Fertile Haploid Sectors by Partial Merogony in Mozaics of Drosophila pseudo-obscura

Abstract

A STRAIN of D. pseudo-obscura gives a high proportion of mozaic offspring. In the majority of those obtained, only the X-chromosomes of the parents were marked and the phamotype of the mozaics consisted of a regular area showing the expected constitution and an exceptional area (usually less than half) in which the recessive genes of the paternal X were expressed. Of the fifty obtained in the first experiments forty-six were female with no suggestion of male tissue. The fact that they were completely normal as to sex suggested that the exceptional area was haploid in constitution, the complex, 1X: 1A, being known to be female. In addition, the exceptional tissue showed the greatly reduced size of bristles, eye-facets and wing-cells characteristic of haploid tissue. To test this hypothesis, a series of matings was made in which the autosomes as well as the X-chromosomes were differently marked in the two parents. Six more mozaics, all female, were obtained, and in each of these the genome of the father only was manifested in the exceptional region. This result suggests that the basis of mozaicism in these flies is some peculiarity following fertilization which permits of a division of the sperm nucleus before the fusion with the egg-nucleus becomes complete, so that a group of haploid cells is formed bearing the paternal genome which eventually form part of the adult fly. (There is also evidence that two separate sperms occasionally take part in development and also that two female pronuclei are sometimes involved, but the mozaics resulting from these contingencies are extremely rare, and are not here considered.)