Many bacteria live inside other organisms. These endobacteria usually evolve functions that enhance the fitness of their partner. It was once perceived that facultative endobacteria were an intermediate, transitional stage leading to more obligate mutualisms. Contrary to what we once thought, this research shows that not all facultative endobacteria are on route to becoming an obligate mutualist. Instead, these facultative relationships can persist for millions of years, especially when their interactions vary greatly with different environmental conditions.
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