Fig. 3. Comparative analysis of rst and kirre expression patterns. Anterior is towards the left. Dorsal is upwards for A,I,K (lateral view) and F,H,L (ventrolateral view). (B-E,G) Dorsal view. (A-F) rst in situ hybridization. (A) Expression of rst starts during stage 4 in seven stripes enveloping the embryo. Soon thereafter this pattern fades and rst is expressed dorsally in procephalic regions and in the future amnioserosa, while ventrally the striped pattern remains present during invagination of the ventral furrow (data not shown). (B) At stage 8, rst labels a segmental pattern in mesectodermal cells. (C) During stage 9, rst is expressed in the midline (arrow) and mesodermal cell clusters (arrowhead). (D) At stages 10 to 11, the staining in the midline increases and rst starts to label progenitors of the visceral muscles (arrowhead). (E) At stage 12, the midline staining fades and rst is expressed strongly in the majority of mesodermal cells. (F) At stages 13-14, rst labels segmental stripes of mesodermal cells close to the epidermis. (F'out) Focal plane close to the epidermis: rst expression includes regions where founder cells reside. dl, dorsolateral; l, lateral; v, ventral clusters (d, dorsal cluster out of focus in F’out). (F’in) Focal plane somewhat further interior within the mesoderm: rst is expressed where fusion-competent myoblasts reside. (G-L) kirre in situ hybridization. (G) Expression of kirre starts during stage 11 in the progenitors of the visceral muscles (arrowhead). (H) kirre expression can be detected up to stage 16, when kirre labels a ventral u-shaped structure at the posterior end of the head region (arrowhead) and two regions more anterior (arrow). (I) During stages 12 to 13, kirre is expressed in segmental clusters close to the epidermis at positions where founder cells reside. (I’) Higher magnification of the boxed region in I, showing kirre expression in the dorsal group of muscle founder cells (d). (J) Scheme of the larval muscle pattern. Depicted in red are muscle groups that can be identified in K',L'. (K,K',L,L') Expression of kirre in outgrowing muscle founder cells/precursors in stage 13-14. lt, lateral transversal muscle precursor; vt, ventral transversal precursor. (M-O) Rst antibody staining of abdominal hemisegments of embryos of the rP298lacZ enhancer trap line. Green labels ß-galactosidase-expressing nuclei and red highlights Rst signal. (M) Single confocal plane within somatic mesoderm of a stage 13-14 rP298lacZ embryo. Arrowheads indicate cells labelled for Rst and ß-galactosidase. (M') Reconstruction of one hemisegment viewed from anterior. Position and morphology of Rst-labelled cells that do not express ß-galactosidase strongly suggests that these cells are fusion-competent myoblasts. vm, visceral muscles. (N,N') Staining of stage 13 to 14 rP298lacZ embryos in a mbcC1 background are roughly comparable with M,M'. (O) In older embryos of the same genotype, Rst expression can be detected in fibrous cells expressing ß-galactosidase (founder cells), while it is very weak in fusion-competent myoblasts. v, ventral founder cells; sb, founder cell of segment border muscle. Scale bars: 20 µm.
