The large DNA viruses – pox-, irido-, herpes-, asfar-, mimi- and baculoviruses and their ilk – have a deep and possibly complex evolutionary history, and there is considerable evidence to suggest long histories of co-speciation with host lineages. Indeed, the herpesviruses – including those of particular interest to humans , 1 and 2 and varicella-zoster (chickenpox) and 8 (associated with Kaposi’s sarcoma) – seem to have co-speciated along with vertebrates, with human and simian cytomegaloviruses, for example.
Now there comes evidence from a metagenomic study (which should, of course, be metaviromic) that invertebrates such as corals also have a a variety of herpesviruses, and that these may well be stress-activated: Rebecca Thurber and colleagues, in the 25th November issue of PNAS, have published an analysis of the incidence and effects of hitherto-unknown herpesvirus-like agents found in finger corals in Hawaii. Their abstract:
Metagenomic analysis indicates that stressors induce production of herpes-like viruses in the coral Porites compressa
During the last several decades corals have been in decline and at least one-third of all coral species are now threatened with extinction. Coral disease has been a major contributor to this threat, but little is known about the responsible pathogens. To date most research has focused on bacterial and fungal diseases; however, viruses may also be important for coral health. Using a combination of empirical viral metagenomics and real-time PCR, we show that Porites compressa corals contain a suite of eukaryotic viruses, many related to the Herpesviridae. This coral-associated viral consortium was found to shift in response to abiotic stressors. In particular, when exposed to reduced pH, elevated nutrients, and thermal stress, the abundance of herpes-like viral sequences rapidly increased in 2 separate experiments. Herpes-like viral sequences were rarely detected in apparently healthy corals, but were abundant in a majority of stressed samples. In addition, surveys of the Nematostella and Hydra genomic projects demonstrate that even distantly related Cnidarians contain numerous herpes-like viral genes, likely as a result of latent or endogenous viral infection. These data support the hypotheses that corals experience viral infections, which are exacerbated by stress, and that herpes-like viruses are common in Cnidarians.
Yet another demonstration – after the viromes of whole oceans – of the sheer brute power of modern sequencing technology and bioinformatics techniques, applied this time to a disease problem of oceanic invertebrates. The authors again:
Environmental stress often results in coral bleaching, disease, and death. Increased temperature, nutrient loading, dissolved organic carbon pollution, and reductions in ambient seawater pH are of particular concern due to their effects on the coral-symbiont relationship, host homeostasis, microbial overgrowth, and skeletal deposition. To determine whether environmental perturbations shift the eukaryotic viral assemblage present in corals, these 4 parameters were manipulated, and the resulting viral consortia characterized through the generation of 6 metagenomes [!]
Basically, they purified virus-sized particles from seawater, extracted DNA, verified there was no cellular DNA present (by PCR for 16S and 18S rDNA), and amplified the viral DNA using a GenomiPhi phi29 plymerase-based genome amplification kit from GE Healthcare, and sent to 454 Life Sciences for pyrosequencing.
What came back was obviously a mountain of sequence data – which was archived at the San Diego State Center for Universal Microbial Sequencing – and then analysed for contigs or assemblable runs of sequences which looked like herpesvirus or other genomes. The supplemental data to this paper gives a relationship dendrogram – which isn’t that informative, as it contains just one coral thymidylate kinase sequence, and that is grouped within a clade with human viruses with other vertebrate herpesviruses outside of those.
The final story, then, is that the intrepid coral viromicers (horrible, I know, but come up with a better one?) succeeded in showing that stresses do indeed activate herpes-like viruses in coral. In their words:
The metagenomic and temporal experiments presented here demonstrate that exposure to stressors results in the production of a herpes-like virus or a consortium of herpes-like viruses in P. compressa corals. Thermal stress, eutrophication, and decreasing seawater pH have each been shown to disrupt coral health. Increases in sea surface temperature causes coral bleaching and increased coral disease incidence (58, 59). Nutrient addition exacerbates coral diseases, and reduced pH results in loss of corallite deposition (60, 61). This study demonstrates that, in addition to symbiont loss and bacterial and fungal disease (62), temperature and nutrient elevation and pH reduction result in increased HLV production.
Another, more fundamental speculative comment:
Herpesviruses typically infect nervous tissue, and it is tempting to suggest that the herpes-like viruses in Cnidarians may have ancient origins, because Cnidarians are the first metazoans to develop rudimentary nervous systems….
So be kind to our poor fronded friends…they may be our close cousins in discomfort when stressed.