The ultrastructure of zoospores ofAphanomyces euteiches and of their encystment and subsequent germination

Summary The ultrastructure ofAphanomyces euteiches during the periods of zoospore motility, encystment, and germination has been studied. The motile spore has two heterokont flagella inserted laterally into the groove of the zoospore body where each is attached to a kinetosome. The kinetosomes and flagella are anchored into the zoospore body by rootlets comprised of two rows of microtubules with up to 12 microtubules in the outer row and are attached by fine threads to a striate fiber bundle. Secondary microtubules are attached at right angles at regular intervals along the rootlets. An unidentified body, 1.25m in diameter, containing helical fibers 16 nm in diameter is present in each zoospore. This body is situated near the two kinetosomes on the side of the pyriform nucleus opposite the contractile vacuole. The Golgi complex is between the nucleus and the contractile vacuole. The latter is surrounded by a 0.5–1.0m wide zone of Golgi proliferated vesicles. Ribosomes are generally absent from this region. Endoplasmic reticulum containing tubules within the expanded cisternae are also present. Vesicles with striated electron opaque inclusions and vesicles containing a granular cortex and center that developed in previous stages of zoosporogenesis were also present. During encystment of the zoospore the latter vesicles disappear. The two flagella are shed at this time leaving a membrane-bounded granular knob protruding from each of the kinetosome terminal plates. The contractile vacuole becomes disorganized and the zoospore assumes a spherical shape. Cyst wall deposition begins immediately and is completed in 30 minutes. The spore begins to germinate 1 hour following initiation of encystment with the appearance of a bulge in the cyst wall which elongates into a germ tube. Mitotic nuclear division follows.Research supported by the College of Agricultural and Life Sciences Station Project No. 1281.Research assistant and Professor. The advice and assistance of G. A. deZoeten, G. R.Gaard, and S.Vicen are most gratefully acknowledged.