| Sumario: | In this study, we addressed the morphological changes of eyed eggs of Atlantic salmon, Salmo
salar L. infected with Saprolegnia from a commercial hatchery and after experimental infection.
Eyed eggs infected with Saprolegnia spp. from 10 Atlantic salmon females were obtained. Egg
pathology was investigated by light and scanning electron microscopy. Eggs from six of ten
females were infected with S. parasitica, and two females had infections with S. diclina clade
IIIA; two Saprolegnia isolates remained unidentified. Light microscopy showed that S. diclina
infection resulted in the chorion in some areas being completely destroyed, whereas eggs
infected with S. parasitica had an apparently intact chorion with hyphae growing within or
beneath the chorion. The same contrasting pathology was found in experimentally infected eggs.
Scanning electron microscopy revealed that S. parasitica grew on the egg surface and hyphae
were found penetrating the chorion of the egg, and re-emerging on the surface away from the
infection site. The two Saprolegnia species employ different infection strategies when colonizing
salmon eggs. Saprolegnia diclina infection results in chorion destruction, while S. parasitica
penetrates intact chorion. We discuss the possibility that these infection mechanisms represent a
necrotrophic (S. diclina) versus a facultative biotrophic strategy (S. parasitica).
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