A cold-sensitive zygotic lethal causing high frequencies of nondisjunction during meiosis I in Drosophila melanogaster females.

The X-linked, cold-sensitive zygotic lethal, l(1)TW-6(cs), both in homozygous and heterozygous females, induces nondisjunction of all four chromosomes at Meiosis I at both 25 degrees and 17 degrees . Nondisjunction frequencies approaching 0.5 for the X and fourth chromosomes have been observed at 16 degrees -18 degrees . The disjunction of the X chromosomes in males is not affected. The mutant causes mitotic irregularities in zygotes at both 25 degrees and 17 degrees . Mortality of all zygotes produced by the crosses 6(cs)/6(cs)x6(cs)/B(s)Y and FM7/6(cs)x6(cs)/B(s)Y is respectively 86% and 67-74% at 25 degrees and 99.8-99.9% and 94% at 17 degrees . The mortality of 6(cs) hemizygotes derived from females carrying no doses of 6(cs) (C(1)DX,y f/yx6(cs)/B(s)Y) is 45-55% at 25 degrees and 98% at 17 degrees . The length of the temperature-sensitive period for 6(cs) homo- and hemizygotes is affected by the maternal dosage of 6(cs); 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.