Esterase isoenzyme variation in the genus Saprolegnia, with particular reference to the fish-pathogenic S. diclina-parasitica complex

Esterase isoenzyme patterns determined by slab gel electrophoresis were compared for nearly 60 Saprolegnia isolates, particularly from the S. diclina-parasitica complex. Consistent differences were found between S. diclina and S. parasitica isolates, with the latter being characterized by having between one and five very fast moving esterase bands that were generally absent from the former. A range of asexual (unidentifiable) isolates, taken from the vicinity of fish hatcheries or from fish lesions, were also examined and their esterase isoenzymes shown to be similar to those of S. parasitica, although usually showing fewer bands. The use of esterase isoenzymes for screening potential fish pathogenic isolates of Saprolegnia is briefly compared with results obtained using oogonium morphology or cyst ornamentation, and the relative merits of each method are discussed. It is proposed that, on the basis of their distinctive cyst coat ornamentation (bundles of long 'boathooks') and esterase isoenzymes, fish lesion isolates can be distinguished from saprophytic isolates, and that analysis of isoenzymes should provide a useful method for the screening of potential pathogenic isolates.