Collaborative Research: Developmental Mechanisms Underlying Diversity in Arthropod Limbs

他们的研究比较了Triops的多分支肢体和果蝇的无分支肢体的分子和细胞发育。这两种肢不仅在分支数量上不同，而且在它们的基本结构上也不同：果蝇的腿是有关节的杆状肢，而Triops的腿是无关节的桨状肢，在边缘处有八个分支。因此，他们之间的比较应该揭示出非常不同的四肢是如何产生的。他们的研究结果表明，果蝇模型的三个关键特征在不同物种之间有所不同。1)在果蝇中，前向（A/P）和后向（D/V）模式轴的交点定位肢体原基。他们的数据表明，Triops中D/V位置的变化导致了更大的肢体原基的形成，他们假设这是构建多分支肢体的第一步。2)在果蝇中，分泌的信号因子，无翼（wg）， decapentaplegic （dpp）和hedgehog （hh）的交集指定了腿生长的近端/远端（P/D）伸长点。不同水平的Wg和Dpp信号直接激活和维持腿部远端区域的细胞命运；在腿的近端区域，它们由不同的基因网络激活，至少部分由外展肌（exd）基因调节。他们的数据表明，Triops的多分支肢体确实有一个单一的P/D轴，可能由相同信号基因的同源物控制。然而，Triops分支的模式并不是与果蝇肢体模式相同的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独特的基因表达模式来识别。3)果蝇P/D单点生长的建立与细胞分裂模式没有直接联系。然而，在Triops中，他们发现有丝分裂活性细胞集中在假定的Triops肢体组织者区域。因此，他们的模型表明，与果蝇不同，Triops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。

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.