The bobtail squid Euprymna
scolopes is home to the bioluminecent bacterium Vibrio fischeri,
present in small 'crypts' in the squids light organ. The bacteria are key
symbionts of the squid and play an important role in the fitness of the animals. However, the way in
which the diversity of V. fisheri is
structured within squid populations was poorly understood. Some evidence, such as differing levels of bioluminescence,
suggested that more than one strain may be present in an individual squid. Furthermore,
it may be that V. fischeri strains
are different between squid populations. Wollenburg et al. confirmed these suspicions
by examining the phenotype and genetics of bacteria in two populations of squid
from Kaneohe and Mannalua Bay on Oahu island, Hawaii.
Squid were collected from the two
bays and tissue dissected from the lobes of the light organ. Symbionts were
then isolated from homogenised tissue. It was found, using a battery of
physiological tests, that populations within a single light organ are
physiologically heterogeneous. Differing in pigmentation, bioluminescence and
other traits such as motility, growth rate and siderophore production, with one
to three different types of bacteria present within a single lobe. Genetic differences
were then assessed using PCR, which amplified the repetitive genetic sequence
VfRep. This produced a genetic ‘barcode’ for each genotype that was used to
categorise them. Seventeen banding types were revealed that fell into three
major types and multiple patterns were found per lobe, per animal and per
population. It was found that each genotype possessed specific physiological
traits such as bioluminescence level, motility and colony pigmentation. This information
clearly indicates that multiple distinct bacteria initially colonise the light
organs of juvenile squid. But the methods available to the researchers could only
reveal course levels of diversity. It would be interesting to apply
single-celled genomics to further delve into within squid strain variation.
The frequency of genetic and physiological
characteristics were then compared between populations. This revealed that
VfRep type, bioluminescence, motility rate, colony pigmentation and growth rate
were significantly different between each site. Differences between Vibrios show host population biology
plays a role in structuring the Vibrio populations. However, one of the weaknesses of this
paper is that they did not speculate what aspect of the host biology are
driving these changes. For example, are differences in strain bioluminescence
being driven by differing site turbiditys etc.?
The presence of multiple strains within
the squid makes symbiosis no walk in the park. Competition between different bacteria
could lead to a weakening of the symbiotic relationship at the detriment to the
squid. Two ways in which the squid may deal with this are by the host
'sanctioning' the non-cooperative symbionts, or by repeatedly adjusting the symbiotic
interactions based on the outcome of previous interactions between host and
symbionts. Theoretical models dealing with the second aspect predict that
spatial structuring in symbionts, as shown above, can stabilise these interactions.
Interestingly the daily ejection of the symbionts was not discussed. It could
be that this behaviour could act to level the playing field between competing
bacteria. All in all, the extremely well written paper provided a fascinating insight
into the diversity of Vibrio fisheri and
that not all model organisms have to be lab bound mice or flies.
Wollenburg M.S. & Ruby E. G.
(2009). Populations Structure of Vibrio
fishcheri within the Light Organs of Euprymna
scolopes Squid from Two Oahu (Hawaii) populations. Applied and Environmental Microbiology, 75(1), 193-202.
Hi Tom,
ReplyDeleteThanks for the interesting paper, I agree that competition between bacteria may be detrimental to the host. One of the first papers to use molecular systematic and experimental colonisation by Nishiguchi et. al. (1998) on Squid-Vibrio symbiosis suggested a hierarchy between symbionts existed. This would limit competition between symbionts meaning the squid may have little need to self regulate bacterial composition. Perhaps comparing the population numbers of the Vibrio symbionts may help to show this hierarchical pattern.
Hi Ben, Yea that sounds like a interesting idea, perhaps physical cell size and growth rate may also be worth investigating? By hierarchy did they mean that there was a difference in their ability to compete between the bacteria or perhaps using slightly different niches? I do think however, that the periodic expulsion of the symbionts by the squid must be important in this kind of area did Nishiguchi et al. discuss that?
DeleteBy hierarchy I think they meant that a certain bacteria will outcompete others and perhaps forcing them into a smaller niche. Nishiguchi has produced quite a few papers and reviews on the topic and from what I read after a commented I think he did talk about the importance of expelling bacteria as well so my thoughts on the last comment may have been slightly wrong.
DeleteA review by Nyholm and Nishiguchi (2008) states that the strategy of accommodating a number of ecologically adaptable Vibrio strains not only allows for squid hosts to select for the best-fit symbionts, but also permits the bacteria to rapidly change in response to both the host and the environment in which the association is found. The review also refers to the paper Ben mentioned where they found that native strain vibrios are better suited to colonize the crypts of their own squid host light organ when compared to non-native Vibrio competitors. This suggests that communities can differ even on a small spatial scale.
ReplyDeleteHi Tom, Did they also look into the diversity of the bacteria present in the water around? This would be also interesting for the armoured snails I thought... maybe there is a particular high or low diversity of the bacteria in the particular environments?
ReplyDelete