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An organism lives and reproduces inside the body of other animals. This organism pays back its host by protecting the DNA from viruses and radiation damage in 100% of all cases. The organism itself is also immune to viruses, bacteria and parasites.

Additionally, it boosts the immune system, supports the gut flora, and decreases cholesterol.

As a side effect it causes some spots to appear on the shoulders and backs of its hosts. Usually the spots are from green to bluish and in rare occasions yellow.

This symbiotic organism also attaches to fetuses in the womb or eggs.

How would such an organism evolve?

What can explain the colored spots on the host?

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    $\begingroup$ You should review my edits regarding English. $\endgroup$ – JDługosz Aug 14 '16 at 4:54
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Take inspiration from the anglerfish:

Some anglerfish, like those of the Ceratiidae, or sea devils, employ an unusual mating method. Because individuals are locally rare, encounters are also very rare. Therefore, finding a mate is problematic. When scientists first started capturing ceratioid anglerfish, they noticed that all of the specimens were female. These individuals were a few centimetres in size and almost all of them had what appeared to be parasites attached to them. It turned out that these "parasites" were highly reduced male ceratioids.

Your symbiotic species began as a parasite, probably one that fed on blood from a variety of much larger species. Over time it and your designated host species' ancestor became codependent: the parasite's natural abilities to filter blood benefitted the host, and the successful proliferation of the host benefitted the parasite.

The parasite eventually evolved to burrow under the skin and lay its eggs within the host directly (hosts could support multiple parasites, and parasites are relatively short-lived so this secured a food source for multiple generations). Being attached to the host made mating difficult, but they evolved to rely on contact between hosts spreading their sperm or spores or whatever through sweat.

As eons passed the parasite, now a symbiote, became flatter and flatter under the skin of the host, as to be less intrusive, and harder to dislodge. The coloured patches seen on hosts mark the location of their subdermal symbiotes; the colouration corresponds to minor genetic variations (think racial skin colours). Here they have co-opted the dermis to serve as their reproductive organ, injecting pheromones and sperm into the sweat glands, and using pores to receive the pheromones and sperm of other symbiotes.

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  • $\begingroup$ Despite this making sense and happening in nature.. I can't help but think how gross this actually is :/, also plus one for a good explanation and answer $\endgroup$ – Xylius Aug 14 '16 at 5:35
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The spots are a social signal sent to other members of the host species to flag up the fact it is a good mating prospect.

Hosts of the symbiote are protected from disease and parasites. This is a major advantage. They pass on the symbiote to their offspring. This is another major advantage. Therefore members of the opposite sex should always pick a mate who carries the symbiote.

The spots are therefore an honest signal of the potential mate's health and ability to produce healthy offspring. They may have started as a side effect of the symbiote, but they will rapidly become something that the infected species is quick to notice and react to.

In short, the spots will become - like the peacock's tail or the iridescence on the wings of a butterfly - a sexually attractive trait to members of their own species.

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I think it's an good and fun idea to play around. But an organism like that seems to border on implausibility to me.

You can come up with a story on how it prevents Tumors in its own genes, no problem. But preventing them in the cells of the host in an entirely different hurdle. For the organism to do that it must be INSIDE the host cells, or have a way for the large number of incredibly complex protection proteins it produces not only to migrate into every cell of the host's body but also to pass through the nucleus membrane to the center of the cell where the DNA is kept. Proteins that pass through this membrane get 'deconstructed' and 'rebuild' on the other side of the membrane, but your own cell doesn't know how to rebuild this strange protein, it wouldn't even pass through the first gate of the membrane to begin with because it isn't 'tagged'properly.

This is all stuff you can pseudoscientificly explain, but I suggest you make it a little less hard on yourself and simply say: "The organism can perfectly detect tumors and destroy them." Instead of preventing them.

Small single cell tumors are pretty common in everyone I understand, and they constantly get obliterated by your immune system, only very very few survive because they copy so much behavior and characteristics of stem-cells. But your micro-organism can still detect them! It will probably work best as a Bacterium, and have colonies under the skin on the shoulders. The Bacteria will move through the bloodstream and function as an extension of your own immune system.

You can tack on gut flora and cholesterol somehow if you want but I don't think it's necessary.

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