Differential gene expression in red abalone (Haliotis rufescens) hyperparasitized by bacteriophage pCXc provides insights into the potential role of bacteriophages in Withering Syndrome dynamics.

Withering syndrome (WS) is a chronic disease of abalones caused by the intracellular bacterium Candidatus Xenohaliotis californiensis (CXc), which is itself susceptible to infection by the bacteriophage hyperparasite (pCXc). However, the relationship between phage hyperparasitism, host transcriptional responses and disease expression remains poorly understood. Here, we integrated histopathological analyses with tissue-specific transcriptional profiling of genes associated with digestive metabolism, antioxidant defense, innate immunity and cellular stress to investigate naturally occurring WS in farmed red abalone (Haliotis rufescens). Histopathological and transcriptional analyses consistently distinguished asymptomatic (AWS) and symptomatic (SWS) abalones. pCXc-associated inclusions predominated in AWS individuals, whereas non-hyperparasitized CXc inclusions were more frequent in SWS abalones. Symptomatic individuals exhibited consistent upregulation of GST and Catalase, together with significant SeGPx upregulation restricted to the post-esophagus, whereas Alginase expression remained significantly higher in AWS abalones in both tissues. These patterns indicate that WS is characterized by tissue-specific transcriptional responses associated with digestive metabolism, antioxidant defense and innate immunity rather than uniform transcriptional responses across the genes examined. Collectively, these findings provide new insights into the potential role of phage hyperparasitism in withering syndrome dynamics within an integrative ecological framework in which disease expression is proposed to emerge from interactions among host physiology, bacterial infection dynamics and phage hyperparasitism. This work provides a foundation for future investigations of host-pathogen-phage interactions in marine mollusks.