A transmissible dicentric chromosome in Drosophila melanogaster

A transmissible dicentric chromosome was recovered in Drosophila melanogaster. The radiation-induced secondary chromosome rearrangement consists essentially of the entire Y and fourth chromosomes joined by 2R heterochromatin. The Y^S · Y^L2Rh4 · chromosome pairs with the X and the free fourth chromosome to form a trivalent in meiosis that is unusual because it forms few chromosome bridges in primary spermatocytes and is transmitted at high frequency. We suggest that the orientation of the weaker fourth chromosome kinetochore eventually fails when opposing the stronger Y kinetochore so that the Y^S · Y^L2Rh4 · moves to the pole to which the Y kinetochore is oriented. There is however an increased frequency of sex chromosome nondisjunction (14%) and of chromosome laggards (6%) in primary spermatocytes; the frequency of exceptional progeny of males containing the Y^S · Y^L2Rh4 · was 7.44% compared with 0.25% in the controls. Disruption of normal sex chromosome disjunction also occurs in females containing the Y^S· Y^L2Rh4 · and a compound X chromosome; the frequency of exceptional progeny was 2.55% versus 0.91% in the controls. Chromosome nondisjunction appears to occur when orientation of the X and Y kinetochores to the same pole is stabilized through tension by the orientation of one or both fourth chromosome kinetochores to the opposite pole. During anaphase, the orientation of the fourth chromosome kinetochore of the Y^S· Y^L2Rh4 · appears to fail and the X and Y^S · Y^L2Rh4 · chromosomes move to the same pole. Y^S· Y^L2Rh4 · chromosome laggards occur with both the Y and fourth chromosome kinetochores amphitelically oriented. This orientation appears to be stable as a result of equal opposing forces toward opposite poles.