Department of Zoology, Southern Illinois University, Carbondale, IL 62901, USA.
Evol Dev. 2013 Jul-Aug;15(4):280-92. doi: 10.1111/ede.12038.
The modular organization of arthropod limbs has lead to the evolution of a diversity of appendages within this phylum. A conserved trait within the arthropods is the utilization of a conserved set of regulatory genes that specify the appendage podomeres along the proximo-distal axis, termed the limb gap genes. These include extradenticle, homothorax, dachshund, and Distal-less. The deployment of these genes in the most basally branching arthropod group, the chelicerates, has only been studied in detail in two chelicerate groups, the harvestmen and spiders. Given the broad range of appendage diversity within the chelicerates, comparative studies of gap gene deployment in other chelicerates groups is needed. We therefore followed limb gap gene expression in a member of the largest chelicerate group, Acari, the oribatid mite Archegozetes longisetosus. We show that in contrast to many arthropod species, A. longisetosus expresses homothorax and extradenticle exclusively in the proximal portion of the appendages, which refutes the hypothesis of a sister-group relationship between chelicerates and myriapods. We also provide evidence that mites posses the ancestral chelicerate condition of possessing three-segmented chelicerae, which also express the gene dachshund. This adds support to the hypothesis that a cheliceral dachshund domain is ancestral to arachnids. Lastly, we provide evidence that the suppression of the fourth pair of walking legs, a putative synapomorphy for Acari, is accomplished by repressing the development of the medial and distal regions of the limb.
节肢动物附肢的模块化组织导致了这个门内各种附肢的进化。节肢动物的一个保守特征是利用一组保守的调节基因来指定沿近-远轴的附肢分节，这些基因被称为肢体缺口基因。这些基因包括 extra-denticle、homothorax、dachshund 和 Distal-less。这些基因在最基础的分支节肢动物——螯肢动物中的部署，仅在两个螯肢动物群体——盲蛛和蜘蛛中进行了详细研究。鉴于螯肢动物中附肢多样性的广泛，需要对其他螯肢动物群体中缺口基因部署进行比较研究。因此，我们在最大的螯肢动物群体之一——蜱螨目动物中的一个成员——长须螨 Archegozetes longisetosus 中研究了肢体缺口基因的表达。我们发现，与许多节肢动物物种不同，A. longisetosus 仅在附肢的近端部分表达 homothorax 和 extra-denticle，这驳斥了螯肢动物和多足动物具有姐妹群关系的假说。我们还提供了证据表明，螨类具有三节螯肢的祖先螯肢动物状态，这也表达了基因 dachshund。这增加了一个假设的支持，即一个螯肢 dachshund 结构域是蛛形纲动物的祖征。最后，我们提供的证据表明，第四对步行腿的抑制，这是螨类的一个假定的共衍征，是通过抑制肢体的中间和远端区域的发育来完成的。
