Division of a Nucleus lacking a Nucleolus

Abstract

IT has been suggested from McLeish's cytological studies of the broad bean, Vicia faba, that a nucleus lacking a nucleolus cannot reproduce¹. Similarly, Gaulden and Perry² have shown that mitosis in the grasshopper neuroblast is permanently inhibited when one of two nucleoli is irradiated with an ultraviolet microbeam. The attached X attached XY system ($\overline{XX}$, $\overline{XY}$) constructed by Lindsley and Novitski³ in Drosophila provides an opportunity for investigating in another organism the behaviour of a nucleus that is deprived of a nucleolus. Nucleolus organizer regions exist on both the X and Y chromosomes of Drosophila, and none exists on the autosomes. When an $\overline{XX}$ female is crossed to an $\overline{XY}$ male, four types of zygotes are obtained, each containing the complete set of autosomes, but varying in sex chromosome constitution. With respect to the sex chromosomes, the zygotes are $\overline{XX}$/O, $\overline{XY}$/O, $\overline{XX}$/$\overline{XY}$, and O/O, where O represents the absence of a sex chromosome. The $\overline{XX}$/O become normal females, the $\overline{XY}$/O become normal males, the $\overline{XX}$/$\overline{XY}$ become superfemales, and the O/O die during embryogenesis. Hence, 25 per cent of the total embryos should be O/O in character.