A Cold-Sensitive Zygotic Lethal Causing High Frequencies of Nondisjunction during Meiosis I in DROSOPHILA MELANOGASTER Females

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The X-linked, cold-sensitive zygotic lethal, l(1)TW-6cs, both in homozygous and heterozygous females, induces nondisjunction of all four chromosomes at Meiosis I at both 25° and 17°. Nondisjunction frequencies approaching 0.5 for the X and fourth chromosomes have been observed at 16°–18°. The disjunction of the X chromosomes in males is not affected. The mutant causes mitotic irregularities in zygotes at both 25° and 17°. Mortality of all zygotes produced by the crosses 6cs/6cs x 6cs/BsY and FM7/6cs x 6cs/BsY is respectively 86% and 67–74% at 25° and 99.8–99.9% and 94% at 17°. The mortality of 6cs hemizygotes derived from females carrying no doses of 6cs (C(1)DX,y f/y x 6cs/BsY) is 45–55% at 25° and 98% at 17°. The length of the temperature-sensitive period for 6cs homo- and hemizygotes is affected by the maternal dosage of 6cs; the shortest TSP is for zero and the longest is for two maternal doses. Mortality takes place primarily during embryogenesis with some larval and little pupal mortality. Analysis of sectioned embryos indicates that the large array of different patterns of damage observed could have arisen from abnormal cleavage divisions and the incomplete population of the blastoderm with nuclei.

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