ultrastructure of the dinoflagellate polykrikos

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J. Cell Sci. 63, 245-261 (1983) 245 Printed in Great Britain © The Company of Biologists Limited 1983 ULTRASTRUCTURE OF THE DINOFLAGELLATE POLYKRIKOS I. DEVELOPMENT OF THE NEMATOCYST-TAENIOCYST COMPLEX AND MORPHOLOGY OF THE SITE FOR EXTRUSION JANE A. WESTFALL* Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506, U.SA. PHYLLIS C. BRADBURY Department of Zoology, North Carolina State University, Raleigh, North Carolina 27650, U.SA. AND J. W. TOWNSENDf Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas 66506, U.SA. SUMMARY Development of the nematocyst-taeniocyst complex in the four-zooid stage of a dinoflagellate, Polykrikos kofoidi, was studied by electron microscopy. We observed the following stages: formation of large spherical bodies in islets of cytoplasm containing extensive rough endoplasmic reticulum and Golgi complexes; differentiation of an anlage of first the nematocyst and then the taeniocyst into a tandem pair; and, maturation of the complex into a nematocyst with operculum and capsule, and a taeniocyst with head, neck and body. In the intermediate stages of dinoflagellate cnidogenesis the structurally elaborate pattern of development differed from that of coelenterate nematocysts but in certain features the mature organelles of both groups were similar. Nematocyst-taeniocyst com- plexes migrated into chutes on zooids two and four near the junction of the annulus and sulcus at the flagellar bases. The specialized chute was partially lined by thimble-shaped organelles of un- known function. The taeniocyst protruded from the surface in association with a striated fibre whose structure and position were those of a trigger to discharge the two organelles. We found no cytostome in this holozoic colony; the structure of the chute suggested that it might also function as a cyto- stome. INTRODUCTION More than a hundred years ago, Biitschli (1873) described a barrel-shaped protozoon, Polykrikos schwartzi, that contained nematocysts (organelles with projec- tile filaments). It is general knowledge that cells containing nematocysts are charac- teristic of coelenterates, but it is less widely known that three different kinds of protists, including Polykrikos, endogenously form nematocysts. The projectile filaments of the protists Myxospora and Microspora are essential for the transmission of these parasites to new hosts and cnidogenesis is only part of the complex process of forming a spore (Lorn, 1969; VaVra, I976a,b). In coelenterates •Author for correspondence. t Present address: Department of Pathology, University of Arkansas Medical Center, 4301 West Markham, Little Rock, Arkansas 72205, U.S.A.

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Page 1: ULTRASTRUCTURE OF THE DINOFLAGELLATE POLYKRIKOS

J. Cell Sci. 63, 245-261 (1983) 245Printed in Great Britain © The Company of Biologists Limited 1983

ULTRASTRUCTURE OF THE DINOFLAGELLATEPOLYKRIKOS

I. DEVELOPMENT OF THE NEMATOCYST-TAENIOCYSTCOMPLEX AND MORPHOLOGY OF THE SITE FOR EXTRUSION

JANE A. WESTFALL*Department of Anatomy and Physiology, Kansas State University, Manhattan, Kansas66506, U.SA.

PHYLLIS C. BRADBURYDepartment of Zoology, North Carolina State University, Raleigh, North Carolina27650, U.SA.

AND J. W. TOWNSENDfDepartment of Anatomy and Physiology, Kansas State University, Manhattan, Kansas66506, U.SA.

SUMMARY

Development of the nematocyst-taeniocyst complex in the four-zooid stage of a dinoflagellate,Polykrikos kofoidi, was studied by electron microscopy. We observed the following stages: formationof large spherical bodies in islets of cytoplasm containing extensive rough endoplasmic reticulumand Golgi complexes; differentiation of an anlage of first the nematocyst and then the taeniocyst intoa tandem pair; and, maturation of the complex into a nematocyst with operculum and capsule, anda taeniocyst with head, neck and body. In the intermediate stages of dinoflagellate cnidogenesis thestructurally elaborate pattern of development differed from that of coelenterate nematocysts but incertain features the mature organelles of both groups were similar. Nematocyst-taeniocyst com-plexes migrated into chutes on zooids two and four near the junction of the annulus and sulcus atthe flagellar bases. The specialized chute was partially lined by thimble-shaped organelles of un-known function. The taeniocyst protruded from the surface in association with a striated fibre whosestructure and position were those of a trigger to discharge the two organelles. We found no cytostomein this holozoic colony; the structure of the chute suggested that it might also function as a cyto-stome.

