Hox genes and the evolution of arthropod heads

Modern arthropod classes have a more conserved organization of head segments than of trunk segments, with the exception of the chelicerates. Arachnids, for example, have paired walking appendages on their prosomal segments, the anterior body region of chelicerates. These prosomal segments exhibit patterns of Hox gene expression that correspond to the expression of the anterior Hox genes lab, pb, Dfd, and Scr in crustaceans and insects (Fig. 5.4). In arachnids, these genes have broadly overlapping expression domains, including a segment-ally restricted Hox-like pattern of expression of the Hox3/zen ortholog. These Hox gene expression patterns suggest that the chelicerate prosoma is roughly equivalent to the head segments of insects and crustaceans.

The conservation of insect and crustacean head segmentation and patterning correlates with generally similar patterns of expression of anterior Hox genes (Fig. 5.4). The primary exception involves the regulation of the pb gene, which is expressed in different segments in insects and crustaceans and even among different insect orders. The insect gnathal head segments where pb is expressed form the mouthparts—namely, the mandible, maxilla, and labium. In crickets, beetles, and flies, the maxilla and labium are jointed and bear distal palps (compared with the stubby mandible); pb is expressed in both appendages. By contrast, hemipteran insects (including milkweed bugs and bedbugs) are characterized by specialized piercing-sucking mouthparts, where the mandibular and maxillary appendages form morphologically similar stylets. The milkweed bug exhibits pb expression only in the labium. This more limited pattern of pb expression correlates with the coordinated development of the milkweed bug's mandible and maxilla and reflects the loss of a patterning role for pb in the

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