No abstract

Chromosome aberrations have marked effects upon the development of an organism. Many such aberrations are lethal. In Drosophila this is particularly true of chromosomal deficiencies, as first described by Bridges.2 A more detailed study of the effects of such aberrations made by Li' shows that in all the cases he investigated deficiencies are lethal in the homozygous condition-the organisms dying in the egg or larval stages. Heterozygous deficiencies result in death in later stages, although many are not lethal. A few exceptional cases of viable homozygous deficiencies have been described.4'5 In such instances the deficiencies are among the smallest known, probably for very few genes.These facts emphasize the importance of the chromosomes in the developmental processes. Just what the chromosomal functions may be, however, is by no means clear; nor is the r6le of the individual gene evident. An ideal approach, such as the removal of one gene at a time, then of combinations of genes, to determine the part played by each and its interactions with others, presents many practical difficulties. The existence of numerous deficiencies, however, makes an approximation to this approach possible by the study of the effects of larger or smaller blocks of genes.The present study is concerned with the effects of certain deficiencies upon the embryonic development of D. melanogaster. The deficiencies used involved greater or lesser portions of the X-chromosome, ranging from the total absence of the X to a small deficiency involving relatively few "bands" as seen in the salivary gland chromosome. The technique of Huettner and Rabinowitz6 was used, in a somewhat modified form, for the observation of living eggs, and details of internal structure were studied by means of sectioned material.' Only timed eggs were used. Usually females were allowed to lay for half an hour and the eggs allowed to develop to the desired stage at a temperature of 22-23°C.

Several lines of evidence implicate small spirochetes, presumably treponemata, as etiologic agents in the production of the maternally transmitted "sex ratio" condition (SR) in Drosophila nebulosa, in D. willistoni, and in strains of D. melanogaster into which the SR condition has been artificially transferred. The presence of treponemata in the hemolymph of adult females of these species is completely correlated with the production of unisexual progenies and like this condition is dependent on the genotype of the host and of the infectious agent.