Tardigrades have a miniaturized body plan. Miniaturization in tardigrades is associated with the loss of several organ systems and an intermediate region of their anteroposterior (AP) axis. However, how miniaturization has affected tardigrade legs is unclear. In arthropods and in onychophorans, the leg gap genes are expressed in regionalized proximodistal (PD) patterns in the legs. Functional studies indicate that these genes regulate growth in their respective expression domains and establish PD identities, partly through mutually antagonistic regulatory interactions. Here, we investigated the expression patterns of tardigrade orthologs of the leg gap genes. Rather than being restricted to a proximal leg region, as in arthropods and onychophorans, we detected coexpression of orthologues of homothorax and extradenticle broadly across the legs of the first three trunk segments in the tardigrade Hypsibius exemplaris. We could not identify a dachshund orthologue in tardigrade genomes, a gene that is expressed in an intermediate region of developing legs in arthropods and onychophorans, suggesting that this gene was lost in the tardigrade lineage. We detected Distal-less expression broadly across all developing leg buds in H. exemplaris embryos, unlike in arthropods and onychophorans, in which it exhibits a distally restricted expression domain. The broad expression patterns of the remaining leg gap genes in H. exemplaris legs may reflect the loss of dachshund and the accompanying loss of an intermediate region of the legs in the tardigrade lineage. We propose that the loss of intermediate regions of both the AP and PD body axes contributed to miniaturization of Tardigrada.

中文翻译：

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垂直体轴中间区域的丢失有助于缓步动物的小型化

缓步动物有一个小型化的身体计划。缓步动物的小型化与几个器官系统及其前后 (AP) 轴的中间区域的丧失有关。然而，小型化如何影响缓步动物的腿尚不清楚。在节肢动物和有甲动物中，腿间隙基因在腿中以区域化的近端 (PD) 模式表达。功能研究表明，这些基因在各自的表达域中调节生长并建立 PD 身份，部分是通过相互拮抗的调节相互作用。在这里，我们研究了腿间隙基因的缓步动物直系同源物的表达模式。而不是像节肢动物和有甲类动物那样局限于腿部的近端区域，我们在缓步动物 Hypsibius exemplaris 的前三个躯干节段的腿上广泛检测到同胸和齿外直向同源物的共表达。我们无法在缓步动物基因组中识别出腊肠犬的直系同源基因，该基因在节肢动物和有爪动物的发育中的腿的中间区域表达，这表明该基因在缓步动物谱系中丢失了。我们在 H. exemplaris 胚胎的所有发育中的腿芽中广泛检测到 Distal-less 表达，这与节肢动物和有甲类动物不同，在节肢动物和有甲类动物中，它表现出远端受限的表达域。H. exemplaris 腿中剩余腿间隙基因的广泛表达模式可能反映了腊肠犬的丧失以及缓步动物谱系中腿中间区域的伴随丧失。