Distributive pairing in the grasshopper Chorthippus binotatus

Abstract

Six males of the grasshopper Chorthippus binotatus were mosaic for the presence of extra (E) chromosomes in the germ line, but lacked them in the somatic cells of gastric caeca. E chromosomes were very similar to the X chromosome in size and meiotic properties (heteropycnosis and autopairing). X and E chromosomes associated frequently at diplotene, but the associations never persisted until metaphase I, which indicated that they were not chiasmate. When one E was present, X and E univalents segregated preferentially to opposite poles. In cells with two E, they formed a bivalent in almost all cells, and decreased the frequency of X–E associations by 20%. These cells showed a high frequency of nondisjunction between the two E chromosomes, such that they segregated independently despite the high persistence of their association at metaphase I. These results are interpreted and discussed in the light of the distributive pairing model.Key words: distributive pairing, X chromosome, E chromosome, Chorthippus, male meiosis.