Initial investigation of viruses infecting copepods suggests that viral infection may be ubiquitous in oceans

Following Collins lecture on the diseases in marine organisms I decided to investigate whether any further research had been conducted in relation to viral infections in mesoplankton. The mesoplankton provide an important link in the trophic transfer of energy up the food web. Copepods are the most ubiquitous and abundant members of the mesoplankton yet very little is known of their interactions with viruses. Predation is often thought to be the primary factor involved in copepod mortality, but this is only accountable for 65-70% of the mortality observed in copepods (Hirst & Kiorboe, 2002). Only one study exists on viral interaction with copepods in the literature which observed no negative impacts on the copepod Acartia tonsa (Drake & Dobbs, 2005).

Dunlop & colleagues (2013) investigated viral infection of the calanoid copepods Labidocera aestiva and Acartia tonsa using molecular and microscopic techniques isolated from natural populations in Tampa Bay, Florida.

Results indicated that two new ssDNA viruses, AtCopCV and LaCopCV, were present A. tonsa and L. Aestiva, respectively.  Both viruses show similarities to Circoviridae family which have the characteristics of a small genome size, two non-overlapping ORFs, and a stem loop with conserved non-anucliotide motif, but in contrast to normal circoviruses the genomes are orientated in the same direction and are not ambisense ( A single stranded genome that contains both positive-sense and negative-sense). These characteristics have been isolated in Circoviridae from environmental and faecal genomes. Previously Circoviruses have only been isolated from vertebrates and one study in dragon flies, so this is an important find suggesting a broader environmental scope for this group of viruses. Phylogenetic analysis suggests that although both LaCopCV and AtCopCV cluster with the circoviridae they are highly divergent from other known circoviridae. Dunlop and Colleagues suggest that these viruses represent a unique lineage in the circoviridae that infect marine invertebrates.

Following this Dunlop and colleagues investigated the viral load by replicating the putative capsid gene of LACopCV using quantative PCR.  This revealed loads of 77-7.5x10⁵ copies per L. aestival individual with 100% of individuals infected in three out of four locations sampled. The viral loading of these copepods is similar to that of WSSV detected in other arthropods. Modified quantative PCR revealed that there was replication of these viruses in the host, but the transcription observed was only minimal. Furthermore, environmental samples revealed that the viruses were detected in the sediment and not in the seawater, suggesting that the sediment is a reservoir for these viruses. This is surprising, because you may expect that a virus infecting a copepod would be present in the water column increasing the chance of virus-host interaction.  

To further document the viral existence in tissue sections of A. tonsa and L. labidocera were observed under via transmission electron microscopy. This revealed that the virus-like particals were present in the connective tissue of both copepods and not the parasites, symbionts or gut material. The viruses were significantly larger than any known Circoviridae, with diameters of roughly 40nm, as-opposed to 17nm, but whether these particles really are the AtCopCV or LACopCV remains to be determined.  

This investigation opens a new door for investigation into food web dynamics and nutrient cycling in ocean and coastal process and likely to lead to many more research opportunities.   

 

REVIEWED: Dunlop. D. S., Fei Fang. Ng. T., Rosario. K., Barbosa. J. G., Greco. A. M., Breibart. M., Hewson. I.,(2013). Molecular and microscopic evidence of viruses in marine copepods. PNAS. 1216595110.

  

Drake LA, Dobbs FC (2005) Do viruses affect fecundity and survival of the copepod
Acartia tonsa Dana? J Plankton Res 27:167–174.

Hirst A, Kiørboe T (2002) Mortality of marine planktonic copepods: Global rates and

patterns. Mar Ecol Prog Ser 230:195–209.