Chromosomal Deficiencies and Embryonic Development

Citation

Poulson, Donald Frederick (1936) Chromosomal Deficiencies and Embryonic Development. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/j2q0-9a33. https://resolver.caltech.edu/CaltechTHESIS:12012024-022824645

Abstract

The progress of genetics has been so rapid in a number of respects that many fields of inquiry have been entirely neglected or left for a future in which fields now so fertile may be exhausted. Much of this progress has been concerned with the relations between genes or groups of genes and the chromosomes bearing them, i.e., with the mechanism of transmission of characters. The problems of what the gene itself does in development and the manner in which it acts have received recent attention (Schultz 1935), but are still completely open.

The relative roles of gene and cytoplasm have long been known. An egg is incapable of further development without a nucleus, although some division is not impossible in the absence of the latter. That at least one set of chromosomes is required for development and two for complete normal development was found in early experiments with Echinoderm eggs. That the character of the embryo is determined by the genes carried in its chromosomes has also long been known. A certain amount of information is available concerning what happens when extra chromosomes or sets of chromosomes are added to the normal diploid number - upset genic balance. Likewise the exaggeration effects of heterozygous small deficiencies have been studied. One approach has only recently been made use of: the behavior of lethal factors and of homozygous deficiencies. Lethal types have been studied in some forms, e. g., the mouse and the fowl. But in only one case in the mouse is it very definitely known that the lethal is a deficiency (Snell and others - 1934). Nearly all homozygous deficiencies have been shown to be lethal at earlier or later stages. The most favorable material for such studies exists in Drosophila melanogaster. The behavior of many small deficiencies has been studied there by Demerec (1934) using stocks of flies which give a high frequency of chromosome elimination and thus patches· of “deficient tissue.” No studies have been made on the whole organism homozygous for deficiencies. It was proposed, therefore, to determine what happens to such “deficient” eggs. This seemed not impossible, as techniques for observing living eggs throughout development had just been introduced by Huettner and his students. Such studies proved not too difficult in cases where normal development ceased very early, but in cases where development became abnormal in the later stages, the situation was more complex. This was principally because the embryology of Drosophila beyond the time of inclusion of the pole cells was unknown. In the developmental studies which had been made, especially those by Sturtevant (1929) on gynandromorphs, the processes had been assumed to be similar to those of the related Muscids. Although the validity of this assumption has since been shown, it was essential that a careful study of embryonic development be made not only as a standard of comparison for deficient types, but also to give detailed information concerning the origin of the various larval organs and the anlagen of the adult, the imaginal discs.

The results of these investigations are embodied in the present work, the first section of which consists of a survey of the literature on insect embryology, particularly that of the Diptera. This is essential as a background for the account of Drosophila embryology to follow. Later sections deal with the literature on deficiencies and the behavior of a number of the deficiencies studied.

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