Multicellularity/The Origin of the Eukaryotic CellJen TrinhLife is pretty darn great. Fantastic. Absolutely marvleous. Sure, you may have finals to study for, graduate school to consider, a job to find, etc., but at least you can breathe. You can eat an apple and appreciate the sweet crispness, you can sleep and you can dream, and it's (partly) because a long time ago, two unicellular organisms came together and formed a symbiotic relationship.It is mind-boggling to consider the path that led from a unicellular organism to the organism you see in the mirror every day; in fact, we have no definitive explanation for even the beginning of this process (mostly the why remains unexplained; the how is marginally clearer). Therefore, my goal in this “handout” is to lay before you some proposed theories for the birth of the eukaryotic cell, and the reasoning behind them, in hopes that you will take a deep breath, give yourself (or someone/thing else) a hug, exhale, andbe happy to be alive, and capable of all that you can do! I may be blowing this a bit out of proportion, but this is all stuff that you should be doing already anyway.Organisms associate, and the conditions under which these associations occur determine whether or not the organisms will develop higher-level relationships. For example, if you fight with someone, you can either solve the conflict and become good friendswho help each other, or you can continue to fight with them and find more and more malicious ways to torment them, or completely cut ties with them. I feel like Ari will talk about cooperation a lot, so I will steer clear of the topic, and instead, we'll mainly focus on the origin of the eukaryotic cell.In each of these theories for why the “cell-in-which-the mitochondria-resides” (we'll call this the nctb, short for “nucleo-cytosol-to-be”) took in the mitochondrion cell (mtb, short for “mitochondria-to-be”), there are three stages: initial interaction, conflict,and conflict mediation. I will refer to these stages throughout.Hypothesis 1: Hydrogen HypothesisDue to a decrease in hydrogen levels in the initial stage, nctb cells surrounded mtb cells to take in the hydrogen released by the mtbs. Nctb cells crowded around these mtbs, a harmful move for both the nctbs and the mtbs because it limited the surface area available to import nutrients from the environment. This leads to further reduction in hydrogen levels (all the nctbs around!), a further dependenceon the mtbs, and thus, an incentive to keep them alive. Somehow (there was no real explanation, and I'm no biologist), the conflict was solved by transfer of the mtb genes to the nctb, and thus, the presence of mitochondria in cells.Hypothesis 2: Units of SelectionThe mtbs were mutants who were unable to break down glycose for food (“damaged glycolytic mechanisms”), and so they fed on the excreted metabolites of the nctbs, and mtbs are capable of reducing NADH (which is used to produce ATP), so the two were happy in the initial stage. However, an increase in oxygen levels meant that the aerobic mtb produced more ATP than the nctb, and so the mtb leaked ATP to the nctb, which benefited them both. They became more and more dependent on each other, and eventually became a single unit.Hypothesis 3: SyntrophicThe two fed on each other's waste, but one produced more than the other, and rather than eating each other's waste, the mtb's aerobic capabilities replaced methanogenesis (the production of methane) in the nctb (genes were transferred). Hypothesis 4: Predatory-preyEither the nctb fed on the mtbs, or vice versa (I would think it was the mtbs being fed upon, but the essay did not actually say one or the other), but the one that was being fed upon escaped digestion. The mtbs provided energy in order to persist.Hypothesis 5: Pathogen-hostMtbs were accidentally engulfed, and surrounded by vacuolar membrane, they busted out by secreting an enzyme that could break down proteins at a critical ATP level, and they abandoned the dying bodies of the nctbs. The nctbs created a protein that they inserted into the membranes of the mtbs in order to leech ATP from the mtbs. This kept the ATP level high and away from the critical level necessary to initiate the protein breakdown process. The nctbs benefited from the stolen ATP, and in return, fed the mtbs. Eventually gene transfer occurred, and we had a eukaryotic cell.Now that you've read some of the “why” for the development of the eukaryotic cell, you can see how complex and ridiculously cool this topic is. Similar mechanisms are used to explain the origin of multicellularity. Volvox are a great specimen to study multicellular development, and here are some notes on Volvox that may show you why.Volvox:-unicellular ancestor: C. reinhardtii...most studied multicellular form: v. carteri-green algae (chlorophytes); vascular plants are descendants of chlorophytes; had a common single-celled ancestor 50+20 million years ago, which is relatively young-almost like it's unicellular, but live in colonies that behave like multicellular organisms-does not produce through binary fission; instead of growing twice as big and splitting in half, grow 2n -fold in volume, then have ndivisions and end up with 2n daughter cells that are attached to each other by “extracellular materials”, holds them in place; whole thing called a “coenobium”...cytoplasmic bridges break down when time for cell division-each colony has many cells, some are mortal somatic cells, others are “gonidia”, potentially immortal asexual reproductive cells-each cell has two flagella, a cis and trans (cis is younger than trans), when cell divides, each daughter gets an old and a new basal body that attaches to the flagella and controls it; when Ca2+ levels are high or low, controls which flagella dominates, allows to turn towards or away from light to photosynthesize -”flagellation constraint”: have a wall or use your flagella; multiple fission is an evolutionary adaptation to having a cell wall; Volvox have some cells that don't divide, just keep using their flagella (during division, they take in their flagella, colony can't move)-since Volvox can get very large, they need to specialize or whatever (see end of ch. 4)-indistinguishable sex in asexual stage, but can produce sperm and egg if under certain conditions (unfavorable/harsh). Sexual males produce a pheromone that will cause others to undergo sexual