Evidence for a novel role for dachshund in patterning the proximal arthropod leg

The branchiopod crustacean Triops longicaudatus has paddlelike thoracic appendages with few joints and multiple marginal lobes. Here, we explore the degree to which the Triops limb is patterned by the same network of genes known to pattern the uniramous, multi-jointed insect appendage. Insect leg patterning proceeds through a process of subdividing the leg into proximal, intermediate, and distal regions by the activity of the transcription factors hth/exd, dac, and Dll. The immature Triops limb is subdivided into large, discrete regional domains (proximal and distal) as defined by nuclear-EXD and DLL. We show that HTH expression in Triops overlaps cell-to-cell with n-EXD expression. In addition, dac is expressed in two domains: (1) adjacent to and partially overlapping the distal Dll domain and (2) along the medial margin of the developing leg. The DAC domain adjacent to the distal Dll domain supports the early establishment of the expected intermediate domain of DAC expression. The medial expression domain resolves over time into a series of reiterated stripes located on the lower side of each medial lobe. Later, this expression pattern correlates with the sclerotized regions associated with limb flexion. We propose that these stripes of DAC expression play a role in forming reiterated medial lobes. Unlike Drosophila, where the proximal distal patterning of the leg is coincident with patterning of reiterated structures (segments), we hypothesize that the patterning in Triops may reflect an ancestral state where the patterning of reiterated medial structures was not coincident with proximodistal limb patterning.