Sequence of meiotic events in female Drosophila melanogaster Temperature-sensitive periods of autosomal crossing-over and secondary nondisjunction of X chromosomes

In autosomal translocation/inversion bearing XXY females of Drosophila melanogaster the autosomal crossing-over frequency is very sensitive to heat shock in an early meiotic stage which is coincidental with premeiotic DNA synthesis. The increase in autosomal crossing-over caused an increase in X chromosome nondisjunction in the same oocytes. Following this stage a heat sensitive period more directly affecting nondisjunction occurs. At the end of this period there is a minor heat sensitive stage of crossing-over. It is suggested that the sequence of meiotic events is as follows: 1) Determination of preconditions of crossing-over, 2) non-homologous and homologous chromosome pairing, 3) final determination of crossing-over.