FIGURE 4.5 Similarity in segment polarity gene expression in the ectoderm and developing nervous system of various arthropods. The segment polarity genes function to maintain and refine segments within both the nervous system and the ectoderm of Drosophila. Shown here is the expression of the segment polarity gene gooseberry (gsb) in Drosophila. Similar patterns of striped expression of gsb homologs through both the ectoderm and the neurogenic region are seen in the grasshopper (Schistocerca), a crustacean (Parhyale), two species of spiders (Schizocosa and Cupiennius), and a centipede (Lithobius).
projections (Duman-Scheel and Patel, 1999). Further studies are still needed in myriapods and chelicerates to determine if one-to-one homologies can be extended to these arthropods. In either case, the segmental nature of the neuromeres is apparent for all arthropods. The early steps in the segmentation process vary significantly between arthropod groups, but there is significant conservation at the level of segment polarity gene expression. For example, the segment polarity gene gsb is expressed in the posterior portion of each ectodermal segment and in the underlying neuroblasts of rows 5 and 6. As shown in Fig. 4.5, the expression pattern of this gene is well conserved in insects, crustaceans, myriapods, and chelicerates. Thus, there are clear molecular similarities in the mechanisms that create the pattern of both body segments and neuromeres in all arthropods.