User:Topaz314/sandbox

 Legume-rhizobium symbiosis: Evolutionary Stability  The evolutionary symbiosis between legume s and rhizobium is relatively new to evolution. The fabaceae dates back as far as 94 million years and Rhizobia dates back to 65 million years ago. Some relationships between the two are better than others and force one to become more like parasites.

Symbiosis
Symbiosis is a close relationship between two different species. This relationship can be positive, negative, neutral and anywhere in between. These relationships can give organisms an advantage, but on the other hand can lower one's fitness. Legumes and rhizobium coevolved forcing dependence on the other one's strength and symbiotic interaction. Legumes can grow without rhizobium, but grow better with the help of rhizobium. This dependence happens because rhizobia can fix Nitrogen and take up other nutrients that the plant needs. Rhizobia can't complete their life cycle without a host plant they reproduce using the some of the plants raw materials. This is how dependent one is on the other.

Evolution
Rhizobia could have been cheaters at some point in time and could have infected a legume. Prisoner's dilemma demonstrates how easily a relationship or a team can be mutualistic or pushed to become parasitic. Rhizobia can fix nitrogen and other nutrients and pushed to be mutualistic. Rhizobia would take sugars and pushed this relationship to become parasitic. This simple relationship brought in more earning then they did alone. Legumes cooperation with rhizobia would have to be balanced to create a optimum high. This will have a balance between trading and receiving. This brings in cooperation and conflict, which forces partner choice.

Mutualism
In the beginning rhizobia must have been nearby the plant and happened to be fixation Nitrogen. The plant could have somehow sensed this. This need for the plant probably found a way to take this specific bacteria in. If this type bacteria was cheating the plant would find ways to punish them. Pun by cutting them off from nutrients. This would force the legume to find way to discriminate between a good and a bad bacteria. Bad bacteria would try to find way to mimic the good bacteria.

Prisoner's dilemma
The Prisoner's dilemma refers to how it is more likely that one or both will cheat to gain the advantage in a relationship. If an organism is given the chance to get a bigger reward right now and the cost of someother organism... most of them will take it. We know this happens because there are examples of organism taking benefit at another cost; parasites. The point of evolution is to survive to pass the organism's genes to the next generation. The ones that work together find a different more valuable reward. This reward refers to a long term benefit that has a higher pay in the long run. If both the legume and rhizobium work together then, they both will have a higher net gain (sometimes). The legume will receive a large amount of nutrients, while the rhizobium gains large amounts of sugar. It would be easy for the rhizobium to become a parasite (steal sugar) or for the legume to absorb them (consume them).

Cooperation and conflict
Legumes trade sugars to rhizobia for nitrogen in return. Trades can always go wrong, but evolutionarily natural selection can be more favorable for cheating to occur then cooperation. The mutualism between the legume and rhizobia was more favorable because of the advantage that came with it. Rhizobia can be beneficial in both high and low quality environments. This will come in handy because it can be used everywhere and be helpful. Rhizobia cooperate by expending energy to fix nitrogen for the legume; this comes at a high cost. This cost is forcing the rhizobia to sacrificing the choice to store large amounts of energy, such as polyhydroxybutyrate. Together they will make more when one of them specializes. Economically this makes sense, the legume specializes in making sugars and rhizobia on collecting nitrogen. Then one will trade that has comparative advantage to be better off then they would alone. Cooperation between legume and rhizobia is at its highest when rhizobia fix a large amount nitrogen for a decent amount of sugar.

The root nodule is the cite where the bulk of the interactions take place, including nitrogen fixation. The protein leghemoglobin is a response to the cooperation between the legumes and rhizobium. Leghemoglobin keeps oxygen levels in the nodule low so that the bacteroids can fix nitrogen. It used to be thought that the heme portion of this was synthesized by the bacteria while the globin portion was synthesized by the plants. However, more recent work indicates that it is synthesized solely by the plant.

The mutualism between the legume and rhizobia can turn parasitic. If the rhizobia isn't collecting enough nutrients for the legume, then it's considered as a parasite. Rhizobia is taking up payment in sugar, while the plant isn't receiving anything in return. When rhizobia bacteria are stop from fixing nitrogen by changing the air with argon and oxygen. This forces the rhizobia to become cheaters and doesn't happen in the wild. There are rhizobia that cheat in other ways, this is seen when some rhizobia stores large amount of sugar in a form that the plant can't use. The plant can sense this and will then punish the rhizobia. Also some rhizobia are better are being accepted by the plant, but don't fix nitrogen as well. They punish rhizobia by cutting them from nutrients which will starve them and attempt to take them from the gene pool.

How Legumes are able to recognize rhizobia
Legumes have the ability to discriminate between different species of bacteria. Legumes attract rhizobia to the root with flavonoids. The type of flavonoid attracts only specific rhizobia. This then determines which species will interact with the legumes. This will eliminate most parasitic bacteria and allows only a specific species into the plant. The plant root hair will curl around the bacteria and surround them. This then allows the rhizobia to enter through a root hair or through cracks in the root epithelial tissue. Nod factor molecules are also important in the initial steps of establishing the symbiosis in many species, although not all legume-rhizobia mutualistic relationships require these signals, or even carry these genes. For example, Aeschynomene and Bradyrhizobium initiate symbiosis without nod factors. It is uncertain whether or not nod-factor-independent symbiosis or nod-factor-dependent symbiosis is the ancestral state. One theory is that nod-factor-dependent symbiosis evolved from already extant and widespread endomycorrhizae. (See also Mycorrhiza.)

Competition between rhizobium
Competition between rhizobium occurs in plant root colonization. If there is a mix of rhizobium species that can infect the legume, but aren't closely related to one another, competition may occur. They compete with one another for domination in the nodules. Most legume-rhizobium interactions are very limited and most legumes may only have one (or few) bacterial species. Rhizobia that can be more competitive tends to have a way to infect a legume more efficiently. Some rhizobium that can infect legumes more efficiently tend to be more abundant and sometimes aren't efficient at fixing nitrogen.

Why are parasites like rhizobia still around
This bring the question into play; why are cheating rhizobia still around? They are still around because of a number of things. Such as: mixing themselves in nodules that can escape sanction conflicting selection regimes, some legumes may have weak sanctions and rhizobia are somehow controlling the host biochemically.