MITOTIC DELAY DEPENDENT SURVIVAL IDENTIFIES COMPONENTS OF CELL CYCLE CONTROL IN THE DROSOPHILA BLASTODERM

The Drosophila body pattern is laid down by maternal and zygotic factors which act during the early phase of embryonic development. During this period, nascent zygotic transcripts longer than about 6 kilobases are aborted between the rapid mitotic cycles. Resurrector1 (Res1) and Godzilla1 (God1), two newly identified dominant zygotic suppressor mutations, and a heterozygous maternal deficiency of the cyclin B locus, complement the partial loss of function of the segmentation gene knirps (kni) by extending the length of mitotic cycles at blastoderm. The mitotic delay caused by Res1 and God1 zygotically and by the deficiency of the cyclin B locus maternally allows the expression of a much longer transcript of a kni cognate gene normally aborted between the short mitotic cycles and consequently allows survival of kni mutant progeny. In addition to the practical benefits of identifying mutations in Drosophila cell cycle regulatory genes as suppressors of kni, our results have evolutionary implications regarding the flexibility of the genome to meet sudden selective pressures by recruiting cognate genes to function.