Saprolegnia species in Norwegian salmon hatcheries: field survey identifies S-diclina sub-clade IIIB as the dominating taxon

Saprolegnia isolates within the recognized clades encompassing the taxa S. parasitica and S. diclina act as opportunist and aggressive pathogens to both fish and their eggs. They are responsible for significant economic losses in aquaculture, particularly in salmonid hatcheries. However, the identity, distribution and pathogenic significance of involved species often remain unexplored. In this study, 89 Saprolegnia isolates were recovered from water, eggs and salmon tissue samples that originated from salmon (Salmo salar) hatcheries along the coast of Norway. The cultures were characterized morphologically and molecularly in order to provide an overview of the species composition of Saprolegnia spp. present in Norwegian salmon hatcheries. We demonstrate that S. diclina clearly dominated and contributed to 79% of the recovered isolates. Parsimony analyses of the nuclear ribosomal internal transcribed spacer (ITS) region split these isolates into 2 strongly supported sub-clades, S. diclina sub-clade IIIA and IIIB, where sub-clade IIIB accounted for 66% of all isolates. A minor portion of the isolates constituted other taxa that were either conspecific or showed strong affinity to S. parasitica, S. ferax, S. hypogyna and Scoliolegnia asterophora. The unique sub-clade IIIB of S. diclina was most prevalent in water and salmon eggs, while S. parasitica isolates were more frequently isolated from post hatching stages. The study demonstrated that morphological criteria in many cases were insufficient for species delimitation due to lack of sexual structures or incoherent morphological expression of such features within the tested replicates.