Formation of K₂-Bodies in Primary Cysts of Saprolegnia Ferax

ABSTRACT

Secondary zoospores of many Oomycetes contain kinetosome-associated organelles (K₂-bodies), single-membrane bounded organelles with distinctive matrices composed of multiple zones. Because ontogeny of K₂-bodies can provide insights into their functions and their validity as a character in systematic analyses, stages in formation were reconstructed ultrastructurally in synchronously developing Saprolegnia ferax. Within five minutes after induction of primary zoospores to encyst, precursor K₂-body vesicles with coated surfaces formed from smooth-surfaced, fenestrated cisternae near the centrally located kinetosomes. A precursor K₂-body vesicle contained three morphologically distinct components: a loosely fibrillar matrix (A1), an electron-dense globule (A2), and plate-like fragments (A3). Later, uncoated precursor K₂-body vesicles were found in the peripheral cytoplasm of primary cysts where they apparently fused with each other and enlarged into nascent K₂-bodies. Zones composing the matrix of nascent K₂-bodies resulted from concentration and morphological rearrangement of material contributed by precursor K₂-vesicles. As the structure of the matrix changed, coated patches were scattered on the cytoplasmic surface of the nascent K₂-body. Successively, two additional morphologically distinct areas appeared within the matrix and were apparently derived from transformation of material in the A1 area. Tubules filled a hemispherical cavity (A4) at one side of the organelle, and coarsely fibrillar material (A5) occupied the central region. Sometimes a fibrous, crystal-like structure with a regular periodicity was also in this central region. Finally, K₂-bodies were again found at the center of the cell near the kinetosomes, nucleus and microtubular rootlets, an association which is maintained in motile secondary zoospores. Microtubules were located along the path where precursor K₂-body vesicles migrated to the cell's cytoplasmic periphery and nascent K₂-bodies returned to the cell's center. Thus, K₂-bodies are assembled by the fusion of endomembrane-derived vesicles, rather than being fully formed while still continuous with cisternae of the endoplasmic reticulum or Golgi apparatus.

Key Words:

• Saprolegnia ferax
• Oomycetes
• zoospore
• ultrastructure
• K₂-bodies
• kinetosome-associated organelle