Growth and sporulation differences of genetically distinct Saprolegnia parasitica strains in vitro

Saprolegnia species are major oomycete pathogens of freshwater fishes, causing significant economic and ecological losses in aquaculture and wild populations. Most saprolegniosis disease outbreaks have been attributed to single or very few genetic clones, but potential differences among strains and the covariation of their virulence and transmission traits have remained largely unresolved. In this study, we investigated differences in laboratory growth and sporulation among 9 pathogenic S. parasitica isolates originating from infected salmonids from 6 aquaculture facilities and 1 wild waterbody across Finland. All isolates were assigned to distinct strains using either specific qPCR or by sequencing the internal transcribed spacer, species identification was confirmed and unified by genome reassembly, and phylogenetic analysis was performed to characterise genetic relationships among the strains. Hyphal growth, time to initiate spore production, spore numbers, and count of repeated zoospore emergence cycles were measured, and potential phenotypic associations among these functional measures were assessed. The strains formed 3 distinct clades with 7 out of the 9 strains clustering into clade I, whereas clades II and III were singletons. Phylogenetic clade, timepoint, and their interaction all significantly affected hyphal growth, indicating timepoint-dependent clade differences. Strains within clade I were faster to initiate sporulation and produced more spores than the singleton clades II and III. Associations between hyphal growth and sporulation metrics did not support trade-offs between transmission- and virulence-related traits; strains with fastest growth and sporulation produced the most spores.