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

Abstract

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.

Intraspecific Competition Impacts Vibrio fischeri Strain Diversity during Initial Colonization of the Squid Light Organ

“…Recent experimental evolution studies using juvenile squid to propagate nonsymbiotic V. fischeri strains resulted in evolved lines that exhibit phenotypes that differ from that of the parental strain (44). For instance, the passage of the highly visible fish symbiont MJ11 through multiple juvenile squid resulted in strains with in vitro luminescence profiles comparable to that of the control strain ES114 (44). Similar diversification has been reported in the persistent infections of Pseudomonas aeruginosa within the lungs of patients with cystic fibrosis (45).…”

“…However, because the infections associated with juvenile squid can persist over the lifetime of the host (9), we speculate that the populations also evolve over time, thereby contributing to the strain diversity observed in the light organs of adult animals (16). Recent experimental evolution studies using juvenile squid to propagate nonsymbiotic V. fischeri strains resulted in evolved lines that exhibit phenotypes that differ from that of the parental strain (44). For instance, the passage of the highly visible fish symbiont MJ11 through multiple juvenile squid resulted in strains with in vitro luminescence profiles comparable to that of the control strain ES114 (44).…”

Phylogeny and fitness of Vibrio fischeri from the light organs of Euprymna scolopes in two Oahu, Hawaii populations

“…Second, the total number of V. fischeri cells in the planktonic environment has been estimated to be several (Jones et al, 2007) to many (Lee and Ruby, 1995) orders of magnitude lower than in the host light organs, suggesting that the seawater niche may have a relatively minor role in V. fischeri evolution. Finally, recent studies of V. fischeri populations in E. scolopes have demonstrated that the light-organ environment can show strong selective pressure on the physiology of V. fischeri (Schuster et al, 2010). Taken together, this evidence suggests many key features related to the host E. scolopes (that is, local population structure; strong, selective environment; high symbiont population density) that might potentiate the evolution of local V. fischeri strains.…”

Contribution of Rapid Evolution of theluxR-luxIIntergenic Region to the Diverse Bioluminescence Outputs ofVibrio fischeriStrains Isolated from Different Environments

“…Moreover, bioluminescence contributes to fitness of V. fischeri in its symbiotic environment (8,73), and this symbiosis can be reconstituted in the laboratory (55,63), affording the opportunity to assess the consequences of genetic changes to the lux system under ecologically relevant growth conditions. Interestingly, when MJ11 colonized squid, the animals were much brighter than those colonized by native symbionts (39,59), which would theoretically disrupt the advantages of dim counterillumination. Preliminary evidence was recently presented that suggested that when MJ11 was grown for multiple generations in association with its nonnative host E. scolopes, most of the squid-adapted strains became dimmer, resembling natural isolates from E. scolopes (59).…”

“…Interestingly, when MJ11 colonized squid, the animals were much brighter than those colonized by native symbionts (39,59), which would theoretically disrupt the advantages of dim counterillumination. Preliminary evidence was recently presented that suggested that when MJ11 was grown for multiple generations in association with its nonnative host E. scolopes, most of the squid-adapted strains became dimmer, resembling natural isolates from E. scolopes (59). This could indicate a selection against the energetic costs of bright bioluminescence when it confers no advantage; however, the relatively rapid loss of bright bioluminescence in the squid as opposed to findings in culture suggests a positive selection for dimness in this host.…”