Cellular and Molecular Events in Early Leech Development

舌蛭科水蛭，包括不同种类的Helobdella水蛭，对深入分析发育过程很有用，原因有两个：首先，它们是相对简单的动物，具有大而易接近的胚胎，适合细胞和分子结合技术。其次，由于水蛭属于环节动物门，这是一个研究较少的群体，因此获得的结果对于进行种间比较是有用的。这种比较对于区分一般的发育过程和特定门的发育过程至关重要，也有助于了解发育过程的进化修饰如何导致不同动物类型的出现。例如，环节动物和节肢动物彼此相似（不同于软体动物），因为它们都有分段的身体结构。在环节动物和节肢动物中，节段性组织都是由纵向的、腹侧的细胞阵列产生的，称为胚带（在节肢动物中，如果蝇）或生发板（在环节动物中）。在水蛭中，生发板以严格的头-尾（背-尾）进程产生，通过被称为端母细胞的特化胚胎干细胞的重复分裂。果蝇的几个基因的同源物似乎在水蛭的背足模式中以同源模式表达。这表明两个门的分化和后期发育的形态发生的分子遗传过程非常相似。但是早期的Helobdella胚胎与早期的果蝇胚胎有着惊人的不同，而与软体动物的胚胎有着惊人的相似。因此，细胞命运如何在早期水蛭中决定的问题是相当有趣的。该提案的第一部分包含旨在进一步研究胚胎发育中特定细胞如何分配不同命运的实验，重点是将中胚层和外胚层命运分配给胚胎仅包含10个细胞时出生的单个鉴定细胞，并在这方面寻找Helobdella和Drosophila之间的相似性。第二个令人感兴趣的问题是早期水蛭发育的“软体动物”模式与后来发展的“节肢动物”模式是如何结合在一起的。这一过程的一个关键方面是在Helobdella萌发板形成过程中，一层细胞在胚胎的整个表面上扩散。本提案第二部分的实验旨在阐明参与这些形态发生运动的细胞过程。

Glossiphoniid leeches, including various species of Helobdella, are useful for in depth analyses of developmental processes for two reasons: first, they are relatively simple animals with large, accessible embryos that are suited for combined cellular and molecular techniques. Second, since leeches belong to the phylum Annelida, an otherwise poorly studied group, the results obtained are useful for making interphyletic comparisons. Such comparisons are essential to distinguish general developmental processes from phylum-specific ones, and also to learn how evolutionary modification of developmental processes leads to the appearance of diverse animal types. For example, annelids and arthropods resemble each other (and differ from molluscs) in that they both have segmented body plans. In both annelids and arthropods, segmental tissues arise from longitudinally oriented, ventrally situated arrays of cells, termed the germ band (in arthropods such as the fruitfly Drosophila) or germinal plate (in annelids). In leech, the germinal plate arises in strict head-to-tail (rostrocaudal) progression by repeated divisions of specialized embryonic stem cells called teloblasts. Homologs of several genes involved in rostrocaudal patterning of Drosophila seem to be expressed in homologous patterns during rostrocaudal patterning of leech. This suggests that the molecular genetic processes of differentiation and morphogenesis in later development will be quite similar in the two phyla. But the early Helobdella embryo is strikingly different from the early Drosophila embryo and strikingly similar to the molluscan embryos. Thus, the question of how cell fates are determined in the early leech is of considerable interest. The first part of this proposal contains experiments designed to further previous investigations of how distinct fates are assigned to particular cells in the developing embryo, with emphasis on the assignment of mesodermal and ectodermal fates to individual ly identified cells that are born when the embryo contains only 10 cells, and to look for similarities between Helobdella and Drosophila in this regard. A second question of interest is that of how the "molluscan" mode of early leech development meshes with the "arthropodan" mode of later development. A critical aspect of this process is the spreading of one layer of cells over the entire surface of the embryo during the formation of the germinal plate in Helobdella. The experiments in the second part of this proposal are designed to elucidate the cellular processes involved in these morphogenetic movements.