Viruses and Ctenophores

Gelatinous zooplankton plays an important role in marine and brackish ecosystems. Cnidaria and Ctenophores are not only predators and invaders, but their blooms also affect human infrastructure, tourism and aquaculture negatively. Over the last half century an increase in gelatinous zooplankton has been recorded in most marine ecosystems, however, their population regulation mechanisms are still relatively unknown. Ctenophores harbour specific bacterial communities as well as invertebrate parasites and symbionts, yet the presence of viruses had not been previously reported. In their paper, Breitbart et al. (2015) analysed the presence of circular, Rep-encoding single-stranded DNA (CRESS-DNA) viruses in two ctenophore species. 

Specimens of Mnemiopsis leidyi and its predator Beroe ovata were collected from February to October 2013 in an estuary in coastal Georgia, USA. The ctenophores were dissected and the extracted DNA was amplified with rolling circle amplification (RCA). Subsequently, the DNA was digested with restriction enzymes, cloned and Sanger sequenced. The prevalence of viral genomes over the 8-month period was examined using PCR assays. 

Out of the 17 extracted circular DNA molecules (between 1030-2838 NT), nine were not definitively identifiable and were not further analysed. The identified CRESS-DNA molecules were characterized by a Rep-encoding ORF and a supposed ori with a conserved motif. After discarding potential partial genomes and plasmids, Breitbart et al. (2015) identified four ctenophore-associated circular viruses (CtaCV-1 to CtaCV-4). These CRESS-DNA genomes were similar to viral sequences identified in previous metagenomic analyses.

Interestingly, while the CtaCVs were retrieved from the same species, they were as distantly related to each other as they were to CtaCVs in other marine organisms and ecosystems. These results reflect prior observations, that CRESS-DNA viruses seem to show no apparent clusters of location, environment or host organism. 

The analysis of CtaCVs over an 8-month period showed temporal shifts of dominant viral species in M. leidyi. While some viruses (CtaCV-2 and CtaCV-3) were only detected during certain months, others (CtaCV-1 and CtaCV-4) were present over the whole of 8-months. However, ctenophore length and volume did not seem to affect viral presence. Analysis of B. ovata associated viruses showed a smaller viral community without CtaCV-3. To exclude the possibility of copepod prey introducing viruses into their predators, copepod samples were screened for CtaCVs. Moreover, the stomachs of the ctenophores were removed during dissection and not further analysed. The results showed a much lower abundance of CtaCVs in copepod batches compared to individual ctenophores, which would make it unlikely for prey to falsify the results. 

Since the increase of gelatinous zooplankton has many negative effects, more information on its population dynamics is sorely needed. This study aims to address one of the gaps in our knowledge. Moreover, the sequence data provided enables quantitative studies of ctenophore associated viruses and their potential roles in regulating ctenophore populations. It would be interesting to see if follow-up papers might be able to shed more light on this subject. 

However, this paper also has its limitations, namely small sample size and no variation in location. Furthermore, I would be hesitant to discount the possibility that the viruses were introduced via copepod prey, as the viruses could have transferred from the stomach to the comb rows of the ctenophores. I would have liked to have seen an analysis of the CtaCV abundance in the stomach compared to the comb rows.

Reviewed Paper:

Breitbart, M., Benner, B. E., Jernigan, P. E., Rosario, K., Birsa, L. M., Harbeitner, R. C., ... & Berger, S. A. (2015). Discovery, prevalence, and persistence of novel circular single-stranded DNA viruses in the ctenophores Mnemiopsis leidyi and Beroe ovata. Frontiers in microbiology, 6. Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4683175/

Further Reading:

Tijssen, P., Pénzes, J. J., Yu, Q., Pham, H. T., & Bergoin, M. (2016). Diversity of small, single-stranded DNA viruses of invertebrates and their chaotic evolutionary past. Journal of Invertebrate Pathology, 140, 83-96. Link: http://www.sciencedirect.com/science/article/pii/S0022201116301318