Arthropods, chordates and annelids are all segmented animals. It is not yet clear wether their common ancestor was segmented or if segmentation developed independently within each group. Recent molecular characterization of developmental pathways utilized in the formation of segments has been used as evidence to support homology of segments. Different scenarios have been proposed to explain the evolution of segmentation.
Examination of several Drosophila segmentation genes in vertebrates has not yielded any striking evidence for a common segmented ancestor of vertebrates and arthropods, but a recent article, which shows the involvement of Delta and Notch in the segmentation of the spider Cupiennius salei, supports the homology of segments. The Delta-Notch pathway plays a crucial role in the somitogenesis of chordates, but is not involved in the segmentation process of Drosophila. These results suggest that the lack of a role of Notch and Delta in the segmentation of Drosophila reflects developmental differences in the segmentation of Drosophila and more basal arthropods. In Drosophila segments are formed simultaneously in a syncytial stage, where cell-cell communication does not yet occur, while segmentation in vertebrates and basal arthropods both generate segments in a cellular environment, where the Notch-Delta pathway can operate between adjacent cells.
To study a possible ancestral function of the Notch-Delta pathway in segmentation, it is important to examine members of the third major phylum of segmented animals, the annelids. In this thesis, expression patterns of Notch and its components in the signaling pathway were studied during the larval development of the two polychaete annelids Platynereis dumerilii and Capitella sp. I. In addition to the receptor Notch and its ligand Delta, I have isolated homologues of hes/hairy-genes, fringe and Suppressor of hairless. The results show that this signaling pathway has been recruited for multiple functions. The expression patterns observed are consistent with a role in chaetae formation, neurogenesis and foregut development in both polychaetes.
A striped like expression in the future segments and/or already formed segments, such as in arthropods or vertebrates, could not be seen for any of the studied genes during early segmentation of both polychaetes. Thus, even though the expression is also located in the presumptive segmented tissue of both larvae, I do not assume an involvement of the Notch signaling pathway during the early segmentation of polychaetes. Expression of Notch, Delta and hes-genes in the larval growth zone of Platynereis dumerilii and Capitella sp. I in later larval development accounts for the involvement of the Notch-signaling pathway in the terminal addition of segments in those two polychaetes and even annelids.
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