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Slit signaling promotes the terminal asymmetric division of neural precursor cells in the Drosophila CNS

Brijesh Mehta and Krishna Moorthi Bhat*

Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA
* Author for correspondence (e-mail: Kbhat{at}cellbio.emory.edu )



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  		Fig. 1. The GMC-1 in sli and sim mutants symmetrically divide to generate two RP2s. Embryos are stained for Eve. Anterior is up, vertical lines indicate the midline. RP2, larger arrow; sib, smaller arrow; GMC-1, arrowhead. Scale bars, 15 µm. (A-C) Wild-type embryos. (D-F) sli mutant embryos, the symmetric division of GMC-1 to generate two RP2s is shown. (G-I) Symmetric division of GMC-1 in sim mutant embryos. The midline in sim mutants become fused by approx. 8.5 hours of development. Note that RP2, aCC, pCC and other neurons are somewhat misplaced within a hemisegment in the CNS of 13-hour or older mutant embryos, however, in younger embryos (approx. 10 hours or less) these cells are not misplaced and do not cross the midline or segmental boundary.

 


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  		Fig. 2. The nuclear division and cytokinesis of GMC-1 in sli mutants is symmetric. Embryos in A-D (half segments) are double stained for Eve (red) and spectrin (green). RP2 is indicated by large arrow, sib by small arrow. Scale bars, 5 µm. Anterior is up. Both nuclear division and cytokinesis of GMC-1 are asymmetric in wild type (A,C) while in sli mutants (B,D), they are symmetric and yield two equal sized RP2s at the expense of the sib.

 


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  		Fig. 3. The symmetric mitosis of GMC-1 and GMC1-1a in sli mutants. Embryos in A-F are double stained for Eve (red) and Zfh-1 (green; yellow indicates co-localization); G-I are double stained for Eve (red) and 22C10 (green). One segment per panel is shown, vertical lines indicate the midline. RP2 is indicated by large arrow, sib by small arrow, aCC by large arrowhead and pCC by small arrowhead. Scale bars, 15 µm. Anterior is up. (A,B) Wild-type embryos. The Eve-positive sib and pCC are Zfh-1-negative whereas both RP2 and aCC are Eve and Zfh-1 positive. (C,D) sli mutant embryos. The duplication of the RP2 occurs at the expense of the sib (hemisegments on the left). (E,F) sli mutant embryos showing the duplication of the aCC neurons at the expense of pCC (hemisegments on the right). (G) Wild-type embryo. Note the ipsilateral RP2 and posterolateral aCC projections (long arrows). (H,I) sli mutant embryos showing the duplication of the RP2 and aCC neurons at the expense of the sib and the pCC, respectively.

 


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  		Fig. 4. The relationship between sli, insc and nb. Embryos in A-D are double stained for Eve (green) and Insc (red) and in E and F, for Eve. A-D show one hemisegment each; E and F, one segment each. Anterior is up, vertical lines mark the midline. Scale bars, 10 µm. (A,C) Wild-type embryos, Insc is asymmetrically localized (arrowhead) in GMC-1 and GMC1-1a. (B,D) sli mutant embryos, the localization of Insc in GMC-1 and GMC1-1a is non-asymmetric. (E) nb mutant embryo. In nb both the daughters of GMC-1 adopt a sib fate (arrows). (F) sli, nb double mutant embryo; both the daughters of GMC-1 adopt a sib fate.

 


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  		Fig. 5. Symmetrical division of GMC-1 in sli mutants is caused by the up regulation of Nub. Anterior end is up, only half segments are shown in these panels. Embryos in panels A-H are double stained for Nub (red) and Eve (green; Eve only panels are not shown). GMC-1 is marked by arrow; U and aCC/pCC neurons are marked by arrowheads. Scale bars, 10 µm. A,C,E and G show Nub staining; B,D,F and H show Eve, Nub merged images. (A,B) Wild-type embryo; note the high levels of Nub in GMC-1 (arrow). (C,D) sli mutant embryo. (E,F) Wild-type embryo; note the down regulation of Nub in GMC-1. (G,H) sli mutant embryo; note that the level of Nub in GMC-1 is not down regulated. Similar results were also observed with Miti. (I,J) Eve-stained hs-miti embryos, the GMC-1 symmetrically divides to generate two RP2s. (K) Eve and Zfh-1 stained wild-type embryo, the asymmetric division of GMC-1 has generated an Eve-positive but Zfh-1-negative smaller sib and a larger Eve- and Zfh-1-positive RP2. (L) Eve and Zfh-1 stained hs-miti embryo. In the upper right hemisegment, the GMC-1 has generated two RP2s whereas in the lower right hemisegment, the GMC-1 has divided normally to generate RP2 and sib. (M) Eve (green) and Insc (red) stained hs-miti embryo where miti is ectopically expressed before GMC-1 division. Note that Insc expression is non-asymmetric. Similar results were also observed with over-expression of nub.

 


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  		Fig. 6. Enhancement and suppression of the symmetrical division of GMC-1 in sim mutants. Embryos stained for Eve; anterior is up. Each panel is assembled from several photomicrographs of the same embryo to represent a continuous section of the CNS. Scale bars, 15 µm. (A) Wild-type embryo showing a smaller sib and a larger RP2. (B) sim mutant embryo. In 2 out of 10 hemisegments the GMC-1 has divided symmetrically to generate two RP2s (large arrows). (C,D) sim; Dp (2; 2) GYL embryos. In 6 out of 10 hemisegments shown the GMC-1 has divided symmetrically to generate two RP2s. (E) sim; Df (2L) GR4/+ embryo, GMC-1 divides normally to generate an RP2 and a sib.

 


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  		Fig. 7. Role of Robo in GMC-1 asymmetric division. Only half segments are shown, vertical lines indicate the midline. Scale bars, 15 µm. (A,B) Embryos stained for Eve (purple, nuclear) and Robo (red, membrane). Robo is in GMC1-1a (A) and GMC-1 (B). (C) Eve-stained robo null embryo, the GMC-1 symmetrically divides into two RP2s. (D) robo null embryo stained for Eve (red) and Zfh-1 (green; yellow indicates co-localization). The symmetrical division of GMC-1 and GMC1-1a into two RP2s and two aCCs (arrowheads) is shown.

 

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