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Replace mushroom with virus, and maybe that's what actually happened in real life.

Most people are probably already aware, but that’s likely exactly what happened with mitochondria: https://en.wikipedia.org/wiki/Symbiogenesis

A better example is the mammalian placenta. That actually was enabled by viral genes, while mitochondria developed from free-living organisms.

Bacteria too, gut biome is a thing and an interesting symbiosis.

In a sense one can argue that all the processes going on in the body (thinking of e.g. white blood cells) is also a form of symbiosis.