Transcriptomic profile of Manila clam (Ruditapes philippinarum) haemocytes in response to Perkinsus olseni infection

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Ruditapes philippinarum, one of the major contributors to the global mollusc production, is susceptible to perkinsosis, a disease caused by the protozoan Perkinsus olseni (Alveolata, Perkinsidae). Understanding the molecular response of Manila clam defence cells (haemocytes) to this parasite is essential to head towards controlling this disease. Here, we assembled the Manila clam haemocyte transcriptome from healthy and P. olseni infected clams collected from wild and experimental conditions (in vivo and in vitro) using RNA-seq. The Manila clam de novo transcriptome consists of 33,079 transcripts, of which 7300 were significantly annotated (E-value <1.0E−5) with the NCBI non-redundant (nr) protein database. Among these annotated transcripts, many were functionally linked to signalling, cell proliferation, cell adhesion, immune system (e.g. immune response, immune effector process, antigen processing) and response to stress and stimuli. Our study integrates previous transcriptomic information of R. philippinarum haemocytes with present resources, including a notable amount of new transcripts with annotation description expressed after challenge with P. olseni in different infection scenarios, many of them linked to stress-related processes. A preliminary analysis of differentially expressed genes (DEGs) was performed with the sequences from the pool samples of control and challenged haemocytes obtained under similar experimental conditions. A higher number of DEGs were detected in in vitro than in vivo stimulated haemocytes, denoting the more acute and different in vitro challenges with P. olseni trophozoites, zoospores and extracellular products. Most in vitro DEGs were down-regulated and associated with immune and stress response, likely reflecting the immune-suppression occurring at the onset of infection, while DEGs in the in vivo infected haemocytes were mostly related to metabolic process. The preliminary DEG analysis on haemocyte response provides some insight into the molecular mechanism of Manila clam defence cells against perkinsosis. Statement of relevanceHigh Manila clam mortality rates have been reported due to perkinsosis caused by Perkinsus olseni, however, the molecular mechanisms of Ruditapes philippinarum response against perkinsosis are still not fully understood. Our study provided new data to enrich the R. philippinarum haemocyte transcriptome databases from previous reports, and preliminary information on haemocyte response against perkinsosis is provided.