Evolutionary Perspectives in a Mutualism of Sepiolid Squid and Bioluminescent Bacteria: Combined Usage of Microbial Experimental Evolution and Temporal Population Genetics

Abstract

The symbiosis between marine bioluminescent Vibrio bacteria and the sepiolid squid Euprymna is a model for studying animal–bacterial Interactions. Vibrio symbionts native to particular Euprymna species are competitively dominant, capable of outcompeting foreign Vibrio strains from other Euprymna host species. Despite competitive dominance, secondary colonization events by invading nonnative Vibrio fischeri have occurred. Competitive dominance can be offset through superior nonnative numbers and advantage of early start host colonization by nonnatives, granting nonnative vibrios an opportunity to establish beachheads in foreign Euprymna hosts. Here, we show that nonnative V. fischeri are capable of rapid adaptation to novel sepiolid squid hosts by serially passaging V. fischeri JRM200 (native to Hawaiian Euprymna scolopes) lines through the novel Australian squid host E. tasmanica for 500 generations. These experiments were complemented by a temporal population genetics survey of V. fischeri, collected from E. tasmanica over a decade, which provided a perspective from the natural history of V. fischeri evolution over 15,000–20,000 generations in E. tasmanica. No symbiont anagenic evolution within squids was observed, as competitive dominance does not purge V. fischeri genetic diversity through time. Instead, abiotic factors affecting abundance of V. fischeri variants in the planktonic phase sustain temporal symbiont diversity, a property itself of ecological constraints imposed by V. fischeri host adaptation.

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

“…Ten ET (wild isolates from E. tasmanica ) and ten ES (wild isolates from E. scolopes ) V. fischeri strains were previously shown to be genetically distinct from one another [7, 8, 33]. These field isolates were randomly sampled from their Euprymna squid hosts and used as a comparison to the evolved clones for motility (Fig.…”

“…V. fischeri ES114 is chloramphenicol sensitive, while V. fischeri JRM200 is a V. fischeri ES114 derivative which is chloramphenicol resistant [24]. V. fischeri JRM200 was used to create twenty-four lines that were serially transferred through novel squid host E. tasmanica for 500 generations [7] and used in this study. All 24 evolved lines improved in colonization of E. tasmanica relative to ancestor, as no lineage remained the same or became worse.…”

“…Particularly, the mutualism between Vibrio fischeri and Euprymna has become a model for studying associations between eukaryotic hosts and bacteria, since both partners can be maintained independently of each other in the laboratory [4, 5]. Axenic juvenile Euprymna squid hatch from their eggs with sterile light organs and are quickly colonized by symbiotically competent V. fischeri present in bacterioplankton, reaching a light organ carrying capacity (10 4 –10 7 colony forming units (CFUs)/light organ) within 12–24 hours [6, 7]. Sepiolid squids are nocturnal, and just before burying in sand, seed the surrounding water with symbiotic competent V. fischeri at dawn by venting 90–95% of the symbiont light organ population.…”

“…Additionally, Euprymna species are distributed allopatrically throughout the Indo-West Pacific Ocean, and V. fischeri colonizing this genus are host specialists, exhibiting competitive dominance, whereas strains forming mutualisms with several sympatric Sepiola species from the Mediterranean Sea [1, 9–11] are host generalists and display no competitive dominance. Prior evidence from experimentally evolved lines of V. fischeri ES114 (native to Hawaiian E. scolopes ) selected through a non-native host (Australian E. tasmanica ) for 500 generations exhibited a significant increase in mean fitness relative to the ancestral strain in colonizing E. tasmanica [7], yet the traits responsible for this host specificity change remain unknown. Therefore, to determine which phenotypes are subject to host selection, both ancestor and evolved clones were characterized for biofilm formation, motility, carbon source utilization, and in vitro bioluminescence.…”

Bacterial Carbonic Anhydrases as Drug Targets

“…Prokaryotes include several kinds of microorganisms, such as archaea, bacteria, and cyanobacteria (7). Most such microorganisms do not normally cause disease in humans, existing either in a state of commensalism or in mutualism with the host (8)(9)(10). This nonharmful condition occurs when the immune system works well, but the same organisms can cause infection when the latter fails.…”