Investigating the Complex Association Between Viral Ecology, Environment, and Northeast Pacific Sea Star Wasting

Abstract

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.

An initial comparative genomic autopsy of wasting disease in sea stars

“…This physical genetic map links a suspected environmental driver (elevated temperature) and intrinsic differences among individuals (age/size, genotype) and genes underlying organismal responses (e.g., immune response [ TL5A , TL5B ], phagocytosis [ SRCR1 ], cell death and wound healing [ CASP1 ], apoptosis [ WWOX ], muscle contraction [ MYS ], heat shock protein [ HSP71 ]) with presentation of SSWD. These results help define what was becoming known as asteroid idiopathic wasting syndrome (AIWS; Hewson et al, 2018) due to its previously elusive aetiology. We do not claim to circumscribe the entire set of factors involved in SSWD, but to have identified important candidate genes and to have described an approach—an autopsy guided by genomic analyses—for better understanding causes and mechanisms of MMEs.…”

“…The greatest differences reflect the distinct expression needed during development to generate different tissues (Ralston & Shaw, 2008) and then to deliver their complementary functions. These functions have recently become of particular interest given the putative involvement of densoviruses in wasting (Hewson et al, 2014; but see Hewson et al, 2018), that the microbiome of sea stars appears to be anatomically partitioned (Jackson, Pepe‐Ranney, Debenport, Buckley, & Hewson, 2018), and that tissues may display different prevalence of viruses within and between species (Hewson et al, 2018). The pyloric caecum shows the greatest number of transcripts being differentially expressed between symptomatic and asymptomatic P. ochraceus (Figures 3, 4; Appendix : Figure b); the cause is unclear but may help explain the prior association between SSaDV and wasting: elevated transcription could indicate hyperplasia, and densoviruses are favoured by rapidly dividing cells (Tijssen, Pénzes, Yu, Pham, & Bergoin, 2016).…”

“…One of the most intriguing aspects of the 2013 outbreak of sea star wasting disease has been its wide zoonotic impact and yet apparently few consistent responses among species in subsequent studies (e.g., see Hewson et al (2018) and Miner et al (2018)). Our synthesis of prior datasets (Chandler & Wares, 2017; Fuess et al, 2015; Gudenkauf & Hewson, 2015; Schiebelhut et al, 2018) enabled by the common reference of the P. ochraceus genome however begins to suggest some potential commonalities (Figure 5), although we do not consider the role of complex interactions among stressors, which may manifest in unintuitive ways that do not necessarily result in shared differential expression of the same genes.…”

“…These results are particularly interesting as recent long‐term coast‐wide analyses have suggested a link between temperature and the wasting disease outbreak of 2013 (Eisenlord et al, 2016; Harvell et al, 2019; Kohl et al, 2016). Nonetheless, regional stressors may vary (Hewson et al, 2018), and whether wasting is a direct response to temperature stress or is part of a general stress response that increases disease risk, is still unknown (Miner et al, 2018). That the differentially expressed genes for temperature treated stars are distributed across all 22 chromosomes and the six genes that overlap with symptomatic versus asymptomatic DGE are distributed across six different chromosomes (Data set d), does suggest that the sea star response is a general response (i.e., not attributable to a single gene or few genes), fitting its innate‐only immune system.…”

“…Despite coincidence of the outbreak of wasting (i.e., SSWD/AIWS) with formation of a northeastern Pacific “warm blob” (Bond, Cronin, Freeland, & Mantua, 2015), the role of temperature in disease onset and intensification is ambiguous (Hewson et al, 2018; Miner et al, 2018), suggesting that wasting might be caused by a combination of environmental (Hewson et al, 2018; Miner et al, 2018) and biological factors (Menge et al, 2016). Exploration of differential gene expression in P. ochraceus under thermal stress suggested heritable variation in transcriptional response to elevated temperature (Chandler & Wares, 2017).…”