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A NEW METHOD FOR THE STUDY OF CHROMOSOME SOME MAPS I N DROSOPHILA MELANOGASTER

For many years Drosophila melanogaster has been a classical object for genetic studies but, due to the small size of the chromosomes and to the fact that we have been able to make little headway in the study of these elements in meiosis, it has been necessary to turn to less favorable material genetically, in order to study some of the questions which have arisen with the progress of cytogenetics. The past fall the writer began a study of the salivary glands in larvae of this fly with the hope that it would be possible to recognize the same chromosome in any cell, and in this way to get more accurate measurements of translocated pieces and, in general, to gain some insight as to the nature of dislocations of chromatin. It has been found that the chromosomes in the salivary glands have a definite and, within limits to be described, a constant morphology which enables one to recognize the same element in any cell. The elements are so distinct and constant in their morphology that we can follow a translocated segment to its new site, or if we are dealing with a mutual translocation, can determine just how much of each chromosome is involved in the exchange. Needless to say, this places in our hands a method of studying translocations and of plotting cytological maps which is a vast improvement over the study of metaphase plates heretofore used. But what is of far greater importance is the discovery, wholly unexpected, that in the salivary glands of old larvae the chromosomes may undergo somatic synapsis. As a result, homologues pair line for line, band for band, and unite into one apparent element. If, however, one of the homologues has an inverted section, we get typical inversion figures, such as may be seen in the meiosis of corn, and by the bands and other landmarks we can tell, cytologically, just where the inversion occurred. If one chromosome has been deleted, it pairs with its homologue except a t the region of the deficiency; here the normal chromosome buckles and by comparing the lines of the two elements we can tell just how much is missing from the deleted element. This gives us a new and relatively exact method for the study of inversions and deficiencies and enables us to make a direct attack upon many of the problems associated with these aberrations. As new as the facts reported above may appear, the morphological observations upon which they are founded (unknown to the writer a t the