Let’s start out with a little quiz. What examples can you name of endosymbiotic bacteria so tightly packed that they’re nearly wall-to-wall within cells of their host? If you said legume root nodules, you can claim a prize. (Ask someone else for it, as we don’t have any.) You would get bonus points if you also mentioned the bacteria-filled bacteriocytes of certain insects or the celebrated giant tube worms found near the deep sea black smokers. By the way, the root nodule symbioses, though not mandatory, are highly beneficial to the host, but the other two are obligatory.
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