The signal crayfish (Pacifastacus leniusculus) in Lake Tahoe (USA) hosts multiple Aphanomyces species

Abstract The genus Aphanomyces( Oomycetes) comprises approximately 50 known species of water moldsin three lineages. One of the most notorious is Aphanomycesastaci , the causative agent of crayfish plague. In this study, fresh isolates of Aphanomycesastaci were collected from 20 live specimens of the signal crayfish Pacifastacusleniusculus (Dana, 1852) from Lake Tahoe, California, providing 35 axenic culturesof A. astaci as well as two apparently undescribed Aphanomycesspp. isolates. Based on the results of ITS-, chitinase-, mitochondrial rnnS- and rnnL-sequences and microsatellite markers combined, the Lake Tahoe A. astaci isolates were identical to isolates of A. astaci B- haplogroupcommonly detected in Europe, and infection experiments confirmed their high virulence towards noble crayfish. One of the two undescribed Aphanomycesspp. isolates was highly similar to an Aphanomyceslineage detected previously in crustacean zooplankton ( Daphnia) in Central Europe, while the other was distinct and most closely related (ITS sequence similarity of 93%) to either A. astaci or to Aphanomycesfennicus isolated recently from Astacus astacusin Finland. Neither of the two Aphanomycesspp. isolates caused crayfishmortality under experimental conditions. Our results indicate that the populations of North American signal crayfishcan act as carriers of both pathogenic and non-pathogenic Aphanomycesat the same time. Furthermore, considering that a limited number of crayfishindividuals from a single location yielded multiple distinct Aphanomycesisolates, our results suggest that substantial species diversity within this genus remains undescribed.