Collaborative Research: Developmental Mechanisms Underlying Diversity in Arthropod Limbs

使节肢动物成为现存物种最多样化的一个重要特征是其肢体形态的适应性。进化改良导致了肢体在形式和功能上的根本不同，从无分支的行走腿到多分支的游泳桨。Nagy博士和Williams博士的目标是确定分支和非分支肢体形成的发育和遗传机制，以了解节肢动物肢体多样性是如何进化的。他们的初步实验使他们能够建立一个初步的模型，说明甲壳类动物Triop Longicaudatus的多分支肢体是如何发育的。这个模型预测了当前果蝇肢体发育模型的哪些特征在其他节肢动物的肢体进化中发生了改变。他们提出了比较基因表达研究、命运图谱和肢体生长的细胞动力学描述，以确认或扩展我们的Triop肢体发育模型。然后，他们将通过观察正在发育的肢体中靶向细胞消融的后果来实验测试我们的模型。通过与果蝇的比较，他们希望解释如何通过修改相似的潜在遗传网络来产生截然不同的肢体形态。他们的研究比较了Triop的多分支肢体和果蝇的无分支肢体的分子和细胞发育。这两条腿不仅在分支的数量上不同，而且在基本结构上也不同：果蝇的腿是连接的，呈杆状，而Triop的腿有一个不相连的桨形腿，边缘周围有八个分支。因此，他们之间的比较应该会揭示出截然不同的四肢是如何形成的。他们的结果表明，果蝇模型有三个关键特征，这些特征在物种之间是不同的。1)在果蝇中，前后轴(A/P)和背腹轴(D/V)的交叉点定位肢体原基。他们的数据表明，Triop中D/V位置的变化导致了更大的肢体原基的指定，他们假设这是建立多分支肢体的第一步。2)在果蝇中，分泌的信号因子无翼(WG)、十足瘫痪(DPP)和刺猬(HH)的交叉点指定了腿部伸展的近端/远端(P/D)点。腿部远端区域的细胞命运由不同水平的Wg和DPP信号直接激活和维持；在腿部近端区域，它们由不同的基因网络激活，至少部分受牙外(EXD)基因调控。他们的数据表明，Triop的多分支分支确实有一个P/D轴，可能受相同信号基因的同源基因控制。然而，Triop中分支的模式并不是对果蝇肢体模式所涉及的相同基因网络的简单迭代。从它们最早的发育开始，至少有四种不同的分支类型可以通过不同的基因表达模式来识别。3)果蝇P/D突起的单点的建立与细胞分裂模式没有直接联系。然而，在TRIOPS中，他们发现在可能的TRIOPS肢体组织者区域有大量有丝分裂活跃的细胞。因此，他们的模型表明，与果蝇不同，Triop中的P/D组织者可能以类似于脊椎动物肢体发育的方式控制特定的细胞分裂模式。

One important feature that has enabled arthropods to become the most diverse extant animals is the adaptability of their limb morphology. Evolutionary modifications have resulted in limbs that differ radically in form and function, from unbranched walking legs to multibranched swimming paddles. Drs. Nagy and Williams' goal is determine the developmental and genetic mechanisms that underlie the formation of branched versus unbranched limbs in order to understand how arthropod limb diversity has evolved. Their preliminary experiments allow them to formulate an initial model for how a multi-branched limb develops in the crustacean, Triops longicaudatus. This model predicts which features of the current Drosophila model of limb development are modified in the evolution of other arthropod limbs. They propose comparative gene expression studies, fate mapping, and descriptions of the cellular dynamics of limb growth to confirm or extend our model of Triops limb development. They then will experimentally test our model by observing the consequences of targeted cell ablations in developing limbs. Through comparisons with Drosophila, they hope to explain how radically different limb morphologies can be produced by modifications of similar underlying genetic networks. Their studies compare both the molecular and cellular development of a multi-branched limb in Triops to that of the unbranched limb in Drosophila. These two limbs differ not only in number of branches but also in their fundamental structure: Drosophila legs are jointed and rod-shaped, Triops legs have an unjointed paddle-shaped limb with eight branches arising around the margin. Thus, their comparison between them should reveal how very different limbs arise. Their results suggest that there are three critical features of the Drosophila model that vary between species. 1) In Drosophila, the intersection of the anteroposterior (A/P) and dorsoventral (D/V) patterning axes position the limb primordia. Their data suggest that variation in D/V positioning in Triops results in the specification of a larger limb primordia, which they hypothesize is the first step in building a multi-branched limb. 2) In Drosophila, the intersection of the secreted signaling factors, wingless (wg), decapentaplegic (dpp), and hedgehog (hh) specify a point of proximal/ distal (P/D) elongation for leg outgrowth. Cell fates in the distal region of the leg are activated and maintained directly by different levels of Wg and Dpp signals; in the proximal region of the leg they are activated by a different gene network, regulated at least in part by the extradenticle (exd) gene. Their data suggest that the multibranched limb of Triops does indeed have a single P/D axis, potentially controlled by homologues of the same signaling genes. However, the patterning of the branches in Triops is not a simple iteration of the same gene network involved in patterning the Drosophila limb. From their earliest development at least four different branch types are identifiable by distinctive gene expression patterns. 3) Establishment of a single point of P/D outgrowth in Drosophila is not directly linked to cell division patterns. However, in Triops, they find a concentration of mitotically active cells in the region of the putative Triops limb organizer. Their model therefore suggests that, unlike Drosophila, the P/D organizer in Triops may control specific patterns of cell division, in a manner similar to vertebrate limb development.