Cellular and Molecular Events in Early Leech Development

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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.