RNA-Seq comparative study reveals molecular effectors linked to the resistance of Pinna nobilis to Haplosporidium pinnae parasite.

With the intensification of maritime traffic, recently emerged infectious diseases have become major drivers in the decline and extinction of species. Since 2016, mass mortality events have decimated the endemic Mediterranean Sea bivalve Pinna nobilis, affecting ca. 100% of individuals. These events have largely been driven by Haplosporidium pinnae's infection, an invasive species which was likely introduced by shipping. While monitoring wild populations of P. nobilis, we observed individuals that survived such a mass mortality event during the summer of 2018 (France). We considered these individuals resistant, as they did not show any symptoms of the disease, while the rest of the population in the area was devastated. Furthermore, the parasite was not detected when we conducted a PCR amplification of a species-specific fragment of the small subunit ribosomal DNA. In parallel, the transcriptomic analysis showed evidence of some parasite RNA indicating that the resistant individuals had been exposed to the parasite without proliferating. To understand the underlying mechanisms of resistance in these individuals, we compared their gene expression with that of susceptible individuals. We performed de novo transcriptome assembly and annotated the expressed genes. A comparison of the transcriptomes in resistant and susceptible individuals highlighted a gene expression signature of the resistant phenotype. We found significant differential expressions of genes involved in immunity and cell architecture. This data provides the first insights into how individuals escape the pathogenicity associated with infection.

Investigating population dynamics from parentage analysis in the highly endangered fan mussel Pinna nobilis.

Understanding dispersal patterns is a major focus for conservation biology as it influences local survival and resilience in case of local disturbance, particularly for sessile species. Dispersal can be assessed through parentage analyses by estimating family structure and self-recruitment. This study documents the family structure of a pelagic spawner, Pinna nobilis, which is facing a major crisis that threatens its survival as most of its populations have been decimated by a parasite, Haplosporidium pinnae. In this context, we focused on a single population (Peyrefite, Banyuls-sur-mer, France) where 640 individuals were sampled in 2011, 2015, and 2018 and genotyped for 22 microsatellite markers. Genetic diversity was high and homogeneous among years, with mean allele numbers ranging between 13.6 and 14.8 and observed heterozygosities (H o) between 0.7121 and 0.7331. Low, but significant, genetic differentiations were found between 2011-2015 and 2015-2018. A parentage analysis described 11 clusters, including one prevailing, and revealed that 46.9% of individuals were involved in half-sib relationships, even between years, suggesting that source populations were recurrent year after year. There were few individuals resampled between years (30 in 2015 and 14 in 2018), indicating a rapid turnover. Considering the large number of half-sib relationships but the low number of relations per individual, we conclude that P. nobilis exhibit homogeneous reproductive success. Self-recruitment was not detected, making this population highly vulnerable as replenishment only relies on connectivity from neighboring populations. In the context of the pandemic caused by H. pinnae, these results will have to be considered when choosing a location to reintroduce individuals in potential future rescue plans.

Genetic homogeneity of the critically endangered fan mussel, Pinna nobilis, throughout lagoons of the Gulf of Lion (North-Western Mediterranean Sea).

The fan mussel, Pinna nobilis, endemic to the Mediterranean Sea, is a critically endangered species facing mass mortality events in almost all of its populations, following the introduction of the parasite Haplosporidium pinnae. Such a unique pandemic in a marine organism, which spreads rapidly and with mortality rates reaching up to 100%, could lead to the potential extinction of the species. Only few regions, involving lagoon habitats, remain healthy throughout the entire Mediterranean Sea. This study describes the genetic structure of P. nobilis across the Gulf of Lion, including confined locations such as lagoons and ports. A total of 960 samples were collected among 16 sites distributed at 8 localities, and then genotyped using 22 microsatellite markers. Genetic diversity was high in all sites with mean allele numbers ranging between 10 and 14.6 and with observed heterozygosities (Ho) between 0.679 and 0.704. No genetic differentiation could be identified (FST ranging from 0.0018 to 0.0159) and the percentages of related individuals were low and similar among locations (from 1.6 to 6.5%). Consequently, all fan mussels, over the entire coastline surveyed, including those in the most geographically isolated areas, belong to a large genetically homogeneous population across the Gulf of Lion. Considering the ongoing mass mortality context, this result demonstrates that almost all of the genetic diversity of P. nobilis populations is still preserved even in isolated lagoons, which might represent a refuge habitat for the future of the species.