Transcriptomic Analysis Reveals the Response of the Hong Kong Oyster ( Crassostrea hongkongensis ) Digestive Gland to Natural Infection by Perkinsus beihaiensis

Perkinsus spp. are pathogenic protistan parasites that pose a significant threat to the aquaculture of the Hong Kong oyster (Crassostrea hongkongensis). This study investigated the transcriptional response of the Hong Kong oyster digestive gland to natural Perkinsus spp. infection. The parasite species was identified via PCR, while infection prevalence, tissue-specific abundance, and infection grade were assessed using Ray's Fluid Thioglycollate Medium (RFTM) culture. Transcriptomic analysis was performed to compare gene expression profiles among uninfected (Neg), lightly infected (L), and light-to-moderately infected (LM) oyster digestive glands. Results confirmed the presence of P. beihaiensis, with an overall infection prevalence of 85.5%. The digestive gland exhibited the highest infection prevalence (80.1%) and abundance (1880.0 ± 144.2 cells/g) compared to other tissues (gills, mantle, and siphon). Transcriptomic analysis of 12 samples (4 biological replicates per Neg, L, and LM group) using an Illumina Novaseq 6000 platform generated 102,827 unigenes. A total of 531 differentially expressed genes (DEGs) were common to both infected groups relative to the Neg group. GO and KEGG enrichment analyses highlighted key biological processes and pathways: up-regulated DEGs were associated with the cell cycle, antigen processing and presentation, and signal transduction, while down-regulated DEGs were linked to cholesterol metabolism, phagosome function, and fat digestion and absorption. qRT-PCR validation of 8 DEGs confirmed consistent expression patterns with RNA-seq data. These findings elucidate the tissue-specific infection patterns of P. beihaiensis in Hong Kong oysters in the digestive gland and provide critical insights into host-parasite interactions.