HORKA, A DOMINANT MUTATION OF DROSOPHILA, INDUCES NONDISJUNCTION AND,THROUGH PATERNAL EFFECT, CHROMOSOME LOSS AND GENETIC MOSAICS

Fs(3)Horka (Horka) was described as a dominant female-sterile mutation of Drosophila melanogaster. Genetic and cytological data show that Ho rka induces mostly equational nondisjunction during spermatogenesis bu t not chromosome loss and possesses a dominant paternal effect: the X, second, third and the fourth chromosomes, but not the Y, are rendered unstable while undergoing spermatogenesis and may be lost in the desc ending zygotes. The frequency of Horka-induced chromosome loss is usua lly 2-4% but varies with the genetic background and can be over 20%. T he X chromosome loss occurs during the gonomeric and the initial cleav age divisions. Loss of the X and fourth chromosomes shows no correlati on. We propose, based on similarities in the mutant phenotypes with th e chromosome destabilizing mutations nonclaret disjunctional and pater nal loss, that the normal Horka(+) product is required for function of the centromeres and/or nearby regions. Horka is a convenient tool for the generation of gynandromorphs, autosome mosaics and for the study of gene expression in mosaics.