INTRODUCTION

More than a hundred years ago, Biitschli (1873) described a barrel-shapedprotozoon, Polykrikos schwartzi, that contained nematocysts (organelles with projec-tile filaments). It is general knowledge that cells containing nematocysts are charac-teristic of coelenterates, but it is less widely known that three different kinds ofprotists, including Polykrikos, endogenously form nematocysts.

The projectile filaments of the protists Myxospora and Microspora are essential forthe transmission of these parasites to new hosts and cnidogenesis is only part of thecomplex process of forming a spore (Lorn, 1969; VaVra, I976a,b). In coelenterates

•Author for correspondence.t Present address: Department of Pathology, University of Arkansas Medical Center, 4301 West

Markham, Little Rock, Arkansas 72205, U.S.A.

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246 J. A. Westfall, P. C. Bradbury andj. W. Townsend

a single specialized cell forms nematocysts, which are used in catching and subduingprey. Polykrikos generates nematocysts resembling those of coelenterates, but it doesso in single cells (zooids) that also carry on the functions of a whole animal (Faur6-Fremiet, 1913; Hovasse, 1965).

The fine structure of the developmental stages of coelenterate nematocysts andprotozoon polar capsules have been examined and compared (Westfall, 1966; Lorn,1969; Loubes & Maurand, 1976). The morphogenesis of nematocysts in Polykrikoshas only recently been studied at the ultrastructural level (Greuet & Hovasse, 1977),perhaps because it appears unpredictably in plankton samples and until recently hasnot been cultivated in the laboratory (Morey-Gaines & Ruse, 1980).

Chatton (1914) described the formation of nematocysts in Polykrikos as anautonomous cyclic process, but much later (Chatton & Hovasse, 1944) it was deter-mined that the taeniocyst was not a precursor to the nematocyst. Greuet (1972) namedthe taeniocyst and described its ultrastructure, as well as that of the nematocyst.Greuet & Hovasse (1977) reported the independent origin of the two organelles inP. schwartzi.

The present electron-microscopic study of the four-zooid stage of the dinoflagellatePolykrikos kofoidi provides the first detailed description of: (1) the development andmaturation of the nematqcyst-taeniocyst complex and (2) the morphology of a newlydiscovered extrusion site for these unique paired organelles.

MATERIALS AND METHODS

Specimens of a dinoflagellate, P. kofoidi (Chatton, 1914), were collected in plankton samples fromArgyle Lagoon at the Friday Harbor Laboratories, University of Washington. All specimens of thiscolonial dinoflagellate had four zooids as shown by Kofoid & Swezy (1921). They were fixed forapproximately 1 h in a cold solution of 2 % glutaraldehyde in 0-4M-phosphate buffer and 0-375 M-sodium chloride (pH 7-4) and post-fixed for about 45 min in a cold solution of 1 % osmium tetroxidein 0-4M-phosphate buffer and 0-75 M-sodium chloride (Dunlap, 1966). The specimens were thendehydrated in ethanol and embedded in Epon. Serial thin sections were stained with uranyl acetatefollowed by lead citrate; electron micrographs were taken with a Philips EM 301 at 80 kV.

OBSERVATIONS

We observed taeniocysts and nematocysts in various stages of development in everyorganism examined from our sample. The nematocyst anlage was the larger and moreelectron-lucent of the two organelles; the taeniocysts began development as anelectron-dense sphere apical to the nematocyst primordium, to which it was connec-ted by a vacuole shared by both organelles. We could establish a developmentalsequence for the two organelles because of their tandem arrangement. This tandemassociation remained even in the final location of the paired organelles in a specializedchannel (Fig. 1) on the ventral surface of the dinoflagellate.

