The time points chosen for cDNA library construction. The diagram illustrates when, in the context of the symbiont-induced developmental program of the host light organ and major milestones in V. fischeri colonization, tissues for library construction were dissected from juvenile animals. As the symbionts aggregate, migrate into host tissues and take up residence in the light organ crypts, they induce the diagrammed series of developmental changes in the various components of the organ. Most of these changes, including the irreversible loss of the SCE, which doesn't culminate until 96 hours, are triggered by or around 12 hours following first exposure to environmental V. fischeri. In addition, around 12 hours, the symbionts usually fully colonize the crypt spaces and their growth is attenuated and luminescence induced. By 48 hours, significant changes in the proteome occur (Lemus and McFall-Ngai, 2000). In addition, mutants of V. fischeri incapable of persisting in the light organ lose the ability to colonize the organ beginning at around 48 hours.