Euprymna Scolopes Research Articles

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

For microorganisms cycling between free-living and host-associated stages, where reproduction occurs in both of these lifestyles, an interesting inquiry is whether adaptation to stress during the free-living stage can impact microbial fitness in the host. To address this topic, the mutualism between the Hawaiian bobtail squid (Euprymna scolopes) and the marine bioluminescent bacterium Vibrio fischeri was utilized. Using microbial experimental evolution, V. fischeri was selected to low (8°C), high (34°C), and fluctuating temperature stress (8°C/34°C) for 2000 generations. The temperatures 8°C and 34°C were the lower and upper growth limits, respectively. V. fischeri was also selected to benign temperatures (21°C and 28°C) for 2000 generations, which served as controls. V. fischeri demonstrated significant adaptation to low, high, and fluctuating temperature stress. V. fischeri did not display significant adaptation to the benign temperatures. Adaptation to stressful temperatures facilitated V. fischeri's ability to colonize the squid host relative to the ancestral lines. Bioluminescence levels also increased. Evolution to benign temperatures did not manifest these results. In summary, microbial adaptation to stress during the free-living stage can promote coevolution between hosts and microorganisms.

Persistent symbiont colonization leads to a maturation of hemocyte response in the Euprymna scolopes/Vibrio fischeri symbiosis.

The binary association between the squid, Euprymna scolopes, and its symbiont, Vibrio fischeri, serves as a model system to study interactions between beneficial bacteria and the innate immune system. Previous research demonstrated that binding of the squid's immune cells, hemocytes, to V. fischeri is altered if the symbiont is removed from the light organ, suggesting that host colonization alters hemocyte recognition of V. fischeri. To investigate the influence of symbiosis on immune maturation during development, we characterized hemocyte binding and phagocytosis of V. fischeri and nonsymbiotic Vibrio harveyi from symbiotic (sym) and aposymbiotic (apo) juveniles, and wild‐caught and laboratory‐raised sym and apo adults. Our results demonstrate that while light organ colonization by V. fischeri did not alter juvenile hemocyte response, these cells bound a similar number of V. fischeri and V. harveyi yet phagocytosed only V. harveyi. Our results also indicate that long‐term colonization altered the adult hemocyte response to V. fischeri but not V. harveyi. All hemocytes from adult squid, regardless of apo or sym state, both bound and phagocytosed a similar number of V. harveyi while hemocytes from both wild‐caught and sym‐raised adults bound significantly fewer V. fischeri, although more V. fischeri were phagocytosed by hemocytes from wild‐caught animals. In contrast, hemocytes from apo‐raised squid bound similar numbers of both V. fischeri and V. harveyi, although more V. harveyi cells were engulfed, suggesting that blood cells from apo‐raised adults behaved similarly to juvenile hosts. Taken together, these data suggest that persistent colonization by the light organ symbiont is required for hemocytes to differentially bind and phagocytose V. fischeri. The cellular immune system of E. scolopes likely possesses multiple mechanisms at different developmental stages to promote a specific and life‐long interaction with the symbiont.

The Vibrio-Squid Symbiosis as a Model for Studying Interbacterial Competition.

The symbiosis between Euprymna scolopes squid and its bioluminescent bacterial symbiont, Vibrio fischeri, is a valuable model system to study a natural, coevolved host-microbe association. Over the past 30 years, researchers have developed and optimized many experimental methods to study both partners in isolation and during symbiosis. These powerful tools, along with a strong foundational knowledge about the system, position the Vibrio-squid symbiosis at the forefront of host-microbe interactions because this system is uniquely suited to investigation of symbiosis from both host and bacterial perspectives. Moreover, the ability to isolate and characterize different strains of V. fischeri has revealed exciting new insights about how different genotypes evolve to compete for a host niche, including deploying interbacterial weapons early during host colonization. This Perspective explores how interbacterial warfare influences the diversity and spatial structure of the symbiotic population, as well as the possible effects that intraspecific competition might have on the host.

Draft Genome Sequences of Type VI Secretion System-Encoding Vibrio fischeri Strains FQ-A001 and ES401.

The type VI secretion system (T6SS) facilitates lethal competition between bacteria through direct contact. Comparative genomics has facilitated the study of these systems in Vibrio fischeri, which colonizes the squid host Euprymna scolopes Here, we report the draft genome sequences of two lethal V. fischeri strains that encode the T6SS, FQ-A001 and ES401.