Cytoplasmic characteristics and anlagen formation

There were numerous regions of rough endoplasmic reticulum and Golgi com-plexes in the cytoplasm of Polykrikos (Fig. 2). A Golgi complex with flattened saccules

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Nematocyst-taeniocyst complex

Striated fibre

Head -

247

— Taeniocyst

Anterior chamber

Stylet

CapsulePosterior chamber

Filament -

— Nematocyst

Fig, 1. Diagram of tandem arrangement of a mature nematocyst-taeniocyst complexwithin a cytoplasmic chute in P. kofoidi.

and numerous coated vesicles was often associated with a conspicuous patch of roughendoplasmic reticulum. Ribosomes attached to the membranes of the endoplasmicreticulum stained intensely, and the expanded cisternae were filled with a moderatelyelectron-dense material. At the periphery of patches of rough endoplasmic reticulumthe cisternae were swollen and lacked ribosomes.

In some individuals, one or more large ovoid bodies appeared to be earlynematocyst-taeniocyst anlagen (Fig. 3). These structures were close to the nucleus,

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248 J. A. Westfall, P. C. Bradbury andj. W. Townsend

Figs 2 and 3

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Nematocyst-taeniocyst complex 249

where light microscopists had reported that the earliest stages of nematocyst forma-tion occurred. They were conspicuous because of their relative size (approximately5-6fjin in diameter) and homogeneity in a cytoplasm packed with organelles. Asdifferentiation progressed, the anlagen became more granular and contained fibroussubstructures with associated dense materials.

With the first signs of opercular development, the anlage of the taeniocyst appearedjust anterior to the developing nematocyst (Fig. 4). The cytoplasm associated with thetwo anlagen was contained within a common membrane. At the taeniocyst end, thecytoplasm contained accumulations of ribosomes, Golgi complexes, coated vesicles,and a large vacuole filled with fibrous material from which the electron-densetaeniocyst condensed. The membrane enclosed much less cytoplasm around the sidesand posterior of the nematocyst, although this cytoplasm was dense with ribosomesand enclosed mitochondria and multivesicular bodies. An electron-lucent vacuoleabutted the posterior end of the nematocyst primordium and indented the posteriorcytoplasm.

Differentiation of nematocyst and taeniocyst

Nematocyst. Advanced stages of the nematocyst anlage (= nematogene) were en-larged and elongated. The outline of the future capsule was irregular, and beneath itsmembrane was a thin dense layer of primordial capsular wall material (Fig. 4). Theanlagen of the anterior chamber (= introvert or ampulla) and the operculum wereapproximately in their final position, and their separate elements were represented inan ordered design by substances of different densities. These substances formed adiffuse and expanded pattern, not delimited by vacuoles or membranes, but recogniz-able as components of the future operculum and anterior chamber. The opercularregion was separated from the capsular region by an infolding of the capsular wall(arrows, Figs 4,5). The fibrous matrix filling the capsule was denser at the centre andformed a clearly discernible longitudinal fibrous strand (Fig. 4) that extended fromthe posterior pole to the base of the anterior chamber and laterally to the infoldedcapsular wall.

A vacuole bridged the space between the developing nematocyst and taeniocyst andcontributed its substance to both structures. An indentation into the operculumcupped the vacuole, and within the vacuole a dense line paralleled the indentation(Fig. 4). The vacuole also indented the base of the taeniocyst; and where it bulged intothe taeniocyst, it had a narrow dense layer of material resembling the substance of thetaeniocyst. Centrally the opercular anlage had a complex pattern resembling a bell; itformed the opercular valve that occluded an opening into the anterior chamber (Fig.5). A central stylet extended from the base of the double-walled anterior chamber to

Fig. 2. Cytoplasmic region of Polykrikos rich in rough endoplasmic reticulum {rer) andGolgi complexes (g) with coated vesicles (cv). Patches of rer have enlarged peripheralcisternae lacking ribosomes (arrows). Mitochondria (m). X13 300.

