Expression of the mouse labial-Alike homeobox-containing genes, Hox 2.9 and Hox 1.6, during segmentation of the hindbrain

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Abstract
The sequence of a mouse Hox 2.9 cDNA clone is presented. The predicted homeodomain is similar to that of the Drosophila gene labial showing 80 % identity. The equivalent gene in the Hox 1 cluster is Hox 1.6 which shows extensive similarity to Hox 2.9 both within and outside the homeodomain. Hox 2.9 and Hox 1.6 are the only two mouse members of the labial-tike family of homeobox-containing genes as yet identified. Hox 2.9 has previously been shown to be expressed in a single segmental unit of the developing hindbrain (rhombomere) and has been predicted to be involved in conferring rhombomere identity. To analyse further the function of Hox 2.9 during development and to determine if the other mouse labial-tike gene Hox 1.6, displays similar properties, we have investigated the expression patterns of these two genes and an additional rhombomere-specific gene, Krox 20, on consecutive embryonic sections at closely staged intervals. This detailed analysis has enabled us to draw the following conclusions: There are extensive similarities in the temporal and spatial expression of Hox 2.9 and Hox 1.6, throughout the period that both genes are expressed in the embryo ( to 10 days). At 8 days the genes occupy identical domains in the neuroectoderm and mesoderm with the same sharp anterior boundary in the presumptive hindbrain. These similarities indicate a functional relationship between the genes and further suggest that the-labial-tike genes are responding to similar signals in the embryoBy 9 days the neuroectoderm expression of both genes retreats posteriorly along the anteroposterior (AP) axis. The difference at this stage between the expression patterns is the persistence of Hox 2.9 in a specific region of the hindbrain, illustrating the capacity of Hox 2.9 to respond to additional positional regulatory signals and indicating a unique function for this gene in the hindbrain.The restriction of Hox 2.9 expression in the hindbrain occurs at days, approximately the same time as Krox 20 is first detected in the posterior adjoining domain. The mutually exclusive expression of Hox 2.9 and Krox 20 demarcated by sharp expression boundaries suggest that compartmentalisation of cells within the hindbrain has occurred up to 6 h before rhombomeres (morphological segments) are clearly visible.Hox 2.9 expression is confined to the region of rhombomere 4 that shows cell lineage restriction and, unlike Krox 20, is expressed throughout the period that rhombomeres are visible (to days). These data strengthen the evidence that Hox 2.9 participates in conferring segment identity.Migrating neural crest cells that arise from rhombomere 4 are uniquely identified by the expression of Hox 2.9 supporting the idea that neural crest cells are patterned according to their rhombomeric origin.The Hox 1.6 gene product is differentially transcribed; only one of the two alternative transcripts codes for a homeodomain-containing protein. A comparison of the distribution patterns of the two transcripts shows that the relative proportion of homeo-domain-producing message decreases as development proceeds.