Molecular insights of a novel cephalopod toll-like receptor homologue in Sepiella japonica, revealing its function under the stress of aquatic pathogenic bacteria

Toll-like receptors (TLRs) play an important role in defense response to pathogens in mollusk. In this study the first TLR from Sepiella japonica (named as SjTLR) was functionally characterized, and its full-length cDNA consisted of 3914bp (GenBank accession no. AQY56780.1) including an open reading frame of 3582bp, encoding a putative protein of 1193 amino acids. Its theoretical molecular weight was 137.87 KDa and the predicted isoelectric point was 3.69. The derived amino acids sequence comprised of an extracellular domain including 26 amino acids signal peptide and eleven leucine-rich repeats (LRR), capped with LRRCT and LRRNT followed by transmembrane domain and cytoplasmic Toll/IL-1R domain (TIR). In addition, 12 potential N-linked glycosylation sites were present in the ectodomain to influence protein trafficking, surface presentation and ligand recognition. Multiple sequence alignment and phylogenetic analysis revealed that SjTLR shared the highest similarity to that of Euprymna scolopes and they fell into the same clade. Real-time PCR showed SjTLR expressed constitutively in all tested tissues, including gill, liver, brain, muscle, intestine, heart, lobus opticus and stomach, but showed different expression levels with genders. The highest expression was in the liver, and the lowest was in stomach for both genders. The functional domain region sequences encoding LRRs domain protein and TIR domain containing protein (TcpB) were expressed in BL21(DE3) respectively and purified with Ni-NAT Superflow resin conforming to the expected molecular weight. The cellular localization of SjTLR in HEK293 cells was conducted and plasma membrane localization was detected. SjLRRs internalization upon the activation of LPS was also observed, and dramatic redistribution of SjLRRs in the cytoplasm with distinct perinuclear accumulation was found. After SjTLR transfection Toll/NF-κB signaling pathway was active in HEK293 treated with LPS and TNFɑ. The nuclear related genes may also be activated by NF-κB in the nucleus, and the corresponding mRNA was transferred through the intracellular signal transduction pathway, so that IL-6 cytokines could be synthesized and released. After infection by Vibrio parahemolyticus and Aeromonas hydrophila the expression of SjTLR were upregulated with time-dependent manner. These findings might be valuable for understanding the innate immune signaling pathways of S.japonica and enabling future studies on host-pathogen interactions.

Natural Strain Variation Reveals Diverse Biofilm Regulation in Squid-Colonizing Vibrio fischeri.

The mutualistic symbiont Vibrio fischeri builds a symbiotic biofilm during colonization of squid hosts. Regulation of the exopolysaccharide component, termed Syp, has been examined in strain ES114, where production is controlled by a phosphorelay that includes the inner membrane hybrid histidine kinase RscS. Most strains that lack RscS or encode divergent RscS proteins cannot colonize a squid host unless RscS from a squid symbiont is heterologously expressed. In this study, we examine V. fischeri isolates worldwide to understand the landscape of biofilm regulation during beneficial colonization. We provide a detailed study of three distinct evolutionary groups of V. fischeri and find that while the RscS-Syp biofilm pathway is required in one of the groups, two other groups of squid symbionts require Syp independent of RscS. Mediterranean squid symbionts, including V. fischeri SR5, colonize without an RscS homolog encoded by their genome. Additionally, group A V. fischeri strains, which form a tightly related clade of Hawaii isolates, have a frameshift in rscS and do not require the gene for squid colonization or competitive fitness. These same strains have a frameshift in sypE, and we provide evidence that this group A sypE allele leads to an upregulation in biofilm activity. Thus, this work describes the central importance of Syp biofilm in colonization of diverse isolates and demonstrates that significant evolutionary transitions correspond to regulatory changes in the syp pathway.IMPORTANCE Biofilms are surface-associated, matrix-encased bacterial aggregates that exhibit enhanced protection to antimicrobial agents. Previous work has established the importance of biofilm formation by a strain of luminous Vibrio fischeri bacteria as the bacteria colonize their host, the Hawaiian bobtail squid. In this study, expansion of this work to many natural isolates revealed that biofilm genes are universally required, yet there has been a shuffling of the regulators of those genes. This work provides evidence that even when bacterial behaviors are conserved, dynamic regulation of those behaviors can underlie evolution of the host colonization phenotype. Furthermore, this work emphasizes the importance of investigating natural diversity as we seek to understand molecular mechanisms in bacteria.

