The phylogenetic tree shows the relationships between different eukaryotic lineages. Each inset represents an example of a eukaryotic host (left) from the specified lineage that uses a specialized structure (right) to establish and maintain a bipartite association with a microbial symbiont. Image credits: flashlight fish, Stefan Herlitze; flashlight fish light organ adapted from [69] under the terms of the Creative Commons Attribution License; ship worm and gill, Margo Haygood; bobtail squid and light organ, Mark Mandel; southern pine beetle, Erich Vallery under the terms of the Creative Commons Attribution License; mycangia, Kier Klepzig [70]; attine ant, Don Parsons; ant crypt, Cameron Currie; nematode and vesicle modified from [71] under the terms of the Creative Commons Attribution License; legume and root nodule, Jean Michel Ané.