Reproduction by zoospores inOedogonium

Summary

Zoosporogenesis in two species ofOedogonium is described. The earliest sign of incipient differentiation is the appearance of a small diffuse mass situated in a basal invagination of the nuclear envelope. From this, centrioles appear which soon rapidly multiply, forming two adjacent rows close to and running around the nucleus. Concurrently, ciliary “rootlet templates”, three short dense tubular elements, appear between all centrioles. The nucleus soon becomes distorted forming a pronounced ridge upon which lie the rows of centrioles. Then the rows, accompanied by the nucleus, move to the lateral cell wall; ahead of them the peripheral chloroplast is cleaved by microtubules. The center of the rows now separates to establish the circular configuration of the future basal bodies, in the process clearing the chloroplast from this region of the wall; the nuclear ridge simultaneously bifurcates so that the nucleus always remains closely associated with the centrioles. Nuclear distortion and expansion of the ring probably involve nearby proliferating microtubules. Then the centrioles begin extruding flagella (i.e., becoming basal bodies) and the rootlet templates concurrently assemble their characteristic rootlet microtubular systems which run around the cell's periphery. Meanwhile, two very complex systems of striated fibres are being formed. One, the “Fibrous Ring”, interconnects the basal bodies. The other is a set of “Striated Fibres”, each lying close to one set of the rootlet microtubules, connecting them to dense caps of amorphous material that have formed over some specific triplet tubules of the basal bodies. The nucleus by now has withdrawn back into the cell. The cytoplasm it occupied near the flagellar apparatus fills with smaller organelles, especially endoplasmic reticulum and differentiated golgi bodies; this region will form the refractile “Dome” of the zoospore.

Early in zoosporogenesis, the cell starts secreting inside its wall two highly characteristic and important materials containing polysaccharide detected by PAS and PA-Silver-Hexamine staining. The first, the “Hyaline Layer”, is deposited quite evenly around the apical portion of the protoplast and it will become the well-known expanding vesicle that encloses the emerging zoospore. Secretion of the second, the “Basal Mucilage”, causes the basal contraction of the protoplast characteristic of zoosporogenesis; this material too is probably important in effecting zoospore release. Both these different secretions are derived concurrently (in part) from vesicles of two apparently differentiated populations of Golgi bodies. In one species, elements of endoplasmic reticulum are clearly continuous with the plasmalemma specifically where and when the hyaline layer is being formed, but this could be artefactual.

The Golgi also show evidence of other functions as their vesicles vary greatly and consistently in size and content during certain stages of development. During one phase of activity they also probably form the dense granules that collect at the Dome's surface. During another phase in one species, up to four large cisternae of smooth endoplasmic reticulum were invariably seen applied simultaneously to the forming face of all golgi bodies. Small nascent contractile vacuoles appear just before zoospore release.