Squids in for lights up

Symbiosis The nocturnal Hawaiian bobtail squid, Euprymna scolopes , uses a luminescent bacterium to provide camouflage against moon- and starlight. This arrangement is so essential to the squid that it has developed a specialized light organ to house the luminous Vibrio fischeri . Strong day-night patterns of responses then develop, which regulate the symbiosis into an ecologically relevant daily cycle. Moriano-Gutierrez et al. monitored gene expression in the host throughout the daily cycle in three organs: the symbiotic light organ itself and the squid eye and gill. If the lux operon is deleted and the symbiont becomes “dark,” then the daily pattern of gene expression in the eye goes awry. Expression in the light organ is also altered, which results in shutting off nutrients to the symbionts—in effect punishing them for turning the light off. The squid holorganism provides a clear example of the potential for symbionts to influence the biology of distal host organs. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1819897116 (2019).

Critical symbiont signals drive both local and systemic changes in diel and developmental host gene expression.

The colonization of an animal's tissues by its microbial partners creates networks of communication across the host's body. We used the natural binary light-organ symbiosis between the squid Euprymna scolopes and its luminous bacterial partner, Vibrio fischeri, to define the impact of colonization on transcriptomic networks in the host. A night-active predator, E. scolopes coordinates the bioluminescence of its symbiont with visual cues from the environment to camouflage against moon and starlight. Like mammals, this symbiosis has a complex developmental program and a strong day/night rhythm. We determined how symbiont colonization impacted gene expression in the light organ itself, as well as in two anatomically remote organs: the eye and gill. While the overall transcriptional signature of light organ and gill were more alike, the impact of symbiosis was most pronounced and similar in light organ and eye, both in juvenile and adult animals. Furthermore, the presence of a symbiosis drove daily rhythms of transcription within all three organs. Finally, a single mutation in V. fischeri-specifically, deletion of the lux operon, which abrogates symbiont luminescence-reduced the symbiosis-dependent transcriptome of the light organ by two-thirds. In addition, while the gills responded similarly to light-organ colonization by either the wild-type or mutant, luminescence was required for all of the colonization-associated transcriptional responses in the juvenile eye. This study defines not only the impact of symbiont colonization on the coordination of animal transcriptomes, but also provides insight into how such changes might impact the behavior and ecology of the host.

Symbiotic organs shaped by distinct modes of genome evolution in cephalopods

Microbes have been critical drivers of evolutionary innovation in animals. To understand the processes that influence the origin of specialized symbiotic organs, we report the sequencing and analysis of the genome of Euprymna scolopes, a model cephalopod with richly characterized host-microbe interactions. We identified large-scale genomic reorganization shared between E. scolopes and Octopus bimaculoides and posit that this reorganization has contributed to the evolution of cephalopod complexity. To reveal genomic signatures of host-symbiont interactions, we focused on two specialized organs of E. scolopes: the light organ, which harbors a monoculture of Vibrio fischeri, and the accessory nidamental gland (ANG), a reproductive organ containing a bacterial consortium. Our findings suggest that the two symbiotic organs within E. scolopes originated by different evolutionary mechanisms. Transcripts expressed in these microbe-associated tissues displayed their own unique signatures in both coding sequences and the surrounding regulatory regions. Compared with other tissues, the light organ showed an abundance of genes associated with immunity and mediating light, whereas the ANG was enriched in orphan genes known only from E. scolopes Together, these analyses provide evidence for different patterns of genomic evolution of symbiotic organs within a single host.

Sulfur availability for Vibrio fischeri growth during symbiosis establishment depends on biogeography within the squid light organ.

The fitness of host-associated microbes depends on their ability to access nutrients in vivo. Identifying these mechanisms is significant for understanding how microbes have evolved to fill specific ecological niches within a host. Vibrio fischeri is a bioluminescent bacterium that colonizes and proliferates within the light organ of the Hawaiian bobtail squid, which provides an opportunity to study how bacteria grow in vivo. Here, the transcription factor CysB is shown to be necessary for V. fischeri both to grow on several sulfur sources in vitro and to establish symbiosis with juvenile squid. CysB is also found to regulate several genes involved in sulfate assimilation and to contribute to the growth of V. fischeri on cystine, which is the oxidized form of cysteine. A mutant that grows on cystine but not sulfate could establish symbiosis, suggesting that V. fischeri acquires nutrients related to this compound within the host. Finally, CysB-regulated genes are shown to be differentially expressed among the V. fischeri populations occupying the various colonization sites found within the light organ. Together, these results suggest the biogeography of V. fischeri populations within the squid light organ impacts the physiology of this symbiotic bacterium in vivo through CysB-dependent gene regulation.

