Consequences of Evolving With Bacterial Symbionts: Insights from the Squid-Vibrio Associations

The squid-vibrio light-organ symbioses, which have been under investigation for just over 10 years, offer the opportunity to decipher aspects of the dynamics of stable associations between animals and bacteria. The two best-studied partners, the Hawaiian sepiolid squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri, engage in the most common type of animal-bacterial association, i.e., between extracellular, gram-negative bacteria and animal epithelia. Similar to most such symbioses, the squid-vibrio relationship begins anew each generation when thehost animal acquires the symbiont from the surrounding environment. To establish a specific association, mechanisms have evolved to ensure recognition between the host and symbiont and the exclusion of other potential partnerships. Once the association has been established, the bacteria induce significant morphological changes in the host that result in a transition of the light organ from a form associated with initiation of the symbiosis to one characteristic of the mature, functional relationship.