Supplemental Materials

This article contains the following supporting material:

• Figure S1 - Model of the nuclear elastic forces. We model nuclear mechanics with three springs connected at a hinge at the center of the nucleus. The two springs at the sides are connected to the centrosomes and stretched, while the spring at the center is compressed against the actin cortex at the top. B: The computed nuclear elastic force F_nucl(S) (blue curve) is small at small pole-to-pole distance and is approximately a linear spring (red line) when the separation is significant. C: Illustration of the vectors that appear in the model equations (MT forces – green, nucleus forces – blue, schematic centrosome velocities - black).
• Video 1 - Time-lapse movie of GFP-tubulin expressing Drosophila embryo injected with rodamine-actin. Dynamic of centrosomes (green) and actin (red) is shown during interphase-prophase. In order to show both actin and centrosomes, the confocal plane where centrosomes were clearly visible (about 2 μm below the cortex) has been chosen so no actin cap is visible in this movie. The time from the start to the end of the movie is 380 s and the movie plays at 85×real time.
• Video 2 - Actin dynamics of a single "bud" during interphase-prophase. Six confocal planes from cortex (on the left) to 3 μm depth in 0.5 μm steps are shown. The time from the start to the end of each movie is 207 s.
• Video 3 - Computer simulation of Hypothesis I for prophase centrosome separation in the syncytial blastoderm stage of the Drosophila embryo. Centrosomes (pink) nucleate MTs (green, magenta) that interact with dynein and actin (red) at the cortex. Free astral MTs (green) undergo dynamic instability and, if in contact with the cortex, exert a polymerization force that pushes on centrosomes; MTs at the cortex that attach to dynein (magenta) are pulled on by dynein with a force determined by their velocity relative to the cortex and the dynein force-velocity relation. The nucleus (blue) acts as an elastic body, stretching as the centrosomes are pulled in opposite directions. The actin cap (red) grows by a centrosome-regulated mechanism (a centrosome-activated kinase that up-regulates actin polymerization). MTs from opposing centrosomes that “contact” each other are cross-linked into ipMTs (not shown) and contribute to the inward Ncd force. This depletes cortical MTs between the centrosomes leading to an outward bias in the net dynein force.
• Video 4 - Computer simulation of Hypothesis II for prophase centrosome separation in syncytial blastoderm stage of the Drosophila embryo. Simulations are as in Movie 4 except that the asymmetry in cortical dynein force stems from the asymmetry of dynein activation. A hypothetical centrosome-activated kinase down-regulates dynein with the resulting level of dynein activity plotted (yellow curve). This mechanism is successful at explaining separation in wild-type as well as Ncd null embryos.
• Table S1