Should they stay or should they go? Nitric oxide and the clash of regulators governing Vibrio fischeri biofilm formation.

A key regulatory decision for many bacteria is the switch between biofilm formation and motile dispersal, and this dynamic is well illustrated in the light-organ symbiosis between the bioluminescent bacterium Vibrio fischeri and the Hawaiian bobtail squid. Biofilm formation mediated by the syp gene cluster helps V. fischeri transition from a dispersed planktonic lifestyle to a robust aggregate on the surface of the nascent symbiotic organ. However, the bacteria must then swim to pores and down into the deeper crypt tissues that they ultimately colonize. A number of positive and negative regulators control syp expression and biofilm formation, but until recently the environmental inputs controlling this clash between opposing regulatory mechanisms have been unclear. Thompson et al. have now shown that Syp-mediated biofilms can be repressed by a well-known host-derived molecule: nitric oxide. This regulation is accomplished by the NO sensor HnoX exerting control over the biofilm regulator HahK. The discoveries reported here by Thompson et al. cast new light on a critical early stage of symbiotic initiation in the V. fischeri-squid model symbiosis, and more broadly it adds to a growing understanding of the role(s) that NO and HnoX play in biofilm regulation by many bacteria.

Nitric oxide inhibits biofilm formation by Vibrio fischeri via the nitric oxide sensor HnoX.

Nitric oxide (NO) is an important defense molecule secreted by the squid Euprymna scolopes and sensed by the bacterial symbiont, Vibrio fischeri, via the NO sensor HnoX. HnoX inhibits colonization through an unknown mechanism. The genomic location of hnoX adjacent to hahK, a recently identified positive regulator of biofilm formation, suggested that HnoX may inhibit colonization by controlling biofilm formation, a key early step in colonization. Indeed, the deletion of hnoX resulted in early biofilm formation in vitro, an effect that was dependent on HahK and its putative phosphotransfer residues. An allele of hnoX that encodes a protein with increased activity severely delayed wrinkled colony formation. Control occurred at the level of transcription of the syp genes, which produce the polysaccharide matrix component. The addition of NO abrogated biofilm formation and diminished syp transcription, effects that required HnoX. Finally, an hnoX mutant formed larger symbiotic biofilms. This work has thus uncovered a host-relevant signal controlling biofilm and a mechanism for the inhibition of biofilm formation by V. fischeri. The study of V. fischeri HnoX permits us to understand not only host-associated biofilm mechanisms, but also the function of HnoX domain proteins as regulators of important bacterial processes.

Achieving a multi-strain symbiosis: strain behavior and infection dynamics.

Strain diversity, while now recognized as a key driver underlying partner dynamics in symbioses, is usually difficult to experimentally manipulate and image in hosts with complex microbiota. To address this problem, we have used the luminous marine bacterium Vibrio fischeri, which establishes a symbiosis within the crypts of the nascent light organ of the squid Euprymna scolopes. Competition assays in newly hatched juvenile squid have shown that symbiotic V. fischeri are either niche-sharing "S strains", which share the light organ when co-inoculated with other S strains, or niche-dominant "D strains", which are typically found alone in the light organ after a co-colonization. To understand this D strain advantage, we determined the minimum time that different V. fischeri strains needed to initiate colonization and used confocal microscopy to localize the symbionts along their infection track. Further, we determined whether symbiont-induced host morphogenic events also occurred earlier during a D strain colonization. We conclude that D strains colonized more quickly than S strains. Nevertheless, light-organ populations in field-caught adult squid often contain both D and S strains. We determined experimentally that this symbiont population heterogeneity might be achieved in nature by a serial encounter of different strains in the environment.

