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The secondary cysts of the fish pathogen oomycete Saprolegnia parasitica possess bundles of long hooked hairs that are characteristic to this economically important pathogenic species. Few studies have been carried out on elucidating their specific role in the S. parasitica life cycle and the role they may have in the infection process. We show here their function by employing several strategies that focus on descriptive, developmental and predictive approaches. The strength of attachment of the secondary cysts of this pathogen was compared to other closely related species where bundles of long hooked hairs are absent. We found that the attachment of the S. parasitica cysts was around three times stronger than that of other species. The time sequence and influence of selected factors on morphology and the number of the bundles of long hooked hairs conducted by scanning electron microscopy study revealed that these are dynamic structures. They are deployed early after encystment, i.e., within 30 sec of zoospore encystment, and the length, but not the number, of the bundles steadily increased over the encystment period. We also observed that the number and length of the bundles was influenced by the type of substrate and encystment treatment applied, suggesting that these structures can adapt to different substrates (glass or fish scales) and can be modulated by different signals (i.e., protein media, 50 mM CaCl2 concentrations, carbon particles). Immunolocalization studies evidenced the presence of an adhesive extracellular matrix. The bioinformatic analyses of the S. parasitica secreted proteins showed that there is a high expression of genes encoding domains of putative proteins related to the attachment process and cell adhesion (fibronectin and thrombospondin) coinciding with the deployment stage of the bundles of long hooked hairs formation. This suggests that the bundles are structures that might contribute to the adhesion of the cysts to the host because they are composed of these adhesive proteins and/or by increasing the surface of attachment of this extracellular matrix. [ABSTRACT FROM AUTHOR]

The rapid increase in the aquaculture production of salmonids has been followed by a rise in several diseases. In particular, saprolegniosis can account for at least 10% of the annual economic loss in salmonids. In this study, we investigated the main Saprolegnia species involved in saprolegniosis of salmonids in Chile, and their association with specific developmental stages of the host fish. For this purpose, we studied 244 isolates of Saprolegnia -affected Atlantic salmon ( Salmo salar ), rainbow trout ( Oncorhynchus mykiss ), and king salmon ( Oncorhynchus tshawytscha ) from the salmon farming regions, using a recently developed identification strategy based on molecular taxonomical operational units. We found that the Saprolegnia species associated with diseased salmon were Saprolegnia australis , Saprolegnia delica , Saprolegnia diclina , Saprolegnia ferax , Saprolegnia parasitica and two new Saprolegnia species observed during this study. In order to determine whether there were any specific species associations with different stages in the fish life cycle, we applied mosaic plots and correspondence analyses for categorical data. These analyses showed a strong association of S. parasitica with samples from the adult stage of the fish ( χ 2 = 196.29, p < 0.0001), while the species S. australis , S. diclina and Saprolegnia sp. 2 were strongly associated with embryonic stages (eggs or alevins) ( χ 2 = 196.29, p < 0.0001). This work represents the first detailed molecular characterization of Saprolegnia species involved in saprolegniosis in Chile, and the first study showing specific association of different Saprolegnia species with different stages in the salmonid life cycle. [ABSTRACT FROM AUTHOR]

Some species of the genus Saprolegnia , such as Saprolegnia diclina and Saprolegnia ferax are responsible for devastating infections on salmonid eggs. Members of this group cause saprolegniasis, a disease resulting in considerable economic losses in aquaculture. Although both S. diclina and S. ferax have received much attention, the role of other Saprolegnia species in infecting fish eggs is less known. For this purpose, we have investigated the aetiology of chronic egg mortality events occurring in farmed brown trout, Salmo trutta . A total of 48 isolates were obtained from eggs with signs of infection as well as from water samples. A molecular analysis based on nrDNA internal transcribed spacer (ITS) operational taxonomic units indicated that the majority of the isolates correspond to Saprolegnia australis . All isolates of S. australis exhibited the same random amplified polymorphic DNA (RAPD) band patterns suggesting that a single strain is implicated in egg infections. The isolates followed Koch postulates using trout eggs and fry. Under standard concentrations of bronopol commonly used in farms, these isolates could grow, but not sporulate. However, both growth and sporulation were recovered when treatment was removed. This study shows that S. australis can infect and kill salmon eggs, and helps in defining oomycetes core pathogens. [ABSTRACT FROM AUTHOR]