A CONSERVED MODE OF HEAD SEGMENTATION IN ARTHROPODS REVEALED BY THE EXPRESSION PATTERN OF HOX GENES IN A SPIDER

Chelicerates constitute a basic arthropod group with fossil representa tives from as early as the Cambrian period. Embryonic development and the subdivision of the segmented body region into a prosoma and an opi sthosoma are very similar in all extant chelicerates. The mode of head segmentation, however, has long been controversial. Although all othe r arthropod groups show a subdivision of the head region into six segm ents, the chelicerates are thought to have the first antennal segment missing. To examine this problem on a molecular level, we have compare d the expression pattern of Hox genes in the spider Cupiennius salei w ith the pattern known from insects. Surprisingly, we find that the ant erior expression borders of the Hox genes are in the same register and the same relative segmental position as in Drosophila. This contradic ts the view that the homologue of the first antennal segment is absent in the spider. Instead, our data suggest that the cheliceral segment is homologous to the first antennal segment and the pedipalpal segment is homologous to the second antennal (or intercalary) segment in arth ropods. Our finding implies that chelicerates, myriapods, crustaceans, and insects share a single mode of head segmentation, reinforcing the argument for a monophyletic origin of the arthropods.