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Fig. 8. Migration outcomes for inner medial and inner lateral cell clones. IM and IL cells were photoactivated in control and htl^AB42 mutant Drosophila embryos (see Materials and methods for details) and development allowed to proceed until 90 minutes after gastrulation. The embryos were then fixed and immunostained for the mesoderm specific transcription factor Twist. By this stage, the monolayer was established and the photoactivated cells had divided twice to form clones of four cells. (A,C,E,G) Typical clones of photoactivated cells (red) within the mesoderm stained for Twist (green). (B,D,F,H) Schematics of IM/IL clone positions in control and htl^AB42 embryos. Positions within the monolayer were calculated as a fraction of the full extent of the mesoderm, and are depicted as grey circles at proportional positions within the black boxes. The mean extent of the mesdoerm in control (143±15 µm, s.d.) versus htl^AB42 (112±22 µm, s.d.) embryos is reflected by the width of the boxes. Boxes outlined in red correspond to the example embryos shown (A,C,E,G). (A) An IM clone arranged as two pairs of cells within the mesoderm monolayer. (B) IM clones spanned the full extent of the mesoderm and were typically arranged as two pairs of cells. Pairs could be widely separated and located on opposite sides of the midline. (C) An IL cell, to the right of the midline, has produced a clone of cells at the right hand, dorsalmost part of the mesoderm. (D) IL clones in control embryos are grouped into those arising from IL cells initially located to the right of the midline (n=8) and those arising from IL cells to the left of the midline (n=5). In all cases, clones did not cross the midline and were positioned in the dorsal part of the mesoderm. (E) A htl^AB42 embryo in which the mesoderm has failed to spread into a monolayer. The clone of cells from an IM cell has failed to move away from the centre of the embryo. (F) IM clones in htl^AB42 embryos tended to remain fairly central, and did not separate into two distinct pairs. (G) An IL cell to the left of the midline in a htl^AB42 mutant, has produced a clone of cells that have not migrated out to the dorsal edge of the mesoderm. (H) IL clones in htl^AB42 embryos tended to stay on the same side of the midline as the progenitor cell, but did not usually reach the dorsalmost regions of the mesoderm. Scale bars: 20 µm.

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