Laurea specialistica honoris causa in Biodiversità ed evoluzione a Lynn Margulis
Il 17 maggio 2010 l'Università di Pisa ha conferito la Laurea specialistica honoris causa in Biodiversità ed evoluzione alla professoressa Lynn Margulis.
Lectio magistralis di Lynn Margulis
Symbiogenesis, not random DNA mutations, as source of life's heritable novelty
Speciation, the appearance of new species in the fossil record, in the laboratory or field, by "gradual accumulation of random mutations in DNA" has not been adequately documented. By contrast, many high-quality scientific studies have unequivocally shown the importance of symbiogenesis in the origin of species, genera and more inclusive taxa (Margulis and Chapman, 2010).
Symbiosis is an ecological phenomenon; the term refers to "living together", physical association between "differently named organisms", i.e., members of different taxa from strains or varieties to superkingdoms. Symbiogenesis differs: it is an evolutionary phenomenon. Symbiogenesis is detected when long-term physical contact between members of different taxa lead to new behaviors, new organelles (e.g., mitochondria, chloroplasts, kinetosomes), new tissues (e.g., the red stipulate tissue of Gunnera manicata), new organs and organisms (e.g., the hypertrophied hindgut of wood-ingesting kalotermitids or rumen of bovidae; squid nidamental gland; the root nodules of leguminous plants induced by rhizobia). Symbiogenesis generates new inclusive taxa such as myriad animals with larvae. Lichens evolved by association between members of different kingdoms (fungi-protoctist or fungi-cyanobacteria). Dramatically metamorphosing animal larvae probably evolved by merger of two or more once-separate lineages in a symbiogenetic process called hybridogenesis (Williamson, 2009).
The power of symbiogenesis to generate novelty is most pronounced, or at least most easily detected, in members of the Protoctista kingdom. Dinomastigote associations with red, green and chrysophyte algae and the dynamism of the in-process-symbiosis of Hatena arenicola (Okamoto & Inouye, 2006)are examples in the marine environment. In fresh water Stentor polymorphus and Paramecium bursaria (large ciliates that harbor Chlorella-like green algae) proffer conspicuous evidence for symbiogenesis. The acquisition and retention of symbiotic algae in both homologous and analogous associations confer the capacity for oxygenic photosynthesis to fossilize in the "punctuated equilibrium pattern". I infer that speciation can occur, in a "geological instant".
I plan to illustrate the power of symbiogenesis to generate evolutionary novelty for three cases via micrographs and moving pictures: (1) Mixotricha paradoxa from Mastotermes darwiniensis; (2) A cyclical parasexual fusion of a zygomycote with a filamentous nostocalean cyanobacterium that produces unique individuals (Geosiphon pyriforme) and (3) eukaryosis, the origin of the earliest nucleated cells in the sulfidic seas of the mid-Proterozoic Eon (c. 1200 million years ago). Members of at least two prokaryotic domains (a sulfidogenic archaebacterium and a sulfide-oxidizing motile eubacterium) merged in the origin of the first motile eukaryote in (3). Such heterotrophic, phagocytotic motile protoctists were ancestral to all subsequent eukaryotes (e.g., other protoctists, animals, fungi and plants), by hypothesis. The defining seme of eukaryosis, the membrane-bounded nucleus as a component of the karyomastigont evolved as Thermoplasma-like archaebacteria and Perfilievia-Spirochaeta-like eubacteria symbiogenetically formed the amitochondriate LECA (Last Eukaryotic Common Ancestor, Margulis et al., 2006).
Modern co-descendant amitochondriate mastigotes still thrive in organic-rich anoxic habitats such as microbial mat muds and hindguts of xylophagus insects where they remain amenable to study. Brief videos of live cells and animation of the eukaryosis process will be shown.
No missing links have been detected in this scenario. Contemporary photosynthetic or bioluminescent animals (e.g., Elysia viridis, Plachobranchus ocellatus, Convoluta roscoffensis, Hydra viridis, Euprymna scolopes), animals that digest cellulose (cows, termites) and secondarily-photosynthetic protoctists (Kentrophorus fistulosum, Hastigerina pelagica) make us virtually certain that Boris Michaylovich Kozo-Polyansky's (1890-1957) analysis of Symbiogenesis: A New Principle,1924, was and still is correct (Margulis, 2011).
Symbiogenesis accounts for the origin of hereditary variation that is maintained and perpetuated by Charles Darwin's natural selection. The fact that biotic potential, the exponential population growth rate characteristic of any species is not reached, is called natural selection. Natural selection, a process of elimination by failure to perpetuate, by itself does not generate evolutionary innovation. Natural selection can only eliminate or direct an evolutionary course.
The phenomenon of speciation is not directly related to meiotic sexuality; it is limited to eukaryotes. Prokaryotes, of course, have evolved but in a far more fluid way than have uniparental and sexual eukaryotes. Bacteria, whether archaebacteria or eubacteria, do not speciate. Speciation itself co-evolved with eukaryosis.
