A Hox class 3 orthologue from the spider Cupiennius salei is expressed in a Hox-gene like fashion

The class 3 Hox gene orthologue in insects, zerknullt (zen), is not express ed along the anterior-posterior axis, but only in extra-embryonic tissues, suggesting that it has lost its function as a normal Hox gene. To analyse w hether this loss of Hox gene function has already occurred in a basal arthr opod lineage, we have isolated a Hox3 orthologue from the spider Cupiennius salei. In contrast to the insect ten sequences, which have a highly diverg ed homeobox, the spider Hox3 gene orthologue, Cs-Hox3, shows a high sequenc e similarity to the class 3 Hox genes of other phyla, including chordates. In situ hybridization in early embryos shows that it is expressed in a cont inuous region covering the pedipalp segment and the four leg-bearing segmen ts. This expression pattern suggests a Hox-gene-like function for Cs-Hox3. On the other hand, the expression pattern does not strictly follow the coli nearity rule, as it overlaps fully with the expression domain of the class 1 orthologue of the spider, Cs-lab. Still, our data suggest that the ancest or of the arthropods must have had a class 3 Hox gene with a function in an terior-posterior axis specification and that this function has been lost in the lineage leading to the insects.