Figure 1. Zebrafish developmental series. Hours post-fertilization (h) are shown in the lower right of each panel. (a) Early blastula. (b) Midblastula. (c) Early gastrula. (d) Midgastrula. The arrows in (c) and (d) mark the position of the blastoderm margin and indicate the process of epiboly; epiboly is the movement of the blastoderm as it crawls over the yolk toward the vegetal pole. (e) Early segmentation stage embryo. Arrowheads in (e) mark developing somites. (f) Midsegmentation stage embryo. (g) Pharyngula. (h) Hatching embryo. The dark cells scattered over the head, body and yolk and in the eyes are pigment cells.

Figure 2. Zebrafish gastrula fate map and regional gene expression. (a) Live early gastrula. (b) Schematic drawing of the fate map at a similar stage. The embryonic blastoderm is shown in red. Mesodermal fates (notochord, muscle and blood) arise near the blastoderm margin, whereas neural tissue and epidermis arise farther from the blastoderm margin. Only a subset of tissue fates is shown. AP, animal pole; VP, vegetal pole; D, dorsal; V, ventral. (Redrawn from Kimmel CB et al. (1990) Development 108: 581–594.) (c) Early gastrula embryo fixed and processed to detect expression of the floating head (flh) gene. The dorsal cells at the blastoderm margin that express flh (stained blue) are found within the notochord domain of the fate map.

Figure 3. Wild-type and spadetail (spt) mutant zebrafish embryos. (a) Wild-type and spt mutant embryos at 24 h. Arrow marks the abnormal accumulation of cells in the spt tail bud. (b) High-magnification dorsal views of wild-type and spt mutant embryos at 14 h; spt mutants have a notochord, but clearly lack somites.

Figure 4. Zebrafish mutagenesis screening strategies. (a) Traditional screening strategy. (b) Haploid screening strategy. +, Wild-type allele; m, mutant allele. Red embryos represent mutant diploids (m/m) or haploids (m).

Figure 5. Zebrafish mutants at one day of development. The notochord (arrowhead) and an eye (open arrowhead) are indicated on the wild-type embryo. The other eye in the wild-type embryo is in another plane of focus. See text for descriptions of mutants. The boxed inset in the lower right shows high-magnification views of wild-type (wt) and no isthmus (noi) mutant embryo heads, showing the lack of the midbrain–hindbrain boundary (arrow) in no isthmus mutants.

Figure 6. Mutagenesis screening using gene expression patterns. Wild-type and valentino mutant embryos fixed at 22 h and processed to detect expression of three different genes. Comparing head views of wild-type and mutant embryos shows that engrailed3 (eng3) expression at the midbrain–hindbrain boundary and krox20 expression in hindbrain segment 3 is normal in valentino mutant embryos, but krox20 expression in hindbrain segment 5 is severely diminished (*). (Photographs courtesy of Dr Cecilia B. Moens.)