In any given time and place in all environments natural selection acts incessantly to eliminate variant life forms. Selection inhibits the perpetuation of populations of prokaryotes in specific environments. In eukaryotes natural selection eliminates the symbiotic complexes, the co-evolved bacterial communities that we recognize as eukaryotic "individuals".
The genesis, proliferation and natural selection of co-evolved symbionts is the major source of the evolutionary novelty recognized in the fossil record by paleontologists. From the late Proterozoic Eon (2500-542 million years ago) until the present fossiliferous sedimentary rocks have been studied and interpreted worldwide. Conspicuous from the Ediacaran (=Vendian) era until the end of the Holocene are macroscopic body fossils. Many, especially the earliest ones, are enigmatic. The absence of any animals or plants prior to the Varangerian ice ages at c.580 million years ago is a robust observation. Most of the Phanerozoic eon fossils (i.e., from 542 million years ago until now) can be tentatively assigned modern plant, animal or fungal affinities. That, prior to the "Cambrian explosion" members of the familiar eukaryotic kingdoms (Plantae, Animalia, Fungi) are conspicuous by their absence is a fact called "Darwin's dilemma" or "Darwin's lost world" (Brasier, 2009).
Protoctists and lichens, represented today by tens of thousands of species, are not exceptional. They are not curiosities although they are treated as such by most professional microbiologists, zoologists and botanists. The evolutionary importance of protoctists (whether called "protozoans", "fungi", "algae" or just dismissed as "lower organisms") is well known to the University of Pisa. Research on these planetmates of ours intrinsically tell stories of symbiosis and symbiogenesis. The inclusive field of protistology (or better yet, protoctistology) is essential to the understanding the history of life on Earth. We hope to establish a new science of "symbiogenetics" to return protoctists to their central importance in life sciences as you Pisani know so well (Sagan & Margulis, 2011). In the late 19th and first two thirds of the 20th century, mainly from work of Professors Nobili and Dini, their colleagues and students, protistology thrived here.
In summary I claim that professional biological literature details no evidence for the widely touted explanation of new species appearance "by natural selection acting on the accumulation of random DNA mutations". This unscientific belief contradicts overwhelming evidence for rampant horizontal genome transfer (symbiogenesis), horizontal gene transfer (Sapp, 2009) and karyotypic fissioning in mammals (Kolnicki, 2011). The new "molecular phylogenetics" renders untenable major assumptions of neo-Darwinian evolutionary "mechanisms". Species are not isolated "gene pool-entities" that "descend with modification" from two fertile (sexual) parents or a single uniparental reproductive (non-sexual) event. The topology of evolution is a net or web; the concept of an evolutionary tree (a "tree of life") is obsolete and should be abandoned. Let us return "power to the protoctists"!
Bibliography
Brasier, M. 2009, Darwin's Lost World, Oxford University Press, Oxford UK
Kozo-Polyansky, Boris M. 1924, Symbiogenesis: A New Principle of Evolution. Translation from the Russian by Victor Fet (2010). Harvard University Press, Cambridge MA USA
Kolnicki, R. 2011 Chapter 15. Lemurs and Split Chromosomes in Chimera and Consciousness Evolution of the Sensory Self, L. Margulis, W.E. Krumbein and C. A. Asikainen, editors. MIT Press, Cambridge MA USA, in press
Margulis M. J. Chapman, R. Guerrero, and J. L. Hall. 2006 The Last Eukaryotic Common Ancestor (LECA): Acquisition of cytoskeletal motility from aerotolerant spirochetes in the Proterozoic eon. Proceedings of the National Academy of Sciences 103:13080-13085.
Margulis L. and Chapman M.J., 2010. Kingdoms & Domains. An illustrated guide to the phyla of life on Earth. 4th edition. Elsevier, San Diego and London
Margulis, L. 2011 Symbiogenesis, A Principle of Evolution: Rediscovery of Boris Mikhaylovich Kozo-Polyansky in Charles Darwin and Modern Biology. Darwin and the History of Genetics and Evolution Conference, Eduard Kolchinsky, editor. Institute of the History of Science and Technology Russian Academy of Sciences, St. Petersburg, Russia (in press)
Okamoto N. & Inouye, I. 2006 Hatena arenicola gen. et sp. nov., a katablepharid undergoing probable plastid acquisition Protist 157:401-419
Sagan, D. and Margulis, L. Symbiogenetics: Toward a new science. Prototaxis, parasexuality and hybridogenesis in evolution of heritable variation. In Gabriel Trueba, editor, Proceedings of The Evolution Summit, University of San Francisco Quito and Galapagos, Ecuador. In preparation.
Sapp, Jan 2009 The New Foundations of Evolution: On the Tree of Life. Oxford University Press, New York and Oxford
Williamson, D.I. 2009 Caterpillars evolved from Onychophora by hybridogenesis,Proceedings of the National Academy of Sciences 106:15786–15790.
Ultimo aggionamento documento: 17-May-2010