Meeting Abstract
Panarthropoda is composed of three lineages of animals that possess legs—Arthropoda, Onychophora, and Tardigrada. Unlike onychophorans and tardigrades, arthropod legs are characterized by highly distinct morphologies along the proximodistal axis and joints. In both arthropods and onychophorans, the leg gap genes Distal-less, dachshund, extradenticle, and homothorax show regionalized expression patterns during leg development. Distal-less patterns the distal region of the leg, dachshund patterns the intermediate region of the leg, and extradenticle and homothorax work in combination to pattern the proximal region of the leg. These similarities reveal that a highly conserved developmental mechanism can underlie the evolution of very disparate leg morphologies. We investigated the leg gap genes in tardigrades to determine whether the mechanism identified in arthropods and onychophorans regulates development of the tiny legs of tardigrades. We identified single orthologs of Distal-less and homothorax in the genomes of two tardigrade species—Hypsibius exemplaris and Ramazzotius varieornatus. We identified three orthologs of extradenticle in these species. We could not identify a dachshund ortholog in these species, even though it is present in panarthropod out-groups, suggesting that this gene was lost in the tardigrade lineage. Using in-situ hybridization, we detected Distal-less signal in all developing tardigrade legs. Strong homothorax signal was restricted to the anteriormost leg pair. We detected weak signal in the second and third leg pair, and no signal in the fourth leg pair. Of the leg gap genes, only Distal-less appears to be required for development of all tardigrade legs. Therefore, our preliminary results suggest that, relative to the legs of arthropods and onychophorans, the tardigrade leg may only possess distal identity.
