Zoospore Fine Structure of the Parasite Olpidiopsis Saprolegniae Variety Saprolegniae (Oomycetes, Lagenidiales)

ABSTRACT

Zoospore fine structure of Olpidiopsis saprolegniae var. saprolegniae was analyzed with light and transmission electron microscopy and was compared with that of other Oomycetes studied previously. For more reliable basis of comparisons, the per cent area of organelles in zoospores of O. saprolegniae var. saprolegniae was measured morphometrically. Zoospores were of the secondary type and bore two flagella subapically inserted into a groove. Although similar to secondary zoospores of other Oomycetes in overall morphology, their relatively small size corresponded with a reduced complexity of some organelles, such as a water expulsion vacuole system with a single dictyosome, spherical mitochondria, and scant, rough endoplasmic reticulum. These zoospores were also distinct in organellar associations and matrix structures. The kinetosome associated organelle (K-body) was unique because its matrix contained hollow tubules extending from a curved plate as a cone. A granular coat covered the outer surface of the K-body membrane adjacent to the plate. Distinct from the flagellar apparatus of other oomycetous zoospores, one of the four rootlets was composed of five microtubules and the transition region of the axoneme contained stacks of bars separated by space rather than a “concertina body.” Because this structure resembles the transitional helix of chromophytous algae, zoospore fine structure of O. saprolegniae var. saprolegniae supports the phylogenetic link between Oomycetes and algae. Structural differences between genera suggest K-body and flagellar apparatus are characters that may be useful in the taxonomy of Oomycetes and in the elucidation of phylogenetic relationships.

Key Words:

• ultrastructure
• zoospore
• Olpidiopsis
• systematics