Riftia pachyptila are giant tubeworms that form dense aggregations and constitute a major component
of the biomass in the deep-sea. As mentioned in the lectures, they form a
symbiont with bacteria that burrow into the skin of the organism and colonize, removing
the necessity of a traditionally functioning digestive tract (mouth, gut, anus
etc.) due to their production of carbon as a metabolite for respiration. No
global genomic work on the host had been published previous to this work.
This study compared the amount of DNA sequences replicated
(cDNA), DNA sequences replicated once repetitive sequences have been removed,
and the number of DNA sequences present, between different tissue types, using Subtractive Suppression
Hybridization technique (SSH). The tissues compared were the branchial plume tissue (the only organ
in contact with the sea water) and in the trophosome (the organ where the
symbiotic bacteria reside), using the body wall as a control tissue. Traditionally,
SSH involves comparing the same tissue at different time points, i.e. when the
organisms are in symbiosis and when they are not. However this is not possible
in this case as the adult cannot survive out of symbiosis, so this study compares
different tissues instead.
RNA was extracted to obtain the DNA sequences that are
replicated, and amplified using PCR. The results were used to create a library showing each tissue and control,
for example the body wall – plume library, which shows the genes that are
present in the body wall and not in the plume.
Not surprisingly,
the trophosome contained most of the sequences that coded for all processes
overall: the bacteria that have colonized the organism are responsible for a
majority of the genes required in the adult life of the tube worm; they require
more genes since they carry out traditional processed for living, including
providing carbon for the tube worm.
The tissue with
the most amount of mtDNA was the brachial plume tissue, with 60% of gene
sequences being found in this tissue; the carbon produced by the bacteria are
utilized here for respiration. The redundancy rates vary between 80.5 and 95.6%,
with 16S ribosomal mitochondrion sequence being highly redundant in the
trophosome. This suggests that further sequencing would not bring up many more new
sequences.
There are several
homologies between the branchial plume tissue and the trophosome, such as
coding regions for carbonic anhydrase enzyme, however the specific sequence is
different. This is presumably due to the nature of the genes being from
bacteria in the trophosome, and from the worm in the branchial plume tissue,
and the fact that the DNA is either eukaryotic or bacterial is demonstrated by
different sequences. The variation that does occur between the branchial plume
tissue and others is possibly due to the necessity to protect against hydrogen
sulphide in the sea.
In the BW-BR
library, representing sequences involved in the formation of the tube, there
was a sequence of cDNA with high homology with galaxin, a protein present in
the calcified exoskeleton of the coral Galaxea fascicularis, suggesting these organisms are
related.
A majority of sequences in
the TR-BW library could not be identified; more than 56 % of those that could be identified
are respiratory pigment protein transcripts. The BW-TR library was
similar to the BW-BR, with less cDNA sequences.
Protein synthesis
is still active within the Riftia pachyptila, shown by the sequence expression present in the trophosome tissue. This
shows that the worm is not
merely a host for a parasite; the relationship is symbiotic. It would be
interesting to look into the sequences of cDNA that cannot be identified, such
as those in the trophosome, considering they may be unique to this organism.
Sanchez,
S; Hourdez, S; Lallier, F. H (2007) Identification of proteins involved in the
functioning of Riftia pachyptila symbiosis by Subtractive Suppression
Hybridization, BMC Gemonics, 8:337
Megan - I am curious about a couple of comments. Do you think the fact that Riftia contains a protein with homology to the coral protein galaxin is enough to conclude that "these organisms are related"? These worms are evolutionarily very different from corals! Maybe homologues of this protein are involved in cell understructure of various organisms. Secondly, I wonder why the statement about active protein synthesis by the host trophosome tissue gives insight into whether this is a parasitic or symbiotic relationship?
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