DNA base composition and the taxonomy of the Oomycetes

Abstract

The DNA base compositions of seven members of the Saprolegniaceae were determined by cesium chloride (CsCl) gradient centrifugation. In general, base composition data correlated with morphological classification, but in several cases it indicated the necessity for revisions. In particular, Achlya inflata (eccentric group) was 42% guanine–cytosine (GC), while A. racemosa, A. sparrowii, and A. colorata (racemose group) were 51–52% GC.Two isolates of Saprolegnia diclina and one of S. parasitica were 58–59% GC and all contained 10–20% of a lighter density satellite DNA.Analysis of thermal denaturation curves by a modification of De Ley's method (J. Theoret. Biol. 22: 89 (1969)) showed that as little as 7.4% GC difference represents a large difference in DNA sequence homology. Therefore, base composition data by itself can provide a rigorous aid to the classification of the oomycetous fungi at the generic level.