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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Cavostelium

  1. Cavostelium apophysatum Olive, 1964 (ref. ID; 3808, 4116)

Cavostelium apophysatum Olive, 1964 (ref. ID; 3808, 4116)

Descriptions

Cavostelium apophysatum Olive, 1964 is an amoebo-flagellate protostelid that forms minute (8-20 um high), single-spored fruiting bodies. The life cycle of C. apophysatum is simple. The trophic phase consists of flagellate cells and uninucleate amoebae, the latter giving rise to the fruiting bodies. At the onset of fruiting each amoeba withdraws its pseudopodia and assumes a rounded shape. At this point a protective sheath is deposited over the protoplast. As sporogenesis proceeds, the protoplast -called the sporogen- raised above the substrate, at first in a more or less hemispheric configuration, but later a distinct basal constriction develops. This constriction delimits the sporogen into an incipient stalk zone and an incipient spore zone. Eventually a stalk and spore wall are secreted. Transmission electron micrographs of early developmental stages reveal the protoplast to be divided into two distinct regions: a basal hyaloplasmic zone delimiting the stalk proper, and an upper zone delimiting the spore proper. This cytoplasmic differentiation arises through the contraction of the hyaloplasmic zone at the base of the young sporogen. The basal constriction of the protoplast forces the bulk of the cytoplasm up, away from the agar surface. Since the lower portion of the sheath is formed prior to this horizontal contraction, a distinct space forms between the sheath and the hyaloplasmic pre-stalk zone. This is manifested as a distinct gas space surrounding the stalk, especially conspicuous in aqueous amounts of fruiting bodies, but also apparent in sporocarps prepared for transmission electron microscopy. Stalk material is deposited during this horizontal contraction, initially in a wide, thin sheet. As the diameter of the hyaloplasmic zone narrows, however, the area of deposition decreases correspondingly. Stalk deposition appears to occur via exocytosis of stalk material directly from the hyaloplasmic zone. Sites of probable exocytotic activity are common in the stalk portion of the sporogen; the accumulation of stalk material beneath the hyaloplasmic zone coincides with the movement of this cytoplasm toward the spore proper. During this upward migration, stalk material continues to be deposited, forming a stout, cylindrical stalk composed of a homogeneous mix of fibrillar and amorphous material. The final step in stalk deposition is the formation of the apophysis. The last bit of cytoplasm remaining in the stalk region deposits stalk material around itself, and the result is a hollow, cup-like structure at the stalk apex. When the stalk is completed, the immature fruiting body consists of a sheath-covered protoplast perched on a rather prominent knob of cytoplasm lodged within the cup-like apophysis. At this point a narrow constriction ring begins to form between the base of the spore and the top of the apophysis. This constriction is apparently the result of the contraction of actin microfilaments included in the hyaloplasmic basal portion of the sporogen. Numerous organelles (i.e. mitochondria, endoplasmic reticulum) are conspicuous the section closet to the incipient spore while in the section closest to the apophysis the cytoplasm is decidedly microfilamentous. Note the infoldings that mark this region, indicating the extent of contraction between spore and apophysis. These contractions do not completely cleave the cytoplasm into two portions, however. A narrow, thin-walled tube still connects apophysis and spore, apparently providing an escape route for the last bit of cytoplasm in the apophysis. During sporocarp formation the protoplast of C. apophysatum contains conspicuous endomembrane system components. Profiles of dictyosomes and rough endoplasmic reticulum are apparent in the sporogen thought the developmental process, and these are presumably the source of stalk and spore wall material. The formation and behavior of the sheath in C. apophysatum has not been examined in any great detail. Apparently it is formed in its entirely at the onset of sporogenesis and surrounds the sporocarp throughout its development. The spore wall is deposited within the sheath as stalk formation is completed, the sheath resting on the tips of the spore wall ornamentation. The sheath eventually separated near the base of the spore, either late in development or at maturity. The sheath then falls back on the substrate, persisting as a membranous disk. The mature fruiting body of C. apophysatum consists of a single spore borne atop a stout, cylindrical stalk. The stalk is composed of fibrillar and amorphous deposits and is topped by a hollow, cup-like apophysis which articulates with the base of the spore. The spore wall consists of a single electron-dense layer, studded with electron-dense projections of two types: follow conical projections and smaller, solid projections. The cytoplasm of mature spores contains a typical complement of eukaryotic organelles: a nucleus with a single central nucleolus, randomly scattered mitochondria with tubular cristae, autophagic vacuoles, and inconspicuous profiles of other endomembrane system components. The process of sporocarp development in C. apophysatum is present diagrammatically. (ref. ID; 4116)

