An Sp6-9 ortholog is expressed in developing legs in the tardigrade Hypsibius exemplaris

Orthologs of the zinc finger transcription factor-coding gene Sp6-9 are expressed in developing appendages across Arthropoda and in Onychophora. Sp6-9 regulates appendage outgrowth in some arthropods by activating Distal-less (Dll) expression. This function has been proposed to represent an ancestral function of Sp6-9 for Arthropoda. Expression patterns of Sp6-9 and Dll in the onychophoran Euperapotoides kanangrensis suggest the conservation of this function in Onychophora. Here, we investigate Sp6-9 in the tardigrade Hypsibius exemplaris. We identified two Sp6-9 orthologs in this species. One ortholog, He-Sp6-9A, was expressed in the lateral ectoderm where legs develop before He-Dll. Later, both genes were expressed in limb buds at the earliest stages of outgrowth and were broadly expressed in developing legs throughout the remainder of development. Our results present the possibility that an ortholog of Sp6-9 regulates appendage outgrowth in Tardigrada, possibly by activating or maintaining expression of He-Dll. These results raise the possibility that this function was already present in the last common ancestor of Panarthropoda.