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Nowadays, the symbiosis between <i>Vibrio</i>
<i>fischeri</i> and the Hawaiian bobtail squid 


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<i>Euprymna scolopes</i> is well-characterised.

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The luminescence produced by <i>V. fischeri</i> 
symbionts help camouflage their host  


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at night by eliminating its shadow within
 the water column. It's a phenomenon called 


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counter illumination. While this symbiosis
is obligatory for the host, symbionts are 


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horizontally transmitted as the squid host
<i>Euprymna scolopes</i> acquires its <i>V. fischeri</i>


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 luminescent symbionts from the surrounding 
sea water. If this rings a bell, you may have


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 already seen this kind of transmission
in the Acoels online course.


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This association shows a strong species
 specificity initiated within hours after 


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the juvenile squid hatches, provided that
 symbiotically-compatible <i>V. fischeri</i> cells


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 are present in the ambient seawater.

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Interestingly, the <i>E. scolopes-V. fischeri</i>
 model provided the first direct evidence 


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of an animal host controlling the number 
and activity of its extracellular bacterial 


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population as part of a
 circadean biological rhythm.


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<i>E. scolopes </i>mechanically controls the 
emission of luminescence by periodically 


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expelling excess <i>V. fischeri </i>symbionts,
 thereby adjusting bacterial density inside


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 the light organ. As a result, the cell 
abundances of <i>V. fischeri</i> within the squid


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 follow a circadian pattern. At night, 
<i>V. fischeri</i> cells are present at high 


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concentrations in the crypt of the light
 organ - 10^10, 10^11 cells per mL -


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and produce autoinducers which induce
 light emission. At the end of the night,


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 most of the bacterial cells are expulsed
 from the light organ, leading to a dramatic


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 reduction in bacterial concentrations and 
of the autoinducers. Thus, in the <i>V. fischeri</i>


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 -<i>E. scolopes</i> symbiosis, the lowest pro
-duction of bioluminescence is observed 


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just before dawn to early afternoon. This 
coincides with the onset of environmental light.


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During the day, the concentration of <i>V. </i>
<i>fischeri </i>cells that have not been expulsed 


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is very low, the autoinducers are not 
produced, and the squid does not glow. 


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However, this remaining population of <i>V.</i>
<i> fischeri</i> grows steadily under favourable 


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conditions within the squid throughout
 the day and reaches again at night  


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a cell abundance that is sufficient 
to produce bioluminescence. 


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A complex and specific dialogue occurs
 between <i>V. fischeri</i> cells and the


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<i>E. scolopes</i> host given that first, the <i>V.</i>
<i> fischeri </i>cells are typically present at a 


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 concentration of &lt;0.1% of the total
 bacterial population in the Hawaiian waters


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 and secondly, the motility of these bacterial
 cells is required to bring the symbionts 


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towards the pores, the entrance of
 the luminescent organ in formation.


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 Two main mechanisms were found to 
initiate the interaction: first, a close 


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 contact between the surfaces 
of the host and symbiont cells


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for receptor-ligand interactions.

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Secondly, the creation of an environment 
in which only <i>V. fischeri </i>cells are viable.


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Receptor-ligand dynamics, often 
more generally referred to as "Microbe-


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Associated Molecular Patterns", can also 
be essential elements underlying the onset,


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 maturation, and persistence of mutualistic
 animal-microbe partnerships.


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 Different data provided evidence that at
 least a portion of the host response is


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 mediated by lipopolysaccharide-binding
 proteins and peptidoglycan recognition 


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proteins. Numerous gene-encoding proteins
known to be essential for both development


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and symbiosis were identified, such as 
reflectin, actin, myeloperoxidase, aldehyde 


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dehydrogenase, and nitric oxide synthase. 
These findings confirm the molecular


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 dialogue between host squid 
and bacterial symbionts.