Fig. 3.

Crossing-only severing is essential for alignment if induced catastrophes are rare, but is insufficient if they are absent. Bundling occurs for $\theta <40°$. (A) Without severing, the reduced induced catastrophe probability $P_{cat}=0.09$ is insufficient to find an aligned regime. (B) With high severing ($r_x=0.1{\text{s}}^{-1}$), some degree of alignment occurs over the same range of parameters for all three values of $P_{cat}$. (C) Using optical instead of normal $S_2$ to measure alignment, i.e., counting every stretch of MT bundle as a single MT, regardless of the actual number in the bundle, alignment disappears toward $G_c=0$ in the absence of induced catastrophes. Also, for intermediate values of $G_c$, the arrays would appear poorly aligned. Error bars represent SEM, with n = 100 independent simulations per data point. For other values of $r_x$ and buildup of alignment over time, see SI Appendix, Figs. S6–S8.