Xenopus spib identification. (A) Conserved protein motifs in spib and spi1; the PEST degradation domain and ETS DNA-binding domain. (B) ClustalW tree alignment on vertebrate spib and spi1 protein sequences and percentage identity over their entirety and DNA-binding domains. (C) Genome organization around the known vertebrate spib and spi1 genes, showing synteny between various vertebrate genomes. Genes shown in the same color represent putative orthologous genes. Expression pattern of Xenopus tropicalis spib and (D-G) Xenopus laevis spib paralogs (SpiBa and SpiBb, H-J) is indistinguishable and marks a population of primitive myeloid progenitors in the anterior ventral blood islands (aVBIs). spib expression is transient and first detected at stage 17. (F) Cleared embryo shows only mesodermal expression, and (J) spib expression is eventually down-regulated after spib-expressing cells associate with vitelline veins (arrowheads). Anterior is to the left, and dorsal to the top in all lateral views; panels D, G, and I,J represent ventral views. The following database sequences were used: Hs spib Q01892, Mm spib O35906, Xl spiba BC046671, Xl spibb BC130210, Xt spib IMAGE 7023083, Hs spi1 P17947, Mm spi1 P17433, Gg spi1 NP_990354, Xt spi1 BC098077, and Dr spi1 CAD58735 (outgroup NP_001008139 and P14921).