema a certain indiation of which is found in secondary chromonemata. The spiral-within-spiral structures of chromosomes was first described by Fujii (1926) in meiotic chromosomes of Tradescantia and this was confirmed by Kuwada (1932) and Kuwada and Nakamura (1933, '34) in their detailed investigations on the same material. The wire-model (Fig. 5) pressed under a plate was compared with the primary and secondary chromonemata pressed under the cover-glass (cf. Fig. 4). The band, disk or ring structure of chromosomes described by several authors may be ascribed to inadequate treatment, to such factors as over-pressure or insufficient staining in making preparations.
From the results of observations it may be concluded that the salivary chromosomes of Lycoria (Sciara) and Drosophila used in this investigation show a spiral structure which confirms Kaufmann's results (1931) obtained in the case of Drosophila and that in the earlier stages of nucleus the primary and secondary spiral chromonemata are seen in the chromosomes or spiremes. The existence of a tertiary chromonema in the secondary one has been suggested.