Hemocytes released in seawater act as Trojan horses for spreading of bacterial infections in mussels

Abstract

Global warming has been associated with increased episodes of mass mortality events in invertebrates, most notably in bivalves. Although the spread of pathogens is one of multiple factors that contribute to such mass mortality events, we don’t fully understand the pathophysiological consequences of sea warming on invertebrates. In this work, we show that in temperature stress conditions, circulating hemocytes in mussels leave the hemolymph to gain access to the intervalvar fluid before being released in seawater. External hemocytes can survive for several hours in seawater before entering other mussels. When infected by bacteria, externally-infected hemocytes can enter naive mussels and promote bacterial dissemination in the host. These results reveal the existence of a new opportunistic mechanism used by pathogens to disseminate in marine ecosystems. Such mechanisms may explain how thermal anomalies triggered by global warming can favor episodic mass mortality observed in recent years in marine ecosystem.

First description of a widespread Mytilus trossulus-derived bivalve transmissible cancer lineage in M. trossulus itself

“…Noteworthy, the mussels in the Gaydamak Bay were collected from the surface of a mooring buoy in a heavily polluted area, where no mussels were recorded at the sea floor (our observations). Since BTN is presumably transmitted by cancer cells through the water column (see Caza et al 72 for more discussion), we suspect that the mussels fouling the buoy contracted the infection from those that had fouled ships moored to it. We point out that mussel populations on mooring buoys, docks etc.…”

Traits of a mussel transmissible cancer are reminiscent of a parasitic life style

“…Indeed, there is a permanent cell exchange between hemolymph and intravalvular fluids in bivalves. Recently, Caza et al 43 showed that under temperature stress conditions, circulating hemocytes were able to leave the hemolymph to gain access to the intervalvular fluid before entering other mussels. Therefore, hemolymph is a likely route for the leakage of cancerous cells outside the infected host.…”

Group phenotypic composition in cancer

“…Although empirical evidence is currently lacking to support the hypotheses outlined above, future transfection experiments could prove or disprove these theories. For example, mussels have a natural tendency to open their valves and to release and uptake hemocytes ( Caza et al, 2020 ). Placing mussels under different environmental conditions and exposing them to stained BTN cells (e.g., CFSE, a cytosol dye commonly used for tracking cells in vivo by flow cytometry), followed by transcriptome and epitome analyses of both established BTN and host cells may reveal the phenotypic composition of BTN cells that were successfully taken up by and survived in the new host ( Caza et al, 2020 ).…”