Cytopathology and phylogeny of Pinna nobilis Picornavirus (PnPV) infecting haemocytes of the noble pen shell population from Lake Faro (Southern Italy)

Pinna nobilis Picornavirus (PnPv) is an RNA virus affecting the immune cells of the noble pen shell P. nobilis, which has recently been discovered in two Mediterranean countries, Spain and Italy. As part of the LIFE PINNARCA (LIFE20 NAT/ES/001265) project, a survey was performed to evaluate the health status of the P. nobilis population found in Lake Faro in Sicily (Southern Italy), previously reported as a P. nobilis sanctuary. A conservative sampling consisting of both mantle biopsies and haemolymph was performed on 6 individuals during the survey in 2023. Immune cells were observed at transmission electron microscopy (TEM) and cytology and used for RNA based qPCR. Nucleic acid extraction (DNA and RNA) was performed on the mantle biopsy to detect the presence of the parasite Haplospodium pinnae and the virus PnPv in order to establish its tropism. From TEM, we discovered that the PnPv infection was present in 100 % of the samples. The parasite H. pinnae was detected through PCR in 3 out of 6 animals (50 %); acid fast bacteria were detected in cytological preparations in 2 out of 6 individuals (33 %). Phylogenetic analyses of PnPv supported the novel virus being a member of the Marnaviridae family, likely descending from an algal host ancestor. This discovery provided some important information on evolutionary relationships and taxonomy of picornaviruses, increasing our understanding of the disease outcome and bringing new insight to the cytopathology and phylogeny of PnPV.