Differential gene expression patterns among wasting Pisaster ochraceus individuals following organic matter enrichment

Outbreaks of sea star wasting (SSW) have killed millions of sea stars across over 20 taxa in the last decade alone, threatening the health and stability of coastal communities around the world. While the causative agent remains unknown, it has recently been postulated that hypoxia exposure may play a dominant role in the onset of SSW. We leverage a study that subjected ochre sea stars to organic matter amendment in a controlled laboratory setting to induce hypoxia and used a repeated sampling design to collect non-invasive tissue samples from both healthy and wasting individuals. Following tag-based RNAseq (TagSeq), we analyzed differential gene expression (DGE) patterns among and within these individuals sampled strategically throughout the 15-day experiment. Transcriptional profiles reveal a progressive change in gene expression accompanying the advancement of SSW, reflecting a transition from asymptomatic stars to the onset of characteristic SSW lesions that progressively worsen until, in some cases, the star dies of their symptoms. We demonstrate an overall wasting arc or trajectory composed of individual stars tracing a shared transcriptional response to wasting and identify 152 genes that are up- or down-regulated during this journey. Further, we present the first report of constitutive differences in gene expression between orange and purple color morphs and identify 73 DEGs as a potential source of key differences in disease susceptibility attributed to this phenotype, a conclusion drawn from both stars in the source experiment and as reported in natural populations during the height of the most recent SSW outbreak. Finally, we explore compelling expression trends among individual genes of interest that ultimately support previous reports that SSW symptoms reflect a systemic response to context-dependent stressors, one that is influenced by hypoxic conditions facilitated by microorganisms inhabiting the animal-water interface.