Expression of N-terminally truncated cyclin B in the Drosophila larval brain leads to mitotic delay at late anaphase.

We have introduced an N-terminally truncated form of cyclin B into the Drosophila germ-line downstream of the yeast upstream activator that responds to GAL4. When such lines of flies are crossed to lines in which GAL4 is expressed in imaginal discs and larval brain, the majority of the resulting progeny die at the late pupal stage of development. Very rarely (< 0.1% of progeny) adults emerge that have a mutant phenotype typical of flies with mutations in genes required for the cell cycle; they have rough eyes, deformed wings, abnormal bristles, and die within hours of emergence. The brains of third instar larval progeny show an abnormally high proportion of mitotic cells containing overcondensed chromatids that have undergone anaphase separation, together with cells that cannot be assigned to a particular mitotic stage. Immunostaining indicates that these anaphase cells contain moderate levels of cyclin B, suggesting that persistent p34cdc2 kinase activity can prevent progression from anaphase into telophase.