Transcriptome-wide analysis of wild Asari (= Manila) clams affected by the Brown Muscle Disease: Etiology and impacts of the disease

Recently, we reported an emerging pathology named Brown Muscle Disease (BMD) affecting Asari clams inhabiting the most productive area for this species in France, the Arcachon Bay. The main macroscopic feature of the pathology relies on the atrophy of the posterior adductor muscle, affecting the ability of clams to burry. The research of the etiological agent of BMD privileged a viral infection. Contrary to healthy clams, infected animals are always found at the surface of the sediment and exhibit 30 nm virus-like particles in muscle, granulocytic and rectal cells. In order to get more insights on the etiology and impacts of the BMD on clams, we took advantage in the present study of next generation sequencing technologies. An RNA-Seq approach was used (i) to test whether viral RNA sequences can be specifically found in the transcriptome of diseased animals and (ii) to identify the genes that are differentially regulated between diseased and healthy clams. Contrary to healthy buried animals, in diseased clams one sequence showing extensive homologies with retroviridae-related genes was detected. Among the biological processes that were affected in diseased clams, the synaptic transmission process was the most represented. To deepen this result, a new sampling was carried out and the transcription level of genes involved in synaptic transmission was determined in healthy and diseased clams but also in clams with no visible sign of pathology but located at the surface of the sediment. Our findings suggest that muscle atrophy is a latter sign of the pathology and that nervous system could be instead a primary target of the BMD agent