MD, brought together 75 protistologists and cell biologists interested in the origin, evolution, and function of eukaryotic motile systems and in their usefulness in comparative phylogenetic and taxonomic studies. Since most, if not all, eukaryotes contain microtubules at some point in their life cycles (centrioles have now been seen in Pelomyxa palustris, the previous exception) and most, but not all, eukaryotes contain actin-and myosin-like proteins, the prevailing view has been that these microtubular and microfilamentous systems arose in some of the earliest eukaryotes and have survived in most extant eukaryotic organisms. Evidence now indicates that similar systems may be present in some prokaryotes, so the discussions of their origin and evolution began with the prokaryotes and continued into eukaryotic movement systems.
Lynn Margulis (Boston University) contrasted the two theories on the origin of eukaryotic microtubules. In the endogenous theory, microtubules (MTs) evolved as polymerizable structures in early eukaryotes and provided a more successful system for division of such cells. In the exogenous theory, MTs arose in prokaryotic cells and became associated with eukaryotes, first as symbiotic MTcontaining spirochetes, then through integration and differentiation as undulipodia (cilia and flagella), centrioles, and other MT systems. She summarized the evidence for the exogenous theory, which is presented in detail in chapter 10 of her latest book ("Symbiosis in Cell Evolution." San Francisco: W. H. Freeman, 1981). Stephen Fracek (Boston University) presented evidence for a tubulin-like protein in certain spirochetes. If this is indeed a microtubule subunit protein, it would be the first clear exception to the generalization that eukaryotes have microtubules and prokaryotes do not. Tubules are present in sectioned spirochetes but have also been seen in prokaryotes with no detectable tubulin. However, spirochete "tubulin" polymerizes in tubulin reassembly buffer and coelectrophoreses with mammalian tubulin, and spirochetes containing this protein stain with fluorescent antitubulin.
Richard Luduefia (University of Texas at San Antonio) compared the structure and chemistry of tubulins from a wide variety of different eukaryotes. Tubulins have proven useful for wide-ranging phylogenetic studies because they are conservative, widespread if not ubiquitous among eukaryotes, and can be purified by polymerization under similar conditions. From peptide maps of the two tubulin subunits from many