Ecological Diversification of Vibrio fischeri Serially Passaged for 500 Generations in Novel Squid Host Euprymna tasmanica

Abstract

Vibrio fischeri isolated from Euprymna scolopes (Cephalopoda: Sepiolidae) was used to create twenty-four lines that were serially passaged through the non-native host E. tasmanica for 500 generations. These derived lines were characterized for biofilm formation, swarming motility, carbon source utilization, and in vitro bioluminescence. Phenotypic assays were compared between “ES” (E. scolopes) and “ET” (E. tasmanica) V. fischeri wild isolates to determine if convergent evolution was apparent between E. tasmanica evolved lines and ET V. fischeri. Ecological diversification was observed in utilization of most carbon sources examined. Convergent evolution was evident in motility, biofilm formation, and select carbon sources displaying hyperpolymorphic usage in V. fischeri. Convergence in bioluminescence (a 2.5-fold increase in brightness) was collectively evident in the derived lines relative to the ancestor. However, dramatic changes in other properties—time points and cell densities of first light emission and maximal light output and emergence of a lag phase in growth curves of derived lines suggest increased light intensity per se was not the only important factor. Convergent evolution implies gnotobiotic squid light organs subject colonizing V. fischeri to similar selection pressures. Adaptation to novel hosts appears to involve flexible microbial metabolism, establishment of biofilm and swarmer V. fischeri ecotypes, and complex changes in bioluminescence. Our data demonstrate numerous alternate fitness optima or peaks are available to V. fischeri in host adaptive landscapes, where novel host squids serve as habitat islands. Thus, V. fischeri founder flushes occur during the initiation of light organ colonization that ultimately trigger founder effect diversification.

Host-selected mutations converging on a global regulator drive an adaptive leap by bacteria to symbiosis

“…These bottlenecks limit light organ microbial diversity, including variation that impairs symbiosis such as cheater strains that do not contribute to the mutualism (Ruby and McFall-Ngai 1999;Wollenberg and Ruby 2009). However, host-imposed selection that drives evolution of some lineages towards efficient colonization could hinder future adaptation and entail fitness trade-offs in other environments where symbionts must also persist (Caley and Munday 2003;Soto et al 2014). By contrast, planktonic V. fischeri strains that reside in habitats without hosts, or that are unable to compete for prime host niches, may maintain greater adaptability despite their ineffectiveness as symbionts (Takemura et al 2014).…”

Adaptation to temperature stress by Vibrio fischeri facilitates this microbe's symbiosis with the Hawaiian bobtail squid ( Euprymna scolopes )

“…; Soto et al. ). See Supporting Information for additional details on strain maintenance, media recipes, aseptic techniques used, and for the details on how V. fischeri was evolved to establish a baseline “ancestral” environment.…”

“…For the animal experiments, the least significant difference (LSD) with a modified Bonferroni correction using the Dunn–Sidak method (experiment‐wise Type 1 α error = 0.05) for all possible pairwise comparisons ( k ) was calculated (Sokal and Rohlf ; Soto et al. ). All possible pairwise comparisons k = ( a )( a – 1)/2, where “ a ” is the number of groups or classes (means).…”

“…This design of statistical analysis was chosen, because significant differences can easily be determined by visual inspection of the figures that present the data (Soto et al. ). This is helpful when many pairwise comparisons are being made.…”

“…With large sample sizes, enough statistical power can be maintained to conduct many pairwise comparisons after a Bonferroni correction has been applied (Soto et al. ). In the present study, there is one animal per each V. fischeri line for each temperature selection regime.…”