Evolution of karyotype in haploid cell lines of Drosophila melanogaster.

Seven continuous cell lines have been established in vitro from lethal embryos produced by the female sterile mutant mh 1182 of Drosophila melanogaster. Six lines show haploid metaphases. Karyotype analysis revealed a high level of aneuploid cells with frequent chromosome fragments. In three lines, haploid cells were quickly overgrown by diploid cells. Two lines were more stable but the proportion of haploid cells decreased with time. One line was stable, showing 80-90% of haploid cells for over 1 000 cell generations. Stable haploid clones have been isolated from two lines. Crossing of mh 1182/mh 1182 females with males bearing a ring X chromosome shows that the haploid genome retained in the cells is of maternal origin and that the diploid cells derive from pre-existing haploid cells. The appearance of the diploid cells and the conditions of karyotypic stability are analysed.