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Fig. 3. Microtubule distribution at the cortex in live embryos. (A) Normalized number of microtubule ends along the length of the embryo at the time of furrow initiation. The number of microtubule ends in a 5 micron-wide window surrounding each position was divided by the mean of all of the values for that embryo in order to compare different embryos. Average of 10 embryos. 0 is the position where the furrow begins ingression, anterior is left (negative values) and posterior is right (positive values). There are valleys and peaks throughout the length of the embryo and yet the equatorial cortex does not align with the major minimum between the poles. Analysis of individual embryos indicates that the equatorial cortex sometimes aligns with a local maximum of microtubules and rarely aligns with the global minimum (see supplementary material Fig. S4). Similar results were obtained for all timepoints between the onset of anaphase and furrow initiation (see supplementary material Fig. S5). The steep decrease in microtubule ends is a consequence of the way in which microtubule ends are counted. The measurement windows have approximately the same lateral extent in the central region of the embryo, which is roughly cylindrical, and so the counts in this region are equivalent to microtubule density, but the area of cytoplasm fall-off in the poles, which gives rise to a reduction of microtubule end counts in these regions. See supplementary material Fig. S6 for analysis of this data using different window sizes. (B) Number of microtubule ends at the equatorial cortex before and after furrow initiation in four different wild-type embryos. Time 0 is when the furrow first appears. Each frame is 10 seconds apart. There is no consistent increase or decrease in the number of microtubule ends in the equatorial cortex at the time of furrow initiation or 20 seconds earlier when the contractile ring is expected to form. (C) Average relative number of microtubule ends around the equatorial cortex at the time of furrow initiation for various mutant combinations that can furrow. Each line is an average of three embryos. 0 is the position where the furrow begins ingression, anterior is left (negative values) and posterior is right (positive values). There is no consistent increase or decrease of microtubule ends at the equatorial cortex in embryos that furrow.
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