19The loss of functions required for independent life when living within a host gives rise to 20 reduced genomes in obligate bacterial symbionts. Although this phenomenon can be 21 explained by existing evolutionary models, its initiation is not well understood. Here, we 22 32 33 38 Kroiss et al., 2010). Such relationships exist on a continuous spectrum of dependency 39 and exclusivity from the perspective of both the host and symbiont. Symbionts generally 40 become obligate after a prolonged period of exclusive association with the host, and the 41 3 symbionts that become obligate tend to carry out highly important functions for the host 42 (Latorre and Manzano-Marín, 2017; Lo et al., 2016; McCutcheon and Moran, 2012). For 43 example, mitochondria and chloroplasts, organelles that are required for energy 44 production and carbon fixation in eukaryotic and plant cells, originated from 45 endosymbiotic capture of alphaproteobacteria and cyanobacteria ~1.2 Bya and ~900 46