Chromosomal Deficiencies and the Embryonic Development of Drosophila Melanogaster

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.2A 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 de- scribed.4'5In such instances the deficiencies are among the smallest known, probably for very few genes.These facts emphasize the importance of the chromosomes in the develop- mental processes.Just what the chromosomal functions may be, how- ever, 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 com- binations of genes, to determine the part played by each and its interac- tions 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 fe- males 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.