Distribution of ring-X chromosomes in the blastoderm of gynandromorphic D. melanogaster.

Embryos of the D. melanogaster strain producing gynandromorphs by loss of the ring-X chromosome were treated with vinblastine to obtain blastoderms with all mitoses arrested in metaphase, and with tetracaine to improve the resolution of chromosomes. Ring-X and ringless mitoses were recorded in the major part of the blastoderm in 18 eggs. Limits between females and male areas were very irregular and some embryos had several isolated areas of one type or the other. The proportion of male nuclei varied from 80.8 to 0.4%, indicating that there must have been more than one loss of the ring-X in most of the eggs and that losses occurred as late as the ninth division. When the percentages of male nuclei were compared with theoretical values, all the observed percentages could be accounted for by two losses. In early cleavages the lost ring could be found halfway between ringless mitoses. Examination of chromosomes in the three polar nuclei showed that the ring often remained undivided in meiosis. If the resulting ringless haploid group became a female pronucleus, an XO or YO embryo was produced after fertilization. We propose a hypothesis to explain the two losses.