Expression of arthropod distal limb-patterning genes in the onychophoran Euperipatoides kanangrensis.

A current hypothesis states that the ancestral limb of arthropods is composed of only two segments. The proximal segment represents the main part of the modern leg, and the distal segment represents the tarsus and claw of the modern leg. If the distal part of the limb is an ancestral feature, one would expect conserved regulatory gene networks acting in distal limb development in all arthropods and possibly even their sister group, the onychophorans. We investigated the expression patterns of six genes known to function during insect distal limb development in the onychophoran Euperipatoides kanangrensis, i.e., clawless (cll), aristaless (al), spineless (ss), zinc finger homeodomain 2 (zfh2), rotund (rn), and Lim1. We find that all investigated genes are expressed in at least some of the onychophoran limbs. The expression patterns of most of these genes, however, display crucial differences to the known insect patterns. The results of this study question the hypothesis of conserved distal limb evolution in arthropods and highlight the need for further studies on arthropod limb development.