Significance Sea stars inhabiting the Northeast Pacific Coast have recently experienced an extensive outbreak of wasting disease, leading to their degradation and disappearance from many coastal areas. In this paper, we present evidence that the cause of the disease is transmissible from disease-affected animals to apparently healthy individuals, that the disease-causing agent is a virus-sized microorganism, and that the best candidate viral taxon, the sea star-associated densovirus (SSaDV), is in greater abundance in diseased than in healthy sea stars.

Sea Star Wasting Disease (SSWD) describes a suite of disease signs that affected >20 species of asteroid since 2013 along a broad geographic range from the Alaska Peninsula to Baja California. Previous work identified the Sea Star associated Densovirus (SSaDV) as the best candidate pathogen for SSWD in three species of common asteroid (Pycnopodia helianthoides, Pisaster ochraceus, and Evasterias troscheli), and virus-sized material (<0.22 µm) elicited SSWD signs in P. helianthoides. However, the ability of virus-sized material to elicit SSWD in other species of asteroids was not known. Discordance between detection of SSaDV by qPCR and by viral metagenomics inspired the redesign of qPCR primers to encompass SSaDV and two densoviral genotypes detected in wasting asteroids. Analysis of asteroid samples collected during SSWD emergence in 2013-2014 showed an association between wasting asteroid-associated densoviruses (WAaDs) and SSWD in only one species (P. helianthoides). WAaDs were found in association with asymptomatic asteroids in contemporary (2016 and later) populations, suggesting that they may form subclinical infections at the times they were sampled. WAaDs were found in SSWD-affected P. helianthoides after being absent in asymptomatic individuals a year earlier at one location (Kodiak). Direct challenge of P. ochraceus, Pisaster brevispinus, and E. troscheli with virus-sized material from SSWD-affected individuals did not elicit SSWD in any trial. RNA viral genomes discovered in viral metagenomes and host transcriptomes had viral loads and metagenome fragment recruitment patterns that were inconsistent with SSWD. Analysis of water temperature and precipitation patterns on a regional scale suggests that SSWD occurred following dry conditions at several locations, but mostly was inconsistently associated with either parameter. Semi-continuous monitoring of SSWD subtidally at two sites in the Salish Sea from 2013 to 2017 indicated that SSWD in E. troscheli and P. ochraceus was associated with elevated water temperatures, but wasting in P. helianthoides occurred irrespective of environmental conditions. Our data therefore do not support that widespread SSWD is associated with potential viral pathogens in species other than P. helianthoides. Rather, we speculate that SSWD may represent a syndrome of heterogeneous etiologies between geographic locations, between species, or even within a species between locations.

Decades of research have demonstrated the crucial importance of viruses in freshwater ecosystems. However, few studies have focused on the seasonal dynamics and potential hosts of RNA viruses. We surveyed microbial-sized (i.e. 5–0.2 μm) mixed community plankton transcriptomes for RNA viral genomes and investigated their distribution between microbial and macrobial plankton over a seasonal cycle across three temperate lakes by quantitative reverse transcriptase PCR (qRT-PCR). A total of 30 contigs bearing similarity to RNA viral genomes were recovered from a global assembly of 30 plankton RNA libraries. Of these, only 13 were found in >2 libraries and recruited >100 reads (of 9.13 x 107 total reads), representing several picornaviruses, two tobamoviruses and a reovirus. We quantified the abundance of four picornaviruses and the reovirus monthly from August 2014 to May 2015. Patterns of viral abundance in the >5 μm size fraction and representation in microbial-sized community RNA libraries over time suggest that one picornavirus genotype (TS24835) and the reovirus (TS148892) may infect small (<5 μm) eukaryotic microorganisms, while two other picornaviruses (TS24641 and TS4340) may infect larger (>5 μm) eukaryotic microorganisms or metazoa. Our data also suggest that picornavirus TS152062 may originate from an allochthonous host. All five viral genotypes were present in at least one size fraction across all 3 lakes during the year, suggesting that RNA viruses may easily disperse between adjacent aquatic habitats. Our data therefore demonstrate that RNA viruses are widespread in temperate lacustrine ecosystems, and may provide evidence of viral infection in larger eukaryotes (including metazoa) inhabiting the lakes.

Echinoderms are prone to large population fluctuations that can be mediated by pervasive disease events. For the majority of echinoderm disease events the causative pathogen is unknown. Viruses have only recently been explored as potential pathogens using culture-independent techniques though little information currently exists on echinoderm viruses. In this study, ten circular ssDNA viruses were discovered in tissues among an asteroid (Asterias forbesi), an echinoid (Strongylocentrotus droebachiensis) and a holothurian (Parastichopus californicus) using viral metagenomics. Genome architecture and sequence similarity place these viruses among the rapidly expanding circular rep-encoding single stranded (CRESS) DNA viral group. Multiple genomes from the same tissue were no more similar in sequence identity to each other than when compared to other known CRESS DNA viruses. The results from this study are the first to describe a virus from a holothurian and continue to show the ubiquity of these viruses among aquatic invertebrates.

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