Chromosome condensation and the evolution of Drosophila karyotypes. Shaila Kotadia, William Sullivan. Molecular, Cell and Developmental Biology, University of California, Santa Cruz, Santa Cruz, CA.

   Chromosomes must be cleared from the metaphase plate during anaphase to prevent collisions between lagging chromosomes and the cleavage furrow. How cells adapt to clear particularly long chromosomes remains largely unexplored. In yeast and mammals, successful clearance of lagging or long chromosomes from the plate occurs by either a delay in cytokinesis initiation or increased compaction of anaphase chromosomes. In contrast, we find that Drosophila melanogaster neuroblast stem cells containing long armed chromosomes elongate during late anaphase and telophase, thus creating more space to ensure proper chromosome clearance. We found that the extent of elongation directly correlates with the length of the chromosome arm. While these studies reveal a novel pathway in D. melanogaster for chromosome clearance prior to cell cleavage, the tested karyotypes relied on translocations and other lab generated chromosome rearrangements. Thus, the mechanism of clearing naturally long armed chromosome karyotypes still remained a mystery. Therefore, we examined Drosophila virilis and americana, which have greater than twice the normal arm length of D. melanogaster. To our surprise, D. virilis and americana compact their long chromosomes in anaphase to ensure clearance from the metaphase plate. Therefore, Drosophila evolution solves the issue of clearance by compaction rather than elongation or a delay in cytokinesis. We will present our studies examining the role of condensin in regulating anaphase arm length in Drosophila chromosome evolution.