A Cytological Study of W Chromosome Instability in Cleavage Mitoses of Drosophila Melanogaster.

NSTABILITY of the wVe ring-X chromosome of Drosophila melanogaster is I attributable to anaphase bridge formation during the cleavage mitoses where loss of the bridge from w""/+ embryos results in gynandromorphs or XO males and breakage of the bridge results in lethality (HINTON 1955). Rod-w"" chromosomes derived from unstable wuc ring chromosomes also cause lethality, but are not lost during cleavage. If lethality is due to anaphase bridge breakage in both the unstable ring and rod chromosomes, the bridges must arise from sister strznd fusion rather than sister strand crossing over. Structural differences between bridges formed by sister strand fusion may account for the difference with respect to gynandromorph production by the two kinds of chromosomes (Figure 1 ) ; the ring-wVc double chromatid bridge may be subject to either breakage or loss while the rod-wVC single chromatid bridge may always break (HINTON 1957). This model of wvc instability is supported by the differences in bridge frequencies for stable and unstable rings observed in larval brain cells (BRAVER and BLOUNT 1949; HINTON 1955) and in secondary spermatocytes (WELSHONS and HINTON 1955). Because of their limited nature and the questionable relevance of observations on these tissues, the results of a cytological study of ring and rodwvc embryos of the third through eighth cleavages are reported in this paper. These results provide partial confirmation of the anaphase bridge model in that the frequencies of bridges and other mitotic abnormalities are associated with the degree of instability manifested by the ring and rod chromosomes.