Distal-less expression in embryos of Limulus polyplremus (Chelicerata, Xiphosura) and Lepisma saccharina (Insecta, Zygentoma) suggests a role in the development of mechanoreceptors, chemoreceptors, and the CNS

The homeobox gene Distal-less (Dll) is well known for its participation in the development of arthropod limbs and their derivatives. Dll activity has been described for all groups of arthropods, but also for molluscs, echinod erms and vertebrates. Generally, Dll participates in the establishment of t he proximo-distal-axis and differentiation along this axis. During our inve stigation of the expression pattern in the silverfish Lepisma saccharina an d the horseshoe crab Limulus polyphemus, we found several expressions in la te stages which cannot be explained with the "normal" limb-specific functio n. The antenna, cerci and terminal filament of the silverfish show a stripe d expression; single cells on the labrum, mandibles, maxillary palps and an al valves are also strongly stained by the Dll antibody. In addition to cel l groups in the developing ganglia of the CNS, in the coxal endites and sev eral nerve cells in femur and the trochanter of the prosomal limbs, the who le prosomal shield of Limulus polyphemus is surrounded by Dll-positive cell clusters. Furthermore, the lateral processes of the opisthosoma and the ed ges of the opisthosomal appendages are Dll positive. To get an indication o f the cell fate of these regions, we examined hatched larvae and juvenile s tages of both species with the SEM. We found a striking correlation of thes e Dll-positive areas and different sense or gans, especially mechanorecepto rs. Since many sense organs in arthropods are situated on the limbs, interp retation of the Dll expression in limbs is problematical. This has critical implications for comparative analysis of Dll expression patterns between a rthropods and for the claim of homology between limb-like structures. Furth ermore, we discuss the possibility of convergent appendage evolution in var ious bilaterian groups based on the improvement of spatial sensory resoluti on.