Conserved and divergent aspects of leg development in Tardigrada

Honorable Mention Winner Legs are a defining characteristic of Panarthropoda, a lineage of animals that includes Arthropoda, Onychophora, and Tardigrada. Arthropod and onychophoran leg development are well studied, but little is known about the leg development of tardigrades. Our recent investigations revealed several intriguing examples about the conservation and diversification of downstream developmental mechanisms in the appendages of the tardigrade Hypsibius exemplaris. In arthropods and onychophorans, the leg gap genes homothorax, extradenticle, dachshund, and Distal-less regulate appendage growth and specify proximodistal leg identities. However, in tardigrades the leg gap genes are not expressed in the regionalized patterns that are seen in onychophorans and arthropods. Dachshund is missing from the tardigrade genome. A distal leg patterning network is active downstream of Distal-less in arthropods and many features of this network are conserved in onychophorans. However, of the distal leg patterning genes thus far investigated, only aristaless is expressed in all leg buds during tardigrade development. Here, we investigate upstream components of tardigrade leg development. In arthropods, wnt1 activates Sp6-9 to specify ventral appendages. This mechanism is most likely conserved in onychophorans. We identified orthologs of two Sp genes in Hypsibius expemplaris, including two copies of Sp6-9. We did not find a buttonhead ortholog, suggesting that this gene is missing from the tardigrade genome. While Sp6-9 is expressed strongly in the developing legs of tardigrades, wnt1 is missing from the tardigrade genome. Placed in a phylogenetic context, our results reveal that leg development has been secondarily simplified in Tardigrada in association with miniaturization