Bioluminescence may be affected by strain assembly history in a marine symbiont

Symbionts can be highly variable in their species or strain composition in ways that environmental factors alone cannot explain. One potential mechanism that contributes to this variability is priority effects, whereby the arrival order and timing influence how species or strains affect one another in the host. Here, we investigated whether priority effects influence strain composition in the luminous bacterial symbiont Photobacterium mandapamensis, which inhabits the light organ of the cardinalfish Siphamia tubifer. Using three strains isolated from the same individual host, we conducted in vitro experiments manipulating arrival order and initial abundance. Strains introduced earlier or at higher initial abundance consistently dominated the resultant bacterial populations, indicating strong priority effects. The strains differed measurably in growth and light production. Moreover, the bioluminescence of mixed-strain populations was not predictable from the performance of strains grown individually and, in some cases, exceeded monoculture luminosity by up to 4000%. These results suggest that assembly history could create otherwise inexplicable variability in the strain composition of symbionts and their bioluminescent output, calling for in vivo investigations to study priority effects in the symbiosis.