Metatranscriptomic Analysis of Pycnopodia helianthoides (Asteroidea) Affected by Sea Star Wasting Disease.

Sea star wasting disease (SSWD) describes a suite of symptoms reported in asteroids of the North American Pacific Coast. We performed a metatranscriptomic survey of asymptomatic and symptomatic sunflower star (Pycnopodia helianthoides) body wall tissues to understand holobiont gene expression in tissues affected by SSWD. Metatranscriptomes were highly variable between replicate libraries, and most differentially expressed genes represented either transcripts of associated microorganisms (particularly Pseudomonas and Vibrio relatives) or low-level echinoderm transcripts of unknown function. However, the pattern of annotated host functional genes reflects enhanced apoptotic and tissue degradation processes and decreased energy metabolism, while signalling of death-related proteins was greater in asymptomatic and symptomatic tissues. Our results suggest that the body wall tissues of SSWD-affected asteroids may undergo structural changes during disease progression, and that they are stimulated to undergo autocatalytic cell death processes

Graphical representation of pathways that were significantly higher in symptomatic or in asymptomatic sea stars.

<p>Graphical representation of pathways that were significantly higher in symptomatic or in asymptomatic sea stars.</p

Twenty most abundant bacterial (A) and eukaryotic (B) phylogenetic affiliations amongst all four metatranscriptomic libraries prepared from <i>Pycnopodia helianthoides</i> tissues.

<p>Phylogenetic affilitation was determined by BLASTx analysis against the non-redundant (nr) database at NCBI. Affiliations were normalized by dividing the percentage of reads amongst all reads by the length of contigs assigned to that affilitation.</p

Asteroid holobiont metatranscriptome charactertics and assembly statistics for global analysis.

<p>All samples were from <i>Pycnopodia helianthoides</i> collected at the Seattle Aquarium on 10/26/13. A = Asymptomatic, S = Symptomatic.</p><p>Asteroid holobiont metatranscriptome charactertics and assembly statistics for global analysis.</p

Twenty most highly expressed functional annotations represented in sea star transcriptomes across all 4 libraries.

<p>Functional annotations were performed by BLASTx comparison against the non-redundant database at NCBI. Transcript frequencies were normalized by dividing percentage of reads associated with each annotation by contig length.</p

Similarity between overall metatranscriptomic profiles based on Manhattan Distance.

<p>Similarity was calculated based on gene-level annotations, and clustered by multidimensional scaling (MDS).</p

Densovirus associated with sea-star wasting disease and mass mortality

Populations of at least 20 asteroid species on the Northeast Pacific Coast have recently experienced an extensive outbreak of sea-star (asteroid) wasting disease (SSWD). The disease leads to behavioral changes, lesions, loss of turgor, limb autotomy, and death characterized by rapid degradation (“melting”). Here, we present evidence from experimental challenge studies and field observations that link the mass mortalities to a densovirus (Parvoviridae). Virus-sized material (i.e., <0.2 μm) from symptomatic tissues that was inoculated into asymptomatic asteroids consistently resulted in SSWD signs whereas animals receiving heat-killed (i.e., control) virus-sized inoculum remained asymptomatic. Viral metagenomic investigations revealed the sea star-associated densovirus (SSaDV) as the most likely candidate virus associated with tissues from symptomatic asteroids. Quantification of SSaDV during transmission trials indicated that progression of SSWD paralleled increased SSaDV load. In field surveys, SSaDV loads were more abundant in symptomatic than in asymptomatic asteroids. SSaDV could be detected in plankton, sediments and in nonasteroid echinoderms, providing a possible mechanism for viral spread. SSaDV was detected in museum specimens of asteroids from 1942, suggesting that it has been present on the North American Pacific Coast for at least 72 y. SSaDV is therefore the most promising candidate disease agent responsible for asteroid mass mortality