Morphogenesis and the role of microtubules in synchronous populations of Saprolegnia zoospores

Abstract

Synchronous populations of secondary zoospores of an isolate of the Oomycete Saprolegnia (probably S. ferax ) were obtained with a temperature-shock treatment. Light microscopy and serial section ultrastructural analysis of these, and the primary zoospores, provided a detailed description of their microtubular systems and the changes in these systems during the morphogenic processes which accompanied secondary zoospore excystment and encystment. In both zoospore types there was a correlation between the arrangement of the cytoplasmic microtubule systems and zoospore morphology, but the asymmetrical arrangement of microtubules in the symmetrical secondary zoospores showed that morphogenesis is complex. Because there was a close temporal correlation between the presence of maturity in both cytoplasmic microtubule arrangement and secondary zoospore shape, the microtubules are probably the major morphogenic agents. There were two locationally and morphologically distinct sets of microtubules in the primary zoospores and seven sets in the secondary zoospores. Because the sets were synthesized and broken down at different times the spores must possess a complex system for control of microtubule polymerization; a simple monomer-polymer-common pool of subunits system is inadequate. Flagella were synthesized at rates up to 2 μ m/min. Their breakdown in primary zoospores and cysts involved loss of functional interdoublet and central pair links followed by fragmentation prior to ultimate disappearance.