Cortical flow and de novo synthesis of actin filaments in grasshopper spermatocytes.

(A) Time-lapse sequence of actin filaments undergoing cortical flow (from Video S11). Flow of actin was induced by collapsing the spindle and repositioning it near one side of the cortex. The final location of the spindle is marked by “sp”. Actin filaments were labeled by microinjection with Alexa 488 phalloidin. Note the clearing over time of the region closest to the spindle, as the actin flows to the cortex on the opposite side of the cell. Spindle-associated mitochondria were seen as a pair of large bright patches (labeled “m”) that autofluoresced in the FITC channel. Because mitochondria were localized to the cytoplasm, their displacement was not related to cortical flow, but rather was driven by the elongation of dynamic microtubules with which they were associated. Time in minutes. (B) A confocal micrograph showing actin aggregates (red, marked by arrows) localized to the tips of bundled microtubules (green) at the spindle midzone. These aggregates were strikingly similar in their location, timing of appearance and morphology to aggregates found in silkworm spermatocytes [17], which have been shown to be assembled de novo at the midzone. Actin was labeled by microinjection of rhodamine phalloidin during anaphase, and microtubules were labeled with paclitaxel green. A cell division scar (red circle, lower edge of cell) was visible. Bars, 10 µm.