The canonical cell cycle alternates rounds of chromosome replication with chromosome segregation. This process ensures that at metaphase each chromosome is composed of a single pair of chromatids that can be bi-oriented by the spindle and segregated evenly. However, many cells undergo an alternate cell cycle termed the endocycle in which multiple rounds of DNA replication take place without intervening segregation. Endocycled tissues are traditionally thought of as being terminally differentiated. However, as we previously showed in the Drosophila rectum, as well as following treatment with many common mitosis blocking drugs, endocycled cells return to the mitotic cell cycle. How cells respond to the challenge of segregating reduplicated chromosomes is not well understood.
The Drosophila rectum provides a developmentally accessible model to answer this question., During rectal development, cells undergo two rounds of the endocycle and then undergo two rounds of mitosis. We find that despite re-entering mitosis with re-duplicated chromosomes, papillar cell division is relatively error free. Using a combination of live-imaging, pharmacology, genetics, and classical cytology we have defined a process that we call Separation Into Recent Sisters (SIRS). SIRS is a spindle- independent process that separates the developmentally programmed polytene chromosomes in the Drosophila rectum into individual pairs of sister chromatids prior to anaphase.
In contrast to the perfectly executed polytene separation during SIRS, mitosis after aberrant (non-programmed) endocycles is highly error prone and results in aneuploidy and cell death. SIRS is lacking in such cells, and as a result polytenes form conjoined chromosomes at anaphase. Further, we have found that while conjoined chromosomes following induced endocycles activate the mad1/mad2 dependent spindle checkpoint, SIRS instead relies on a checkpoint-independent function of mad2 to regulate the length of mitosis. Together, these data give important insights into how endocycled cells respond when they return to mitosis. These findings have relevance to both developing polyploid tissues as well as the many pathological conditions in which polyploidy plays a role.