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We have isolated the zebrafish ziro7 gene, a novel, divergent member of the Iroquois family. ziro7 is expressed at early epiboly stages in the dorsal half of the zebrafish embryo, with a higher level in the dorso-lateral margin. From mid-gastrulation stages onward, ziro7 is expressed in a large transversal stripe in the future neural plate, which subsequently divides into thinner stripes located in the diencephalon, midbrain and hindbrain.

We examined the pattern of expression in hair follicles of the transcription factor gene Krox-20. During embryogenesis, Krox-20 is first expressed in the upper portion of the hair bud, then in the hair canal, in the sebaceous glands and in the outer root sheath. In the mature follicles, Krox-20, like Shh and TGFbetaRII, is also expressed in a sub-population of matrix keratinocytes located in a ring-like fashion in vibrissal follicles and clustered on one side of the papilla in pelage follicles. This polarized pattern rotates around the papilla as a result of sequential gene expression by individual groups of matrix cells. This peculiar pattern is not linked to follicle angling.

The morphogenesis of the vertebrate hindbrain involves a transient segmentation process leading to the formation of reiterated organisation units called rhombomeres (r). A number of regulatory genes expressed with a rhombomere-specific pattern have been identified, including the gene encoding the transcription factor Krox-20, which is restricted to r3 and r5. We have previously demonstrated that in r3 and r5 Krox-20 directly controls the transcription of Hoxa-2 and Hoxb-2. In the present study, we provide evidence that Krox-20 is required for the expression of another Hox gene, Hoxb-3, in r5 specifically. Furthermore, the regulatory role of Krox-20 is not restricted to the control of Hox gene expression, since it is also involved in the activation of a receptor tyrosine kinase gene, Sek-1, in r3 and r5 and in the repression of the follistatin gene in r3 but not in r5. In conclusion, at least five regulatory genes belonging to different families are under the direct or indirect control of Krox-20 in r3 and/or r5 and this transcription factor therefore appears as a key regulator of gene expression in the developing hindbrain.