Fig. 2. Disruption of primitive hematopoiesis in bls mutants. In situ hybridization analysis of hematopoietic markers scl, gata1, gata2, ikaros and the endothelial marker flk1 in bls mutants. (A) Expression of scl (red) and gata1 (blue) in wild-type and bls embryos. Cells expressing both scl and gata1 appear purple. At eight somites, scl transcripts can be detected in the anterior and posterior lateral plate mesoderm. The expression of scl in the anterior lateral plate does not appear to be perturbed in bls. By contrast, the expression of scl and gata1 in bls is decreased at eight somites, with scl expression appearing less reduced than that of gata1 (arrowhead). By 18 somites, scl expression persists in the ICM but gata1 expression cannot be detected. At 23 hpf, some scl transcripts can be detected in the posterior ICM (arrowhead with asterisk) and gata1 expression remains absent. Note that the scl/gata1 double in situ clearly defines the anterior versus posterior ICM, as the anterior ICM appears dark purple and the posterior ICM appears red. (B) Expression of gata1 (in dark purple) and myosin (in blue) in wild-type and bls embryos. In wild-types, gata1 expression is shown at eight somites in the lateral plate mesoderm (arrowhead). Myosin is expressed in the differentiating somites. gata1-expressing cells in the lateral plate mesoderm converge towards the midline by 18 somites to form the ICM, which expresses gata1 (arrowhead). Expression of gata1 persists in the hematopoietic progenitors at 23 hpf. By contrast, gata1 is absent in bls embryos, from as early as eight somite stage. (C) Expression of gata2, ikaros, and flk1 in wild-type and bls embryos. gata2 transcripts are detectable in bls, although the experession appears to be reduced (arroheads and arrowheads with asterisks), whereas ikaros expression is absent in bls. By contrast, flk1 expression is undisturbed by bls.