THE INDUCTIVE ROLE OF BACTERIAL SYMBIONTS IN THE MORPHOGENESIS OF A SQUID LIGHT ORGAN

The association of the sepiolid squid Euprymna scolopes with its marin e luminous bacterial symbiont Vibrio fischeri is an emerging model sys tem to study the initiation and development of bacterial symbioses in higher animals, in particular the influence of bacteria on the ontogen ic development of symbiotic-specific host tissues. Experiments compari ng the development of juvenile squid infected with symbiotic V. fische ri with that of uninfected juveniles suggest postembryonic development of the light organ requires cell-cell interactions with the bacterial symbionts. The presence of symbiotic bacteria induces specific morpho logical changes by affecting such fundamental processes as cell death and cell differentiation. The surface of the juvenile organ is largely composed of ciliated cells that appear to facilitate infection of the light organ. These cells begin to undergo cell death within hours of infection with symbiotic V. fischeri. Within three days the epithelial cells that form the bacteria-containing crypts of the light organ inc rease in size; these cells do not appear mitotically active, and may r epresent a terminally differentiated state. The light organs of uninfe cted juvenile E. scolopes, however, do not exhibit any of these early postembryonic developmental events but remain in a state of arrested m orphogenesis.