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Possible developmental mechanisms underlying the origin of the crown lineages of arthropods

机译：节肢动物冠状血统起源的可能发展机制

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The extraordinarily preserved, diverse arthropod fauna from the Lower Cambrian Maotianshan shale, central Yunnan (southwest China), represents different evolutionary stages stepping from stem lineages towards crown arthropods (also called euarthropods), which makes this fauna extremely significant for discussion of the origin and early diversification of the arthropods. Anatomical analyses of the Maotianshan shale arthropods strongly indicate that the origin of crown arthropods involved three major evolutionary events, arthrodisation, arthropodisation and cephalization. We try to explore possible evolutionary changes of the developmental mechanism that may have underlain origins of euarthropod appendage and head. Fossil evidence suggests that the formation of a jointed limb known as arthropodisation and formation of multi-segmented head (called cephalization), which characterize euarthropods, is an event after arthrodisation characterized with the formation of segmented-exoskeleton and the joint membrane between tergites. We propose that the Hox complex was already operating at least as early as in the Early Cambrian and is responsible for the formation of the joint membrane between two semgents through Hox gene regulation along the D-V and P-D axis. Fossil evidence indicates that the head in ground state of arthropods consists only of two segments, an ocular and an antennal one. The formation of multiple segmented, euarthropod head (called syncephalon) from the two-segmented head was a separate event, which is called cephalization. Presence of the Hox gene head expression domain and change of developmental mechanism in head segments might be responsible for the formation of the syncephalon and this event has been broadly finished in the Early Cambrian arthropods. The post-oral limbs in the early syncephalons as evidenced from the Lower Cambrian Maotianshan shale arthropods however were almost identical to those in trunk. Therefore we proposed that the Hox genes specified-expression in individual segment was a late evolutionary step postdating the formation of syncephalon.
机译：来自云南中部下寒武统茅田山页岩的特别保存的，多样化的节肢动物区系代表了从茎系到冠状节肢动物（也称为真节肢动物）的不同进化阶段，这使得该动物区系对于讨论其起源和起源极为重要。节肢动物的早期多样化。帽天山页岩节肢动物的解剖学分析强烈表明，冠状节肢动物的起源涉及三个主要的进化事件，节肢动物化，节肢动物化和头型化。我们尝试探索可能有真人节肢动物附肢和头部起源的发育机制的可能进化变化。化石的证据表明，关节节肢的形成被称为节肢动物化，多节头的形成（称为头颅化），这是真肢动物的特征，是关节置换后的一个事件，其特征是节肢性外骨骼的形成和各个尾部之间的关节膜的形成。我们提出，Hox复合体至少早在早寒武纪就已经开始运转，并且负责通过沿D-V和P-D轴的Hox基因调控在两个分支之间形成联合膜。化石证据表明，节肢动物的头处于节律状态，仅由两部分组成，即眼和触角。由两段头部形成多个节段的节肢动物头部（称为合成头）是一个单独的事件，称为头颅化。 Hox基因头部表达结构域的存在和头部节段发育机制的改变可能是形成中脑的原因，这一事件已在寒武纪早期的节肢动物中广泛完成。从下寒武统茅田山页岩节肢动物可以看出，早期中枢神经元的口腔后肢与躯干中的肢体几乎相同。因此，我们认为Hox基因在各个节段中的特定表达是在形成后脑后的晚期进化步骤。

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