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 d ynamics of stable associations between animals and bacteria. The two best-s tudied partners, the Hawaiian sepiolid squid Euprymna scolopes and the mari ne luminous bacterium Vibrio fischeri, engage in the most common type of an imal-bacterial association, i.e., between extracellular, gram-negative bact eria and animal epithelia. Similar to most such symbioses, the squid-vibrio relationship begins anew each generation when the- host animal acquires th e symbiont from the surrounding environment. To establish a specific associ ation, mechanisms have evolved to ensure recognition between the host and s ymbiont and the exclusion of other potential partnerships. Once the associa tion has been established, the bacteria induce significant morphological ch anges in the host that result in a transition of the light organ from a for m associated with initiation of the symbiosis to one characteristic of the mature, functional relationship.