Molecular and Morphological Characterization of Segmentation in Artemia franciscana

Abstract

In the animal kingdom, only the annelids, arthropods and chordates are segmented. Whether the common bilateran ancestor to these three phyla was segmented, remains debated. One way to address the origins of the evolution of segmentation is to compare the molecular mechanisms underlying this complex process between the phyla and across each phylum. This thesis first examines what we already know about segmentation in each of the three phyla, and compares the models of segmentation in each phylum as well as between the three. Then, the role of γ-secretase mediated signaling in segmentation was examined in the branchiopod crustacean, Artemia franciscana. These findings were further compared to another crustacean Thamnocephalus platyurus. Both of these species develop their thoracic segments sequentially from anterior to posterior, and exposure to a γ-secretase inhibitor slows segmentation in a dose dependent manner, but does not affect the overall growth. My results suggest that Delta/Notch signaling is an essential for segment patterning in these two species, although it may not function as a molecular oscillator, as is the case in vertebrates. Similar findings in other arthropods suggest that the role of Notch in segmentation is not as unique to vertebrates as once thought. Finding such similarities in the molecular pathways that pattern segments across segmented phyla suggests that the Urbilaterian may have indeed been segmented.