THE MORPHOLOGY OF THE THIRD CHROMOSOME IN THE SALIVARY GLAND OF DROSOPHILA MELANOGASTER AND A NEW CYTOLOGICAL MAP OF THIS ELEMENT

Abstract

The present article is the second of a series of three papers dealing with the morphology of the chromosomes of Drosophila melanogaster as these appear in the salivary glands of larvae, and to the presentation of a new type of chromosome map. As in my paper on the X chromosome, which appeared recently in this journal, the brief descriptions and the numerous figures have been presented primarily with the view of assisting other investigators in the ready identification of the various landmarks along the two arms of the third chromosome, because it is clear that study of salivary gland material is going to play an important part in the future genetic studies on Drosophila. The cytological map we are able to present now is little more than a skeleton, in spite of the fact that some 33 translocations have been analyzed. The reason for this is two-fold. In the first place, a great many of the breaks are bunched between rather narrow genetic limits. Thus out of a total of 36, 17 fall between the genetic limits of scarlet and pink. In the second place, the markers used in the original experiment were rather widely scattered and neither the time nor, in some instances, have the necessary stocks been immediately available for the finer analyses needed to determine more exactly the genetic limits of these breaks. Miss META SUCHE is making these determinations and by the time this article goes to the printer we should be able to add new data.' I n the meantime, the morphological positions of the breaks will be indicated. No doubt many other investigators have breaks in the third chromosome, the genetic limits of which have been relatively exactly determined, and it is believed that they will have no difficulty in placing these cytologically with the aid of the topographical maps presented in this study. In this way within a few years we should have a large number of gene loci placed within narrow morphological limits. Aside from the usefulness of the topographical maps for the study of all types of chromosome rearrangements and the problems associated with this branch of cytogenetics, perhaps the points of greatest general interest center around the question of chromosome structure. For the present study shows very clearly that the third chromosome is differentiated