Fig. 1. Dynamics of expression of bon/mixer, fau/gata5, casanova and sox17 genes in wild-type embryos at 40% epiboly (A-H), 50% epiboly (I-L), shield (M-P) and 70-80% epiboly (Q-T) stages. (A-D) Animal pole views; (E-T) lateral views, dorsal to the right. At 40% epiboly, whereas bon/mixer (A,E) and gata5/fau (B,F) are homogeneously expressed in large marginal domains, casanova expression is mosaic and preferentially restricted to the most marginal blastomeres of the dorsal side (C,G). At this stage, sox17 is expressed only in the superficial and marginal cells of the dorsal side (D,H). At 50% epiboly, expression patterns of bon/mixer (I), gata5/fau (J) and sox17 (L) are roughly unchanged. The casanova pattern is still mosaic but it is found throughout the margin and in the forerunner cells (K). At the shield stage, bon/mixer (M) and gata5/fau (N) are expressed in more germ ring blastomeres. Cells expressing casanova have begun to involute and abut the YSL, except the forerunner cells, which remain superficial (O). The sox17 gene is expressed in deep cells abutting the YSL in the dorsal axis (P). After the onset of gastrulation, bon/mixer (Q) is no longer expressed, whereas fau/gata5 (R), casanova (S) and sox17 (T) are expressed in the scattered endodermal cells (arrowhead); casanova (S) and sox17 (T) are still expressed in the forerunner cells. (U,V) Close up of the mosaic pattern of casanova at the dorsal margin (U) and lateral margin (V) of embryos at 40% epiboly stage (notice the isolated blastodermal cas-positive cells (arrowheads); the dotted lines mark the YSL-blastoderm frontier). (W-Y) Cross sections of embryos following whole-mount in situ hybridization with casanova at 50% epiboly (W), shield (X) and 70-80% epiboly stage (Y), animal pole up (arrows point to YSL nuclei). The cas positive cells involute at the margin, abut YSL and spread over the whole embryos with a scattered pattern.