THE PROTISTAN ORIGINS OF MULTICELLUARITY: TIMING AND EVOLUTION OF CELL ADHESION MOLECULES
Sharpe, Susan Christina
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Multicellularity has evolved many times within eukaryotes. Comparisons between various multicellular groups and between multicellular groups and their unicellular relatives help illuminate how this transition happened. I addressed two aspects of the evolution of multicellularity: the timing of the emergence of eukaryotic multicellular groups, and the evolutionary history of integrins, metazoan cell adhesion receptors. Using phylogenomics and relaxed molecular clock dating methods (which employ paleontological calibrations), I estimated the timing of the emergence of eukaryotic multicellular groups. My results show that Metazoa, Fungi and two of the major multicellular red algal taxa first emerged during the mid-Neoproterozoic, whereas the dictyostelid aggregative slime moulds arose during the Paleozoic. I found that the unicellular breviate Pygsuia biforma expresses both subunits of the metazoan integrin receptor, as well as several associated scaffolding proteins. In Metazoa, these proteins function in cell adhesion and signaling. To further study these proteins, I developed antibodies against the Pygsuia homologs of the integrin receptor proteins and the associated scaffolding protein talin, and demonstrated that they specifically recognize recombinant target proteins expressed in Escherichia coli. These tools will facilitate the elucidation of the role of integrins in Pygsuia, furthering our understanding of the ancestral functions of proteins associated with multicellularity.