Comments

The method of sporocarp formation in C. apophysatum is quite different from those reported for other protostelids. To date, the most detailed studies of fruiting body development in the protostelids have been performed on Planoprotostelium aurantium Olive & Stoianovitch, 1971. In this species, a long, tubular stalk composed of longitudinally oriented fibrils is formed. The stalk fibrils are apparently polymerized outside the protoplast, adjacent to the hyaloplasmic contractile region often called the steliogen. A number of similarities and differences are apparent in the mechanisms of sporocarp development in C. apophysatum and P. aurantium. The internal organization of the sporogen is similar in both species. An apical, globose spore proper is subtended by a hyaloplasmic, contractile zone, and in these taxa, this hyaloplasmic region is involved in stalk formation. In P. aurantium the contraction of the steliogen is thought to orient the externally-polymerized stalk fibrils longitudinally, ultimately producing the narrow, tubular stalk. In C. apophysatum, however, the basal hyaloplasmic zone seems to play a more direct role in the formation of the stalk. After its initial contraction the hyaloplasmic zone begins deposition of stalk material, with drawing towards the spore proper as the stalk grows in height. When it has achieved its full height, the sporogen of P. aurantium is cradled in a reticulum of randomly oriented stalk fibrils. At this point, the unornamented spore wall is deposited via exocytosis of wall material. The mature sporangium consists of a single uninucleate spore atop a narrow, tubular stalk formed primarily of longitudinally oriented fibrils, occasionally with protoplasmic inclusions in the stalk lumen. In contrast, the final stages of stalk deposition in C. apophysatum involve the formation of a hollow, cup-like apophysis at the tip of the stalk. A second horizontal contraction forms an infolding between the top of the apophysis and the base of the spore. A narrow tube remains between stalk and spore, through which the hyaloplasm remaining in the apophysis migrates into the spore proper. The stalk of C. apophysatum (not including apophysis) is solid, containing no central lumen or cavity, and the stalk fibrils are more or less randomly oriented. Preliminary studies indicate that the mechanism of sporocarp formation in Schizoplasmodiopsis amoeboidea Olive & Whitney, 1982 also differs from that of C. apophysatum. The early stages of sporocarp formation, though, are similar in both species. Both have an initial hemispheric phase that develops a basal constriction, dividing the protoplast into stalk and spore zone. Instead of, or in addition to, depositing stalk material directly, however, the hyaloplasm in the stalk zone of S. amoeboidea appears to serve as a template around which stalk material is assembled. Once this tubular stalk is secreted, the cytoplasm divides into two separate parts: the upper portion forms the spore, and the lower portion remains behind, nearly filling the lumen of the stalk tube. Olive (1964) described an unusual aspect of C. apophysatum fruiting bodies; as the fruiting bodies aged, the stalks tended to elongate. Although Olive proposed the renewed sporogenic activities by the spore protoplast might be responsible, the behavior of the sheath and ultrastructural aspects of the stalk offer an explanation for post-sporogenic stalk elongation. It seems likely that the initial height achieved by C. apophysatum fruiting bodies is somewhat limited by the developing sheath. Once the fruiting body is complete, however, the sheath ruptures. With the sheath out of the way, stalk elongation can occur, apparently a drying phenomenon. Dehydration of stalk material is known to provide post-sorogenic lift in the sorogenic ciliate Sorogena stoianovitchae Bradbury & Olive, 1980, and a similar process probably occurs in the stalks of C. apophysatum. Just completed stalks of C. apophysatum are short and stout in contrast to those of older fruiting bodies, which are taller and more slender. As the fruiting bodies age and become drier, the stalk material is drawn inward, and the net effect is to push the spore somewhat higher. Thus, in C. apophysatum, the combination of the loss of the restrictive sheath and the dehydration and elongation of the stalk appear to provide some additional fruiting body height. (ref. ID; 4116)