Up in Arms: Immune and Nervous System Response to Sea Star Wasting Disease

Abstract

Echinoderms, positioned taxonomically at the base of deuterostomes, provide an important system for the study of the evolution of the immune system. However, there is little known about the cellular components and genes associated with echinoderm immunity. The 2013–2014 sea star wasting disease outbreak is an emergent, rapidly spreading disease, which has led to large population declines of asteroids in the North American Pacific. While evidence suggests that the signs of this disease, twisting arms and lesions, may be attributed to a viral infection, the host response to infection is still poorly understood. In order to examine transcriptional responses of the sea star Pycnopodia helianthoides to sea star wasting disease, we injected a viral sized fraction (0.2 μm) homogenate prepared from symptomatic P. helianthoides into apparently healthy stars. Nine days following injection, when all stars were displaying signs of the disease, specimens were sacrificed and coelomocytes were extracted for RNA-seq analyses. A number of immune genes, including those involved in Toll signaling pathways, complement cascade, melanization response, and arachidonic acid metabolism, were differentially expressed. Furthermore, genes involved in nervous system processes and tissue remodeling were also differentially expressed, pointing to transcriptional changes underlying the signs of sea star wasting disease. The genomic resources presented here not only increase understanding of host response to sea star wasting disease, but also provide greater insight into the mechanisms underlying immune function in echinoderms.

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

“…We further examined these libraries to identify specific viral genomes which may have contributed to the disease. We found 11 contigs resembling Picornaviruses and chose two (PhaDiV1 and PhaDiV3; both most similar to Dicistroviruses) based on their coverage within metaviromes, completion of genomes, and presence in host transcriptomes prepared from material from the Seattle Aquarium (Gudenkauf and Hewson, 2015) and Friday Harbor (Fuess et al, 2015). We then performed qRT-PCR across a broad range of SSWDaffected and asymptomatic samples from 2013 used to determine pattern of SSaDV association with SSWD (Hewson et al, 2014).…”

“…Resulting contigs (n = 11) were then vetted for completeness of the polycistronic ORF (resulting in 4 contigs). The four contigs were then compared against four transcriptomes prepared as part of a prior study (Fuess et al, 2015). From these four, two recruited reads only from a single transcriptome, while the remaining two (PhaDiV1 and PhaDiV3) produced alignments from all transcriptomes (Supplemental Table 5).…”

“…PhaDiV1 and PhaDiV3 were selected for further quantitation by qRT-PCR. PhADiV1 and 3 were also compared against transcriptomes prepared in a separate study (Fuess et al, 2015) examining viral challenge and its impacts on host gene transcription.…”

“…The disease was associated, on a metacommunity level, with the Sea Star associated Densovirus (SSaDV) in P. helianthoides, P. ochraceus, and E. troscheli along its geographic range (Salish Sea to Southern California) when surveyed in 2013-2014 (Hewson et al, 2014). In addition, virus-sized material inoculated into one species (P. helianthoides) repeatedly elicited SSWDlike signs (Hewson et al, 2014) and were accompanied by enhanced expression of immune-related and tissue remodeling genes (Fuess et al, 2015;Gudenkauf and Hewson, 2015) and autocatalyzed cell death processes (Gudenkauf and Hewson, 2015). An apparent conundrum of SSWD is the presence (and in at least one asteroid species the active replication) of SSaDV in a large proportion of asymptomatic asteroids surveyed in 2013-2014, and detection of SSaDV in museum specimens since at least 1942, well before SSWD was first observed (Hewson et al, 2014).…”

Devastating Transboundary Impacts of Sea Star Wasting Disease on Subtidal Asteroids

“…Previous outbreaks of SSWD have been reported in the northeast Pacific, California and northern New England since the 1970s [11–13]; however, these earlier SSWD outbreaks involved single species in localized areas. In addition to the SSaDV, the 2013–14 SSWD epidemic has been linked to an increase [10,13,14], or decrease [15] in sea temperature, while host immunity [16], genetics [17], and a suite of other host or environmental factors are likely involved.…”