Identification and genetic characterization of yeasts isolated from freshwater prawns, Macrobrachium rosenbergii de Man, in Taiwan

Since 1987, farmers in southern Taiwan have reported significant disease-related mortalities in freshwater prawns. Most mortalities have occurred during the winter, and usually almost all the adult prawns die within 4-5 days after clinical signs appear. Histopathological studies show that cells in hepatopancreatic ducts and tubules are vacuolized and degenerated. Large numbers of membrane-bound yeast aggregates are observed in the affected tissues. Two hundred and seventeen yeast isolates were obtained from diseased prawns, pond water and sediments from six areas. DNA restriction fragment length polymorphism (RFLP) was used to analyse the yeast genomes and to categorize them into five groups. Conventional biochemical and metabolic methods were then used to identify the yeasts of each group. The results showed that two strains of Candida sake (I and II), Pichia anomala, Endomyces fibuliger and Candida famata were present. In addition, the RAPD (random amplified polymorphic DNA) method was used to determine their genomic similarities. Two strains (I and II) of C. sake were the most similar (72%). C. sake I appears to be the primary causative agent of disease, based on frequencies of occurrence of the yeasts found in the diseased prawns.