COMPETITION BETWEEN VIBRIO-FISCHERI STRAINS DURING INITIATION AND MAINTENANCE OF A LIGHT ORGAN SYMBIOSIS

Colonization of the light-emitting organ of the Hawaiian squid Euprymn a scolopes is initiated when the nascent organ of a newly hatched squi d becomes inoculated with Vibrio fischeri cells present in the ambient seawater. Although they are induced for luminescence in the light org an, these symbiotic strains are characteristically non-visibly luminou s (NVL) when grown in laboratory culture. The more typical visibly lum inous (VL) type of V. fischeri co-occurs in Hawaiian seawater with the se NVL strains; thus, two phenotypically distinct groups of this speci es potentially have access to the symbiotic niche, yet only the NVL on es are found there. In laboratory inoculation experiments, VL strains, when presented in pure culture, showed the same capability for coloni zing the light organ as NVL strains. However, in experiments with mixe d cultures composed of both VL and NVL strains, the VL ones were unabl e to compete with the NVL ones and did.not persist within the light or gan as the symbiosis became established. In addition, NVL strains ente red light organs that had already been colonized by VL strains and dis placed them. The mechanism underlying the symbiotic competitiveness ex hibited by NVL strains remains unknown; however, it does not appear to be due to a higher potential for siderophore activity. While a differ ence in luminescence phenotype between VL and NVL strains in culture i s not likely to be significant in the symbiosis, it has helped identif y two distinct groups of V. fischeri that express different colonizati on capabilities in the squid light organ. This competitive difference provides a useful indication of important traits in light organ coloni zation.