Unique viruses associated with symbionts of Caribbean corals

There were a lot of words and abbreviations in here that I wasn’t too sure on, so here’s a list of them to start off with:
ss – single stranded
ds – double stranded
NCLDV – nucleocytoplasmic large DNA virus
cDNA – complementary DNA – synthesized from mRNA
EST – expressed sequence tag
Virome – the genomes of all the viruses that inhabit a particular organism or environment
HcRNAV – Heterocapsa circularisquama RNA virus
mcp – major capsid protein
Girus – giant NCLDV

The Caribbean coral Montastrea cavernosa contains endosymbiotic dinoflagellate algae from Symbiodinium clade C and is known to be susceptible to a variety of coral diseases. The particular combination of host and symbiont genotype has previously been shown to influence phenotypic characteristics such as tolerance to thermal and light regimes as well as disease resistance and susceptibility. It has been hypothesized that viral groups that are present in free-living dinoflagellates and eukaryotic algae may be causing bleaching in corals due to viral infection and/or viral-induced lysis of Symbiodinium cells. Correa et al. (2013) analyzed metatranscriptomes from VLP fractions isolated from control (28^oC) and heat stressed (31.5^oC) M. cavernosa and compared them with independent cDNA EST draft libraries from Symbiodinium cultures to investigate whether viruses such as phycodnaviruses (known to be present in Montastrea corals) are present in Symbiodinium as well as their host corals.

Correa et al. (2013) found that many transcripts coding for structural components and enzymes were indicative of NCLDVs (dsDNA eukaryotic viruses). This is interesting given that in previous studies Herpesvirus-like sequences have generally dominated. They constructed phylogenetic trees for three phylogenetically informative genes common among NCLDVs; mcp, DNA polymerase family B and topoisomerase II. Neither mcp  or topoisomerase II sequences were able to be assigned to an existing clade, and were both thought to provide evidence for the presence of Phycodnaviridae while DNA polymerase family B sequences fell into a clade with two unclassified giruses. They also found that the stressed M. cavernosa virome contained unique similarities to HcNRAV that infects free-living dinoflagellates (that are often responsible for toxic algal blooms), however the mcp sequences (known to be relatively highly conserved) clearly differed from published sequences, and it was thus suggested that the sequences represent novel undescribed members of the Alvernaviridae (+ssRNA viruses). These sequences may represent different viral strains within a larger population that could target distinct Symbiodinium clades.

It would be interesting for a future study to isolate these viruses and investigate the effects of inoculating healthy corals at normal and elevated temperatures with them as well as carrying out a slightly longer study with samples at intervals (corals were only subjected to heat stress for 12 hours in this study). It may then be possible to determine if coral diseases are in fact mediated by viruses as some have suggested. This paper was quite a difficult read as there was a lot of vocabulary that was new to me, but I think it was probably quite useful for gaining a bit of a better understanding of marine viruses.  

 

Correa, A., Welsh, R. & Thurber, R. (2013) Unique nucleocytoplasmic dsDNA and +ssRNA viruses are associated with the dinoflagellate endosymbionts of corals. The ISME Journal. 7, 13-27

There was quite a lot in the paper that I didn’t have space to cover, so here’s the link to it if anyone’s interested:
http://www.nature.com/ismej/journal/v7/n1/full/ismej201275a.html