Breaking the language barrier: experimental evolution of non-native Vibrio fischeri in squid tailors luminescence to the host

Although most Vibrio fischeri isolates are capable of symbiosis, the coevolution of certain strains with the Hawaiian bobtail squid, Euprymna scolopes, has led to specific adaptation to this partnership. For instance, strains from different hosts or from a planktonic environment are ineffective squid colonists. Even though bioluminescence is a symbiotic requirement, curiously, symbionts of E. scolopes are dim in culture relative to fish symbionts and free-living isolates. It is unclear whether this dim phenotype is related to the symbiosis or simply coincidental. To further explore the basis of symbiont specificity, we developed an experimental evolution model that utilizes the daily light organ venting behavior of the squid and horizontal acquisition of symbionts for serial passage of cultures. We passaged six populations each derived from the squid-naïve strains of V. fischeri MJ11 (a fish symbiont) and WH1 (a free-living isolate) through a series of juvenile squid light organs. After 15 serially colonized squid for each population, or an estimated 290–360 bacterial generations, we isolated representatives of the light organ populations and characterized their bioluminescence. Multiple evolved lines of both strains produced significantly less bioluminescence both in vitro and in vivo. This reduction in bioluminescence did not correlate with reduced quorum sensing for most isolates tested. The remarkable phenotypic convergence with squid symbionts further emphasizes the importance of bioluminescence in this symbiosis, and suggests that reduced light production is a specific adaptation to the squid.