The Morphology of the X Chromosome in Salivary Glands of Drosophila Melanogaster and a New Type of Chromosome Map for This Element.

In a paper which appeared recently in this journal the writer has described a new method for the study of chromosome structure and chromosome aberrations and has called attention to its application to a number of problems in the field of cytogenetics. In the present paper, the normal morphology of the X chromosome will be described and a new type of chromosome map will be presented showing the approximate position of some 21 gene loci scattered well along the whole active region of the X. It is believed that this type of analysis marks a distinct advance in the technique of cytogenetics and will be of general interest from both practical and theoretical standpoints. On the practical side, the new chromosome map of the X may be used as a yardstick to determine the genetic position of new breaks cytologically, thus greatly simplifying the breeding tests which need be made and saving much time. From a theoretical standpoint, perhaps the most significant point is the demonstration that we have in our hands a qualitative cytological method of chromosome analysis which should be applicable not only to Drosophila but other types of insects and perhaps to other animals as well. For if the X chromosome of D. melanogaster has such distinct topographical features that we can follow a section as it is translocated here or there through the agency of irradiation, there is no obvious reason why the same method of study may not be applied to a study of all the chromosomes not only in this species but in related forms, thus opening up to a direct cytological attack the whole field of the behavior of chromosomes in phylogeny and the hosts of questions clustered about this central theme. Of considerable interest, also, is the discovery that neither the inert region of the X, nor the Y chromosome, except possibly for a short piece, which probably carries the locus for bobbed, are visible as recognizable formed structures in salivary gland cells. The X appears to be made up wholly of the genetically active region.