Integrated study of biology and distribution on red-mark syndrome-Midichloria-like organism in rainbow trout farm.

Introduction Saprolegniosis is a disease caused by oomycetes of the genus Saprolegnia. During the recent years, saprolegniosis outbreaks have resulted in mass-mortalities of salmonids reared in aquaculture facilities. These outbreaks significantly threaten both the maintenance of hatchery broodstocks of endangered salmonids and production of salmonids for human consumption. A single species, Saprolegnia parasitica, is most identified in saprolegniosis outbreaks of fish but even with the substantial risks associated with S. parasitica, the genetic variation and the differences in virulence of different S. parasitica strains remain poorly studied. S. parasitica can cause massive economic damage to aquaculture industry and threaten food security, in addition to causing mass-mortalities in both wild salmonids and endangered broodstocks kept in aquaculture facilities. Even geographically closely related aquaculture facilities can differ in the severity of the annual saprolegniosis outbreaks. It is highly important to investigate whether genetically distinct S. parasitica strains have evolved in different aquaculture facilitites and if these strains show differences in their virulence factors. Methods Nine Saprolegnia parasitica strains, and one Saprolegnia diclina strain, were provided by the Finnish Food Authority. These strains have been collected from aquaculture facilities from different geographical locations in Finland and extracted from multiple different fish host species (Atlantic salmon, Saimaa landlocked salmon, brown trout, rainbow trout). The strains were cultivated in laboratory on PG1 agar in room temperature; five replicates were made of each strain. Circular punch of 5,1mm in diameter was cut from the advancing edge of the hyphal growth and transferred to a new PG1 agar plate. The radial growth of the hyphae was measured every 24 hours for total of three times. Furthermore, a sporulation experiment will be conducted with the same strains, to test for differences in sporulation and potential virulence factor trade-offs between sporulation and hyphal growth. The differences in radial growth were tested, and the differences in sporulation will be tested using linear mixed effect models in R. Results The strains showed significant differences in their radial hyphal growth. Three of the nine strains grew worse than the mean of all the strains, and two of the strains grew better than the mean of all the strains. Conclusions The significant differences in hyphal growth between the Saprolegnia parasitica strains imply high potential for differences in sporulation, i.e., in virulence factors of the pathogen. Further, these differences imply differentiation between strains from geographically distinct locations, suggesting potential for local adaptation and evolution.