Gene-Swapping Mediates Host Specificity among Symbiotic Bacteria in a Beneficial Symbiosis

<div><p>Environmentally acquired beneficial associations are comprised of a wide variety of symbiotic species that vary both genetically and phenotypically, and therefore have differential colonization abilities, even when symbionts are of the same species. Strain variation is common among conspecific hosts, where subtle differences can lead to competitive exclusion between closely related strains. One example where symbiont specificity is observed is in the sepiolid squid-<i>Vibrio</i> mutualism, where competitive dominance exists among <i>V. fischeri</i> isolates due to subtle genetic differences between strains. Although key symbiotic loci are responsible for the establishment of this association, the genetic mechanisms that dictate strain specificity are not fully understood. We examined several symbiotic loci (<i>lux</i>-bioluminescence, <i>pil</i> = pili, and <i>msh</i>-mannose sensitive hemagglutinin) from mutualistic <i>V. fischeri</i> strains isolated from two geographically distinct squid host species (<i>Euprymna tasmanica</i>-Australia and <i>E. scolopes</i>-Hawaii) to determine whether slight genetic differences regulated host specificity. Through colonization studies performed in naïve squid hatchlings from both hosts, we found that all loci examined are important for specificity and host recognition. Complementation of null mutations in non-native <i>V. fischeri</i> with loci from the native <i>V. fischeri</i> caused a gain in fitness, resulting in competitive dominance in the non-native host. The competitive ability of these symbiotic loci depended upon the locus tested and the specific squid species in which colonization was measured. Our results demonstrate that multiple bacterial genetic elements can determine <i>V. fischeri</i> strain specificity between two closely related squid hosts, indicating how important genetic variation is for regulating conspecific beneficial interactions that are acquired from the environment.</p></div

<i>lux</i> operon data.

<p>Colonization assays 48-hour post-infection of juvenile (A) <i>Euprymna scolopes</i> and (B) <i>Euprymna tasmanica</i> by their respective wild-type (ES114 or ETJB1H), mutant, and complement strains of the <i>lux</i> operon for <i>Vibrio fischeri</i>. Infection efficiency data is plotted as the log values of the relative competitiveness index (RCIs), calculated by dividing the ratio of mutant to wild-type by the starting ratio <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0101691#pone.0101691-Hussa1" target="_blank">[28]</a>. If the RCI is <1 the mutant strain was outcompeted by the wild-type, the wild-type strain was outcompeted by the mutant if the value is >1, and a RCI equal to 1 indicates no competitive difference. Data points represent individual animals and the position of the figures on the y axis is merely for spacing. Vertical line represents the median value of each data plot.</p

<i>msh</i> operon data.

<p>Colonization assays 48-hour post-infection of juvenile (A) <i>Euprymna scolopes</i> and (B) <i>Euprymna tasmanica</i> by their respective wild-type (ES114 or ETJB1H), mutant, and complement strains of <i>msh</i> genes for <i>Vibrio fischeri</i>. Infection efficiency data is plotted as the log values of the relative competitiveness index (RCIs), calculated by dividing the ratio of mutant to wild-type by the starting ratio <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0101691#pone.0101691-Hussa1" target="_blank">[28]</a>. If the RCI is <1 the mutant strain was outcompeted by the wild-type, the wild-type strain was outcompeted by the mutant if the value is >1, and a RCI equal to 1 indicates no competitive difference. Data points represent individual animals and the position of the figures on the y axis is merely for spacing. Vertical line represents the median value of each data plot.</p

<i>pil</i> operon data.

<p>Colonization assays 48-hour post-infection of juvenile (A) <i>Euprymna scolopes</i> and (B) <i>Euprymna tasmanica</i> by their respective wild-type (ES114 or ETJB1H), mutant, and complement strains of <i>pil</i> genes for <i>Vibrio fischeri</i>. Infection efficiency data is plotted as the log values of the relative competitiveness index (RCIs), calculated by dividing the ratio of mutant to wild-type by the starting ratio <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0101691#pone.0101691-Hussa1" target="_blank">[28]</a>. If the RCI is <1 the mutant strain was outcompeted by the wild-type, the wild-type strain was outcompeted by the mutant if the value is >1, and a RCI equal to 1 indicates no competitive difference. Data points represent individual animals and the position of the figures on the y axis is merely for spacing. Vertical line represents the median value of each data plot.</p