Genetic differentiation and host specialization among OsHV-1 infecting two oyster species in France
Authors: Pelletier Camille , Chevignon Germain , Faury Nicole , Arzul Isabelle , Garcia Céline , Chollet Bruno , Renault Tristan , Morga Benjamin , Jacquot Maude 
Published: August 2023

Cross-species transmission is one of the most significant causes of disease emergence in humans and other species. The Ostreid Herpesvirus type 1 has mostly been detected in the Pacific oysterMagallana gigasin the context of mortality events. However, it has been detected in other mollusc species including the flat oysterOstrea eduliswhich raise questions about OsHV-1 host specialization. This study looked at the genetic differentiation of OsHV-1 in Pacific and European flat oysters, and the mechanisms that might underlie this genetic differentiation. To this end, high-throughput deep sequencing was used to analyze forty OsHV-1 genomes to characterize the genetic diversity and the degree of genetic isolation of OsHV-1 viruses from bothO. edulisandM. gigasin France as well as to assess the frequency of the virus cross-species transmission. To achieve this, comparative and population genomic, phylogenetic, and phylodynamic approaches were employed. Our findings highlight the significant influence of host species on the genetic diversity and differentiation among OsHV-1. The emergence of OsHV-1 in France seems to be traced back to the introduction ofM. gigas, and the virus has since evolved into two distinct lineages following a cross-species event. Moreover, our analysis revealed selection signals in regions responsible for crucial biological functions and domains, indicating the potential for OsHV-1 to adapt and specialize in response to its respective host species. To get further insights into evolutionary processes and host-virus interactions, future research should focus on investigating the coevolution between OsHV-1 and a wider range of host species using phylogenetic approaches.

Author Summary

In 1972, a herpes-like virus was discovered for the first time infectingCrassostrea virginicain the USA, during the course of laboratory experiments on adult oysters. Since the beginning of 1990s, a herpesvirus identified as Ostreid herpes virus type 1 (OsHV-1) has caused large-scale deaths in the Pacific oyster (M. gigas) and has been detected in other species including scallops, clams, and flat oysters. A recent study examined different lineages of OsHV-1 infecting various host species, raising questions about how often the virus is transmitted from one species to another and whether it is specialized to a particular host, affecting co-culture systems. To investigate this, advanced tools in genetics and computational analysis were used to determine the frequency and impact of cross-species transmission events between the Pacific oyster and the European flat oyster. Our findings indicate that these events are uncommon but have given rise to a distinct lineage of OsHV-1 that specifically infects the flat oyster, suggesting specialization to its host. This study offers valuable insights into the evolutionary history of this viral pathogen and its interactions with different host species.