MUTATIONS IN THE DROSOPHILA-MELANOGASTER GENE 3 ROWS PERMIT ASPECTS OF MITOSIS TO CONTINUE IN THE ABSENCE OF CHROMATID SEGREGATION

We have cloned the three rows (thr) gene, by a combination of chromoso me microdissection and P element tagging. We describe phenotypes of em bryos homozygous for mutations at the thr locus. Maternal mRNA and pro tein appear to be sufficient to allow 14 rounds of mitosis in embryos homozygous for thr mutations. However, a small percentage of cells in syncytial blastoderm stage thr embryos sink into the interior of the e mbryo as if they have failed to divide properly. Following cellularisa tion all cells complete mitosis 14 normally. All cells become delayed at mitosis 15 with their chromosomes remaining aligned on the spindle in a metaphase-like configuration, even though both cyclins A and B ha ve both been degraded. As cyclin B degradation occurs at the metaphase -anaphase transition, subsequent to the microtubule integrity checkpoi nt, the delay induced by mutations at the thr locus defines a later po int in mitotic progression. Chromosomes in the cells of thr embryos do not undertake anaphase separation, but remain at the metaphase plate. Subsequently they decondense. A subset of nuclei go on to replicate t heir DNA but there is no further mitotic division.