Divergent immune dynamics in European perch (Perca fluviatilis): Population-specific responses of peritoneal cells to Aeromonas hydrophila.

Fish populations provide valuable genetic resources for sustainable breeding programs. For species like the European perch (Perca fluviatilis), which are early in domestication, systematic characterization is key to developing strong breeding lines. This pilot study assessed the immune response to the bacterium Aeromonas hydrophila in six different perch populations. Bacterial injection triggered two distinct patterns of immune reactions. Fish originating from Finland (FIN) and Slovakia (SVK-LM and SVK-RV) exhibited a rapid immune response, with a dramatic influx of cells, including 4 million myeloid cells, into the peritoneal cavity during the first 24 h. Conversely, fish from the Czech Republic (CZE), Italy (ITA), and Poland (POL) displayed a weaker response, with only 1-2 million myeloid cells detected. By 3 days postinoculation, the number of myeloid cells in the FIN and SVK groups had declined to about 500,000, whereas levels in the POL, ITA, and CZE groups remained elevated at 1-1.5 million cells. Nanofluidic Real-Time PCR identified the genes encoding interleukin-1 beta (il1b) and immunoglobulin heavy (igh) chain as robust indicators of peritoneal immune activation in perch. Expression of il1b was rapidly induced across most populations, while igh expression increased more gradually, consistent with early B-cell-mediated responses. Gene-expression and flow-cytometry data suggested divergent immune strategies among perch from different ecological and evolutionary backgrounds, highlighting a central role for myeloid and B cells, while canonical T -cell-associated markers were not strongly induced during the early infection stage. These pilot findings demonstrate how environmental conditions shape immune responsiveness, providing a starting point for future breeding programs.