Recent studies show that dinoflagellates have acquired a
number of important things from cyanobacterial horizontal gene transfer, for
example RuBisCo; a crucial component in CO2 fixation, histone-like
proteins and other plasmid-related genes. As certain dinoflagellates are also
known to produce harmful compounds known as saxitoxins, Hackett et al. carried out genomic analysis of
the dinoflagellate Alexandrium tamarense
to look for saxitoxin genes (sxt),
testing the hypothesis that they too had been passed on by horizontal gene
transfer.
The researchers first grew A. tamarense in media and harvested RNA which was prepared and
sequenced. The genetic material was then screened for a particular domain found in cyanobacterial
stx genes.
The study showed that A.
tamarense had 13 homologous genes for cyanobacterial saxitoxin production,
however the genes were not structurally similar. In cyanobacteria, sxtA is composed of two domains fused
together, in the dinoflagellate however, the two domains are separate proteins,
suggesting the two are not related phylogenetically. Other proteins that showed
similarities to cyanobacterial proteins were not calculated to have arisen by
horizontal gene transfer to the dinoflagellate.
These results tie in nicely with another article Laura
blogged about recently. Her article said that the dinoflagellate Gymnodium catenatum did not produce
saxitoxin without its bacterial community. G.
catenatum is mentioned briefly in the discussion of this article as having sxt homologues. So how do we interpret
these results? Without being too speculative, I think it may be possible that the
dinoflagellate doesn't have all the proteins required to produce saxitoxin and
therefore requires its bacterial community to create it instead.
Another idea put forward in this paper is that the
dinoflagellate enzymes have evolved independently of the cyanobacteria and are
functionally different despite structural similarities, that is, the
dinoflagellate sxts are involved in other
processes separate from production of saxitoxin. A next step would be to test the
functionality of the genes from these dinoflagellates to see if they do or don’t
produce the toxins.
Here’s a link to Laura’s blog, if you want to read it
yourself:
And here’s the reference for this article:
Hackett, J.D. Wisecaver, J.H. Brosnahan, M.L. Kulis, D.M.
Anderson, D.M. Bhattacharya, D. Plumley, F.G. and Erdner, D. (2013) Evolution
of Saxitoxin Synthesis in Cyanobacteria and Dinoflagellates. Molecular Biology and Evolution 30(1) pp.70-78.
If you have any questions or comments, do write them below
and I’ll respond as best I can.
Interesting! Just a quick question really, are we certain the cyanobacteria can produce the saxitoxins without the presence of the dinoflaggelate? Could it be that it is in fact the cyanobacteria associated with the dinoflaggelate producing the toxin by taking advantage of the dinoflaggelates enzymes? Perhaps, as you said, the dinoflagellate evolved its genes independently for a different function but it is the local bacterial community exploits them to produce saxitoxins.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteHi Kat,
ReplyDeleteI've certainly not read that the dinoflagellates make the toxin on their own, and the paper Laura gives evidence that they don't, or at least, the ones tested don't. That would be an interesting notion about the bacteria using dinoflagellate enzymes (assuming I've understood you correctly?) but the bacteria still produce their own toxin encoding genes. Though I do wonder now if there are any genes that are hijacked by symbiotic bacteria.