Crayfish plague dilemma: how to be a courteous killer?

Aphanomyces astaci is among the most serious pathogens affecting European aquatic ecosystems. We demonstrate that both virulence of A. astaci isolates and resistance of native European crayfish stocks vary notably. Some native European crayfish stocks latently carry crayfish plague, indicating adaptation and contemporary co-evolution between host and pathogen. The earliest introduced A. astaci genotypes have adapted to novel, susceptible native European crayfishes, likely under an evolutionary pressure to maintain a necessary host population as an essential habitat. Then, highly virulent genotypes that were introduced together with their original American hosts, have more resistant host populations present in Europe. This creates a dilemma for A. astaci: whether to increase virulence to better utilize invasive American hosts or to reduce virulence to better utilize the native European hosts. All A. astaci genotypes are potent killers, but they already show lowered virulence similarly to previous examples of virulence evolution in novel pathogens.