Fig. 3. Presumed Golgi (g)-derivcd primordial body with a condensed substructure(arrows). X16600.

9 CEL63

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250 J. A. Westfall, P. C. Bradbury andjf. W. Townsend

Fig. 4

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Nematocyst-taeniocyst complex 251

the opercular valve at its apex. It was bounded laterally by electron-dense wings (Fig.4), which became the walls of the anterior chamber in a later stage (Fig. 5). The bandof fine filaments at the base of the anterior chamber (Fig. 4) formed a hollow internalfilament (Fig. 6) that presumably could be everted at discharge of the maturenematocyst. The outer wall of the anterior chamber was continuous with the thickdouble-walled capsule at its junction with the operculum (arrows, Fig. 5). When thefilament appeared the capsular wall was thickened; the body was greatly elongated andundulating, so that it was difficult to obtain a single longitudinal section of the entireintermediate stage (Fig. 7). The posterior end of the developing capsule wasthickened and abutted a fibrous body (= posterior cap) (Fig. 8), presumably formedfrom the posterior vacuole associated with the nematocyst primordium (Fig. 4).

Taeniocyst. The developing taeniocyst (= taeniogene) originated in a largevacuole with an irregular outline (Fig. 4). Initially it was a large electron-dense sphere(approximately 6-4/im in diameter), which condensed from granular material in thevacuole. The size and general appearance of the sphere resembled the large lipidstorage droplets in the cytoplasm, except that the sphere was slightly ovoid with strataof less dense material at either pole. The large and most electron-dense stratumoccupied two-thirds of the sphere (Figs 4, 9, 10). This homogeneous dense areabecame the body of the mature taeniocyst. The light granular bands anterior to it

Fig. 4. Early differentiation of a paired nematocyst-taeniocyst complex with a connectingvacuole (va) and cytoplasmic region (cy) common to both primordia. Note condensationof material from vacuole around taeniocyst anlage (T) and infolding of capsule wall ofnematocyst (arrows) with differentiating operculum (o), anterior chamber (a) and capsule(c). A fibrous strand (fs) extends from base of anterior chamber to base of capsule and toinfolded capsular wall. Posterior vacuole (pva). X7600.

Fig. 5. Advanced differentiation of nematocyst-taeniocyst complex with spherical anlageof taeniocyst (T), central valve in operculum (o) and stylet (s) attached to base of double-walled anterior chamber (a), connected (arrows), in turn, to capsule wall (cw). X11 200.

Fig. 6. Filament (/) attached to base of anterior chamber (a) in serial section to Fig. 5.X 11 200.

Fig. 7. Elongated capsular region of developing nematocyst with coiled filament (f) cuttransversely. X11200.

Fig. 8. Posterior end of developing nematocyst with double-walled capsule (civ) in con-tact with fibrous body (fb). Filament (/). X 11 200.

Fig. 9. Spherical anlage of taeniocyst with three major strata of different densities andperipheral condensation of granular material. X11 200.

Fig. 10. Differentiating complex with large primordial taeniocyst (T) and opercularregion (o) of nematocyst connected by a vacuole (va) in cytoplasm common to bothorganelles. X5600.

Fig. 11. Advanced differentiation of taeniocyst with head (h) and neck (n) separated fromthe body (b) by a dense ring. Note fibrous material in vacuole (va) between taeniocyst andoperculum (o) of nematocyst. Xll 200.

Fig. 12. Operculum of nematocyst with cap (ca) over valve (v), dense collar (co) andjunction to capsule (arrows). Compare with earlier stage of operculum in Fig. 11. X 20 400.

Fig. 13. Cross-section of mature nematocyst capsule with hollow internal filament (/).X15 000.

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251 J. A. Westfall, P. C. Bradbury andj. W. Townsend

Figs 5-8. For legend see p. 251.

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Nematocyst-taeniocyst complex 253

Figs 9-13. For legend see p. 251.

