[MUSIC] The origin of the eukaryotes is one of the most important events in the evolution of the life on Earth. Even being intensively studied with a lot of intriguing theories and regular discoveries of really interesting facts, it mostly remains a mystery. What makes the eukaryotes distinct from bacteria and archaea? Not just the absence of the nucleus or presence of the nucleus, of course. Briefly mentioning, these are the size, not strict difference if we take into account smallest eukaryotes and the largest prokaryotes. Intracellular membranes and compartmentalization, other than the nucleus, sophisticated chromatin organization, partitioned genome, a rare thing in prokaryotes, introns and splicing machinery. Eukaryotes have elaborated regulation networks such as kinase-phosphatase machinery for protein function control, Ubiquitin system for protein localization and turnover, and micro-RNAs for translation control. Of course, eukaryotes are characterized with the presence of a cytoskeleton and endocytosis machinery as well as mitosis and meiosis. And finally, all the eukaryotes ancestrally have their eukaryotic type of flagellum. Traditional view of the tree of life, since the discovery of archaea by Carl Woese in 1977, included three domains of life. One eukaryotic domain and two prokaryotic, archaea and bacteria. But then the data started to accumulate, that eukaryotes are closer to archaea than to bacteria. And actually, their genome is a mixture of archaea-like, bacteria-like, and unique genes. The genes of archaeal origin are mostly responsible for informational processes such as transcription, translation, and replication. That means that original eukaryote, the common ancestor of eukaryote was closer to archaea than to bacteria. In that case, the tree would look like this. But one then could be curious of where the bacterial signal is coming from. The most popular and justified answer to that question is: because of symbiosis with mitochondria. The endosymbiotic origin of mitochondria was hypothesized by some early researchers and substantially justified by Lynn Sagan Margulis and now this has been proofed with many independent lines of evidence. Of course, the main evidence is the presence of the circular, like in bacteria, mitochondrial genome. All known mitochondrial genomes are considerably reduced, with most of the genes transferred to the nucleus. The analysis of this genome, as well as of nuclear-coded mitochondria targeted genes, allows to pinpoint a particular bacterial group, alphaproteobacteria which mitochondrial ancestor most probably were belonging to. This is interesting to note that alphaproteobacteria also include some endobiotic species such as Wolbachia and Rickettsia. Mitochondria multiply by binary fission, exactly like in the case of bacteria. You can see here dividing mitochondria from the myxomycete, Physarum Polycephalum on the transmission electron micrograph. Mitochondria have double membrane, like gram negative bacteria. Mitochondrial and bacterial membranes share similar transport proteins, porins and lipids, cardiolipins. Mitochondria contain ribosomes similar to bacteria, sharing the same initiation mechanism for protein synthesis. Protein synthesis inhibiting antibiotics can affect mitochondria. Here on this slide, mitochondrial ribosomes in heterokont flagellate Ochromonas are shown on the transmission electron micrograph. In certain cases, even immune system of multicellular organism can treat mitochondria as bacteria. All the known eukaryotes have mitochondria or mitochondria related organelles. with the exception of Monocercomonoides, which definitely lost its mitochondria secondarily. But all the eukaryotic features could not be evolved at once and most probably a long way of evolution preceded last eukaryotic common ancestor, or LECA. How the acquisition of mitochondria correlates with other typical eukaryotic traits? There is a lot of debate on this topic but generally two basic scenarios can be hypothesized. The first scenario, blue arrows on the picture proposes that at the beginning, the symbiosis of two prokaryotes triggered the development of eukaryotic features. For example, Martin and Koonin think, that expansion of type two mobile genetic elements led to the emergence of introns in the genomes and the nucleus was evolved as a protection mechanism from them. The problem of that theory is that almost all known prokaryotes are not capable of phagocytosis and it is not obvious that the other way of obtaining mitochondrial exists. The second scenario, pink arrows assumes that archaeal ancestor has evolved phagocytosis early, which require elaborate endomembrane system, as well as cytoskeleton and then became able to obtain future mitochondria. And recently, this idea obtained a strong argument for it. A clade of archaea was discovered which combines a lot of features which used to be considered purely eukaryotic. Endomembrane trafficking, ubiquitin, cytoskeleton, and so forth. This clade have got a name, Asgardarchaea, and unfortunately the data on it are restricted to the environmental sequencing information. Asgardarchaea discovery is groundbreaking and suggests that eukaryotes not just closer to Archaea, than to bacteria but rather they branch within their the Archaea clade and surprisingly this view modifies a tree of life the way, that there are only two branches remain: Archaea and Bacteria. And eukaryotes have got no separate domain like we do not create a new domain for lichens and just treat them as fungi which contain algal symbionts. Similarly, more and more people now start to treat eukaryotes like archaea which contain bacterial endobionts - the mitochondria.
