Sequence analysis and typing of Saprolegnia strains isolated from freshwater fish from Southern Chinese regions

Saprolegniasis, caused by Saprolegnia infection, is one of the most common diseases in freshwater fish. Our study aimed to determine the epidemiological characteristics of saprolegniasis in Chinese regions of high incidence. Saprolegnia were isolated and identified by morphological and molecular methods targeting the internal transcribed spacer (ITS) ribosomal DNA (rDNA) and building neighbor-joining (NJ) and maximum parsimony (MP) phylogenetic trees. The ITS sequences of eight isolated strains were compared with GenBank sequences and all strains fell into three clades: CLADE1 (02, LP, 04 and 14), CLADE2 (S1), and CLADE3 (CP, S2, L5 and the reference ATCC200013). Isolates 02 and LP shared 80% sequence similarity with S. diclina, S. longicaulis, S. ferax, S. mixta, and S. anomalies. Further, isolates 04 and 14 shared 80% similarity with S. bulbosa and S. oliviae. Finally, extremely high ITS sequence similarities were identified between isolates S1 and S. australis (100%); CP and S. hypogyna (96%); and S2, L5, ATCC200013 and S. salmonis (98%). This research provides insights into the identification, prevention and control of saprolegniasis pathogens and the potential development of effective drugs.