XVI.—The Problem of Chromosome Re-arrangements in Somatic Cells ofDrosophila Melanogaster

Abstract

The mechanism by which irradiation produces gross re-arrangements of chromosomes (inversions, translocations, deletions) is now generally believed to occur in two steps: (a) breakage of one or more chromosomes, yielding two or more points of breakage; and (b) reunion of broken ends in a novel combination. Thus the frequency with which a given treatment produces re-arrangements depends not only on the number of breaks produced, but on the circumstances under which reunion takes place. In Drosophila the fertilized ovum seems to offer particularly favourable conditions for breaks produced in the mature spermatozoon to join up into new combinations. In immature germ-cells, on the other hand, re-arrangements are much less readily produced by irradiation (Muller, 1930; Shapiro and Neuhaus, 1933; Shapiro, 1937; Glass, 1939). Evidence for the occurrence of spontaneous or induced re-arrangements in somatic cells of Drosophila is very scanty.

Effect of mustard gas on some genes in Drosophila melanogaster

“…There are two similar mosaics in X-ray literature; one, Panshin (1935) where a son of an irradiated father was phenotypically and gonadically a mosaic for two alleles of lozenge; and a second (Neuhaus, 1935) Auerbach (1946) because of the hypothesized lengthening 4-3 of period when the chromatids were not intact. Patterson (1933) proposed another explanation of mosaics appearing in the F 1 after irradiation as due to a certain proportion of split chromosomes in the sperm.…”

“…These spots are chiefly due to somatic crossing over (Auerbach, 1945;1946). The spots are definitely of smaller area in mustard gas treatment than with X-rays.…”

“…Treatments of female embryos marked with recessive sexlinked genes give a high percentage, about 40 percent with the doses employed, (Auerbach, 1946) of mosaic spots in various areas of the body. These spots are chiefly due to somatic crossing over (Auerbach, 1945;1946).…”

“…The size difference is difficult to explain if chemicals effect chromosomes, as X-rays are assumed to do, at the time of treatment. If a time lag (Auerbach, 1946) . Regular females with unattached X-chromosomes and attached-X females were diagrammed as supplying the maternal X-chromosomes as the type of mosaic produced will depend on the kind of untreated maternal X-chromosome used as well as on the type of the induced chromosomal change, i.e., mutation or chromosomal rearrangement in the sperm.…”

“…This theory as its author stated will not ex plain mosaics due to gene mutations proper, that is, chromo somal aberrations not due to visible chromosome breakage and reunion. The theory would encounter difficulties for example in explaining a male mosaic which resulted from chemical treatment of sperm fertilizing an attached-X female (Auerbach, 1946) which was both phenotypically and gonadically a mosaic for two different allelomorphs of white.…”