Poleward microtubule flux is a major component of spindle dynamics and anaphase A in mitotic Drosophila embryos

During cell division, eukaryotic cells assemble dynamic microtubule-based spindles to segregate replicated chromosomes [1, 2]. Rapid spindle microtubule turnover, likely derived from dynamic instability, has been documented in yeasts [3,4], plants [5] and vertebrates [6]. Less studied is concerted spindle microtubule poleward translocation (flux) coupled to depolymerization at spindle poles [7]. Microtubule flux has been observed only in vertebrates [7], although there is indirect evidence for it in insect spermatocytes [8, 9] and higher plants [10]. Here we use fluorescent speckle microscopy (FSM) to demonstrate that mitotic spindles of syncytial Drosophila embryos exhibit poleward microtubule flux, indicating that flux is a widely conserved property of spindles. By simultaneously imaging chromosomes (or kinetochores) and flux, we provide evidence that flux is the dominant mechanism driving chromosome-to-pole movement (anaphase A) in these spindles. At 18 C and 24 C, separated sister chromatids moved poleward at average rates (3.6 and 6.6 mum/min, respectively) slightly greater than the mean rates of poleward flux (3.2 and 5.2 mum/min, respectively). However, at 24 C the rate of kinetochoreto-pole movement varied from slower than to twice the mean rate of flux, suggesting that although flux is the dominant mechanism, kinetochore-associated microtubule depolymerization contributes to anaphase A.