Bacterial symbionts use a type VI secretion system to eliminate competitors in their natural host

Intraspecific competition describes the negative interaction that occurs when different populations of the same species attempt to fill the same niche. Such competition is predicted to occur among host-associated bacteria but has been challenging to study in natural biological systems. Although many bioluminescent Vibrio fischeri strains exist in seawater, only a few strains are found in the light-organ crypts of an individual wild-caught Euprymna scolopes squid, suggesting a possible role for intraspecific competition during early colonization. Using a culture-based assay to investigate the interactions of different V. fischeri strains, we found "lethal" and "nonlethal" isolates that could kill or not kill the well-studied light-organ isolate ES114, respectively. The killing phenotype of these lethal strains required a type VI secretion system (T6SS) encoded in a 50-kb genomic island. Multiple lethal and nonlethal strains could be cultured from the light organs of individual wild-caught adult squid. Although lethal strains eliminate nonlethal strains in vitro, two lethal strains could coexist in interspersed microcolonies that formed in a T6SS-dependent manner. This coexistence was destabilized upon physical mixing, resulting in one lethal strain consistently eliminating the other. When juvenile squid were coinoculated with lethal and nonlethal strains, they occupied different crypts, yet they were observed to coexist within crypts when T6SS function was disrupted. These findings, using a combination of natural isolates and experimental approaches in vitro and in the animal host, reveal the importance of T6SS in spatially separating strains during the establishment of host colonization in a natural symbiosis.

Involvement of a host Cathepsin L in symbiont-induced cell death.

The cathepsin L gene of the host squid, Euprymna scolopes, is upregulated during the first hours of colonization by the symbiont Vibrio fischeri. At this time, the symbiotic organ begins cell death‐mediated morphogenesis in tissues functional only at the onset of symbiosis. The goal of this study was to determine whether Cathepsin L, a cysteine protease associated with apoptosis in other animals, plays a critical role in symbiont‐induced cell death in the host squid. Sequence analysis and biochemical characterization demonstrated that the protein has key residues and domains essential for Cathepsin L function and that it is active within the pH range typical of these proteases. With in situ hybridization and immunocytochemistry, we localized the transcript and protein, respectively, to cells interacting with V. fischeri. Activity of the protein occurred along the path of symbiont colonization. A specific Cathepsin L, nonspecific cysteine protease, and caspase inhibitor each independently attenuated activity and cell death to varying degrees. In addition, a specific antibody decreased cell death by ~50%. Together these data provide evidence that Cathepsin L is a critical component in the symbiont‐induced cell death that transforms the host tissues from a colonization morphology to one that promotes the mature association.

Ontogenetic and Experience-Dependent Changes in Defensive Behavior in Captive-Bred Hawaiian Bobtail Squid, Euprymna scolopes.

Cephalopod molluscs are known for their extensive behavioral repertoire and their impressive learning abilities. Their primary defensive behaviors, such as camouflage, have received detailed study, but knowledge is limited to intensive study of relatively few species. A considerable challenge facing cephalopod research is the need to establish new models that can be captive bred, are tractable for range of different experimental procedures, and that will address broad questions in biological research. The Hawaiian Bobtail Squid (Euprymna scolopes) is a small, tropical cephalopod that has a long history of research in the field of microbial symbiosis, but offers great promise as a novel behavioral and neurobiological model. It can be bred in the laboratory through multiple generations, one of the few species of cephalopod that can meet this requirement (which is incorporated in regulations such as EU directive 2010/63/EU). Additionally, laboratory culture makes E. scolopes an ideal model for studying ontogeny- and experience-dependent behaviors. In this study, we show that captive bred juvenile and adult E. scolopes produce robust, repeatable defensive behaviors when placed in an exposed environment and presented with a visual threat. Further, adult and juvenile squid employ different innate defensive behaviors when presented with a size-matched model predator. When a 10-min training procedure was repeated over three consecutive days, defensive behaviors habituated in juvenile squid for at least 5 days after training, but memory did not appear to persist for 14 days. In contrast, adult squid did not show any evidence of long-term habituation memory. Thus we conclude that this species produces a range of quantifiable, modifiable behaviors even in a laboratory environment where ecologically-relevant, complex behavioral sequences may not reliably occur. We suggest that the lack of long-term memory in adult squid may be related to their less escalated initial response to the mimic, and thus indicates less motivation to retain memory and not necessary inability to form memory. This is the first demonstration of age-related differences in defensive behaviors in Euprymna, and the first record of habituation in this experimentally tractable genus of squid.

Reproductive System Symbiotic Bacteria Are Conserved between Two Distinct Populations of Euprymna scolopes from Oahu, Hawaii.

