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The genus Saprolegniasensustricto (Dick, 1973) contains 12 ' morphospecies ' (see Scudder, 1974) separated from each other primarily on the basis of five key characters (Seymour, 1970): (i) presence or absence of antheridia; (ii) antheridial origin; (iii) presence or absence of pitting on the oogonial wall; (iv) presence of centric or subcentric oospores; and (v) oospore diameter. The weight given to each of these characters varies for the different species and this has resulted in both broadly (e.g. S. ferax) and narrowly (e.g. S. hypogyna) defined species which may be separated from one another by differences in only one character. Furthermore, some important characters are culturally variable, some isolates apparently lose the ability to form oogonia in culture, and still other isolates have never been induced to form oogonia and therefore cannot be assigned to any species using the presently accepted criteria. There are several problems associated with the accumulation of cytogenetic information for the genus so the extent to which morphological variations indicate genetic relationships is unknown (Dick, 1972 ; Dick & Win-Tin, 1973), but work by Storck & Alexopoulos ( I 970), Green & Dick ( I 972) and Clark-Walker & Gleason (1973) suggests that Saprolegnia species may have DNA base compositions that are sufficiently divergent to make this a useful character for separating isolates at the infrageneric level. In addition, the isolates examined by Green & Dick (1972) and Clark-Walker & Gleason (1973) contain a satellite DNA which might also be a useful taxonomic indicator.

Nuclear and Satellite DNA Base Composition and the Taxonomy of Saprolegnia (Oomycetes)