A thicker chorion gives ova of Atlantic salmon (Salmo salar L.) the upper hand against Saprolegnia infections

Abstract

Since the ban of malachite green in the fish farming industry, finding alternative ways of controlling Saprolegnia infections has become of utmost importance. Much effort has been made to elucidate the mechanisms by which Saprolegnia invades fish eggs. Little is known about the defence mechanisms of the hosts, making some eggs more prone to infection than others. One clue might lie in the composition of the eggs. As the immune system in the embryos is not developed yet, the difference in infection levels could be explained by factors influenced by the mother herself, by either transferring passive immunity, influencing the physical aspects of the eggs or both. One of the physical aspects that could be influenced by the female is the chorion, the extracellular coat surrounding the fish egg, which is in fact the first major barrier to be overcome by Saprolegnia spp. Our results suggest that a thicker chorion in eggs from Atlantic salmon gives a better protection against Saprolegnia spp. In addition to the identification of differences in sensitivity of eggs in a fish farm set-up, we were able to confirm these results in a laboratory-controlled challenge experiment.

Galleria melonella as an experimental in vivo host model for the fish-pathogenic oomycete Saprolegnia parasitica

“…Moreover, it is difficult to replicate the complex conditions under which infections naturally occur ( Kales et al., 2007 , Minor et al., 2014 ). Due to the economic impact of S. parasitica , Saprolegnia diclina and other Saprolegnia species in hatcheries, eggs are also frequently used for challenge experiments ( Songe et al., 2016 , Eissa et al., 2013 ). However, these studies aim more at replicating conditions of outbreaks on eggs specifically than investigating the host–pathogen interaction and insights of the defence system gained from egg experiments cannot simply be transferred to fish.…”

Development of a 3D spheroid cell culture system from fish cell lines for in vitro infection studies: Evaluation with Saprolegnia parasitica

“…Several systems to study different Saprolegnia –host interfaces have been used previously. These have ranged from cell monolayers using established cell lines, that have proven to be a useful tool to study immune responses, Galleria mellonella as an alternative model host to study virulence and innate immune responses, to fish eggs, which are frequently used for challenge experiments (Belmonte et al., 2014; de Bruijn et al., 2012; Eissa et al., 2013; Songe et al., 2016; Wuensch et al., 2018). To complement these tools, we have developed a 3D cell culture system, to study chemotactic and infection behaviour at the cellular level under controlled conditions.…”

Chemical degumming increases larvae size and facilitates the commercial production of Lumpfish (Cyclopterus lumpus) eggs

“…Although a thick egg chorion can make it more difficult for water moulds, such as Saprolegnia, to penetrate the eggs (Songe et al, 2016), it can also favour settlement of water borne particulates that facilitate fungal and bacterial infection and can make eggs more vulnerable. Indeed, an adhesive chorion is thought to be particularly susceptible to viral, fungal and bacterial infection, and one of the benefits of achieving a thinner chorion through chemical degumming may be to reduce pathogen loads and facilitate egg prophylaxis under aquaculture conditions (Krise et al, 1986).…”

“…They were then photographed using a Leica DFC290 digital camera (Leica Microsystems Ltd, UK) mounted to a Nikon SMZ 800 Zoom Stereomicroscope at X20 magnification. Mean chorion thickness (±10 μm) was obtained from photomicrographs of six eggs per replicate (n=18 per treatment) at 2-4 equidistant points of the egg, as described in Songe et al (2016).…”