Female Hawaiian bobtail squid, Euprymna scolopes, harbor a symbiotic bacterial community in a reproductive organ, the accessory nidamental gland (ANG). This community is known to be stable over several generations of wild-caught bobtail squid but has, to date, been examined for only one population in Maunalua Bay, Oahu, HI. This study assessed the ANG and corresponding egg jelly coat (JC) bacterial communities for another genetically isolated host population from Kaneohe Bay, Oahu, HI, using 16S amplicon sequencing. The bacterial communities from the ANGs and JCs of the two populations were found to be similar in richness, evenness, phylogenetic diversity, and overall community composition. However, the Kaneohe Bay samples formed their own subset within the Maunalua Bay ANG/JC community. An Alteromonadaceae genus, BD2-13, was significantly higher in relative abundance in the Kaneohe Bay population, and several Alphaproteobacteria taxa also shifted in relative abundance between the two groups. This variation could be due to local adaptation to differing environmental challenges, to localized variability, or to functional redundancy among the ANG taxa. The overall stability of the community between the populations further supports a crucial functional role that has been hypothesized for this symbiosis. IMPORTANCE In this study, we examined the reproductive ANG symbiosis found in two genetically isolated populations of the Hawaiian bobtail squid, Euprymna scolopes. The stability of the community reported here provides support for the hypothesis that this symbiosis is under strong selective pressure, while the observed differences suggest that some level of local adaptation may have occurred. These two host populations are frequently used interchangeably as source populations for research. Euprymna scolopes is an important model organism and offers the opportunity to examine the interplay between a binary and a consortial symbiosis in a single model host. Understanding the inherent natural variability of this association will aid in our understanding of the conservation, function, transmission, and development of the ANG symbiosis.

Profile of Margaret J. McFall-Ngai

At the 152nd annual meeting of the National Academy of Sciences (NAS) in 2015, newly elected NAS member Margaret McFall-Ngai told attendees, “I happen to be in the right place at the right time as microbiology takes center stage in biology.” The charismatic professor and director of the Pacific Biosciences Research Center, School of Ocean and Earth Science and Technology, at the University of Hawaii at Manoa was referring, in part, to technologies like low-cost, high-throughput gene sequencing that have facilitated greater insight into the identity and function of microbes. Margaret McFall-Ngai. Image courtesy of Kent Nishimura (photographer). Hawaiian bobtail squid ( Euprymna scolopes ). Image courtesy of Chris Frazee (University of Wisconsin–Madison, Madison, WI). When McFall-Ngai’s scientific career began in 1978, microbes were primarily viewed as pathogens or agents of decomposition. Since then, her work using a binary squid–bacterial symbiosis system to characterize animal microbiomes has contributed to expanding the once-narrow view of the microbial world. For example, McFall-Ngai and her team have shown that bacterial symbionts can induce host-animal development and drive circadian rhythms. Her Inaugural Article (1) advances such work by describing the biophysical mechanisms by which a host selects bacteria. Raised in Southern California, McFall-Ngai attended Immaculate Heart High School in Los Angeles during the late 1960s. “It is a phenomenal college-preparatory girl’s high school where students are taught that there is no limit to what they can dream about doing in their careers,” she says. “I will forever be grateful to the faculty there for giving me wings.” McFall-Ngai next attended the University of San Francisco, where she earned her Bachelor of Science in biology in 1973. After returning to Southern California for graduate studies with James Morin at the University of California, Los Angeles (UCLA), McFall-Ngai began to explore …

The squid insurance plan: female Euprymna scolopes add potentially protective bacteria to the egg coats of their clutches

The squid insurance plan: female <i>Euprymna scolopes</i> add potentially protective bacteria to the egg coats of their clutches

Host-selected mutations converging on a global regulator drive an adaptive leap towards symbiosis in bacteria.

Host immune and physical barriers protect against pathogens but also impede the establishment of essential symbiotic partnerships. To reveal mechanisms by which beneficial organisms adapt to circumvent host defenses, we experimentally evolved ecologically distinct bioluminescent Vibrio fischeri by colonization and growth within the light organs of the squid Euprymna scolopes. Serial squid passaging of bacteria produced eight distinct mutations in the binK sensor kinase gene, which conferred an exceptional selective advantage that could be demonstrated through both empirical and theoretical analysis. Squid-adaptive binK alleles promoted colonization and immune evasion that were mediated by cell-associated matrices including symbiotic polysaccharide (Syp) and cellulose. binK variation also altered quorum sensing, raising the threshold for luminescence induction. Preexisting coordinated regulation of symbiosis traits by BinK presented an efficient solution where altered BinK function was the key to unlock multiple colonization barriers. These results identify a genetic basis for microbial adaptability and underscore the importance of hosts as selective agents that shape emergent symbiont populations.