Fig. 3.

sanpodo exerts a context-dependent effect on Notch signaling during lateral inhibition. (A) Model of heart precursors and differentiated (mature) heart cells. Left, in a wild-type hemisegment two cardioblast precursors (red cells labeled ‘P’) each divide to produce two cardioblasts (red cells labeled ‘CB’), and two Svp-lacZ⁺ heart precursors (green cells labeled ‘P’) each divide to produce one Svp-lacZ⁺ pericardial cell (green, ‘A’ daughter cell) and one Svp-lacZ⁺ cardioblast (red, ‘B’ cell). Right, in numb mutant embryos heart precursors form normally but Svp-lacZ⁺ heart precursors produce only pericardial cells (green, ‘A’ cells). Precursors are shown for one hemisegment, whereas mature heart cells are shown for one full segment. (B-F) High-magnification views of two hemisegments of the dorsal mesoderm of embryos of indicated genotype labeled for Svp-lacZ⁺ (green) or Mef-2 (red), which labels mesodermal cells. (B′-F′) High-magnification views of three segments of the heart of embryos of indicated genotype labeled for Svp-lacZ⁺ (green) and Nmr-1 (red), Nmr-1 specifically labels cardioblasts (Leal et al., 2009). (B) In wild-type stage 12 embryos, two Svp-lacZ⁺ heart precursors arise per hemisegment (arrows; dotted lines demarcate segment-sized regions in B-F). (B′) By stage 16, two rows of cardioblasts (red; yellow for Svp-lacZ⁺ cardioblasts) and Svp-lacZ⁺ pericardial cells (green) align on either side of the dorsal midline (thick white line). In B′-E′ one segment is bracketed and white lines indicate defined or inferred sibling relationships. (C) twist-GAL4-mediated expression of spdo in a wild-type embryo generates extra Svp-lacZ⁺ heart precursors (arrowhead) by stage 12, and extra Svp-lacZ⁺ pericardial cells and cardioblasts (arrowheads, C′) as well as extra Svp-lacZ⁻ cardioblasts (arrow, C′) by stage 16. (D) In numb mutant embryos two Svp-lacZ⁺ precursors arise normally (arrows), but each divides to produce two pericardial cells (green cells, D′), no Svp-lacZ⁺ cardioblasts develop in this background. (E) Expressing spdo throughout the mesoderm of a numb mutant embryo inhibits the formation of Svp-lacZ⁺ precursors (arrow), which leads to fewer Svp-lacZ⁺ pericardial cells (green cells, E′); note also the reduction in cardioblasts (red cells, E′). (F,F′) In wild-type mesodermal expression of a constitutively active form of Notch (Notch^ΔECN) leads to a near complete loss of Svp-lacZ⁺ heart precursors (arrowheads, F) and heart cells (F′). Anterior, left.