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254 J. A. Westfall, P. C. Bradbury andj. W. Townsend

formed the neck and head. The narrow fibrous layer at the base of the sphere wasincorporated into a posterior articulation with the common vacuole of the nematocyst.

In early stages of differentiation the sphere occupied a large vacuole filled with aflocculent material that was added to the sphere (Fig. 9). In advanced stages ofdifferentiation the taeniocyst became closely bounded by a membrane; a dense ringseparated the potential body from the future neck and head regions (Fig. 11). Thebody was homogeneous except for several faint, regularly spaced fibrils. The bridgingvacuole (= intermediate piece) between the taeniocyst and the nematocyst containedfibrous strands extending between the two organelles.

Operculum and mature capsule. At an advanced stage of taeniocyst developmentthe nematocyst was compact, with a distinct operculum (Fig. 11). The operculumbecame elaborated into a cap covering a valve surrounded by a dense peripheral wall(Fig. 12). The operculum covered the apical region of the mature nematocyst, whichwas a tough impermeable double-walled capsule approximately 15/um X 5 fim. Themature filament was coiled tightly within the greatly shortened capsule, which wasresistant to fixative and embedments and thus difficult to preserve in sectionedmaterial (Fig. 13).

Taeniocyst-nematocyst chute

Near the flagellar insertions we observed a passage, which we termed a chute,enclosing a mature nematocyst and taeniocyst (Figs 14—16). The two organelles wereseparated by a distance of 5 fim. The taeniocyst, enveloped by the outer thecalmembrane, protruded from the body and appeared as if in a position to fire, uponreceiving a proper stimulus (Figs 14, 17). The nematocyst was situated deeper in thechute.

Chute wall. At the opening of the chute the theca was interrupted except for itsouter membrane (Figs 14, 17). A striated fibre parallel to the chute had a shape andposition suggesting a trigger; it was attached to the fibrillar sheath that formed thechute wall (Fig. 15). The fibre was 0-3 fim in diameter and had a periodicity of 47 nm.Thimble-like protrusions (0-3 ^m in diameter) were embedded in the fibrillar sheathof the chute (Figs 14, 16, 18). They represented a new kind of organelle (chuteorganelles) found nowhere else in the body. In cross-section, each organelle had acentral clear disc with a dense collar (Fig. 18 inset); in longitudinal section, a mem-brane covered the lateral and apical surface of each chute organelle (Fig. 18). These

Fig. 14. Longitudinal section of mature taeniocyst with head (h), neck (n), and body (6)lying in chute (ch) above theca (t) and nematocyst (N) at base of chute. Chute organelles(•); anterior wall (aw). X5800.

Fig. 15. Longitudinal section through striated fibre (sf) on anterior wall of chute. Noteperpendicular dense fibre (arrow) at end of chute and thin double-membrane (dm) form-ing posterior wall of chute with taeniocyst (T). X17 200.

Fig. 16. Base of taeniocyst (T) within chute (ch) lined by fibrous wall with chute organ-elles (•). Note posterior fibrous skirt (arrows) associated with taeniocyst. Xl l 200.

Fig. 17. Oblique section of taeniocyst (T) in chute above annulus with transverseflagellum (tf) and sulcus with longitudinal flagellum (If). Theca (t). X5000.

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Nematocyst-taeniocyst complex 255

18 i p.Figs 14-17

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2S6 1. A. Westfall. P. C. Bradbury andjf. W. Townsend

21

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Nematocyst-taeniocyst complex 257

organelles occurred at intervals on the fibrillar sheath as well as deeper in the chute,where they were restricted to the wall nearest the annulus.

The part of the chute extending into the cytoplasm contained a fibrillar matrix,numerous chute organelles, a few multivesicular bodies and, in its centre, close-packed vacuoles (Fig. 16). The substructure of the chute and its organelles positioneda taeniocyst in readiness to fire. At the base of the chute a nematocyst was in line toenter after the taeniocyst was extruded (Fig. 14). To date, chutes have been identifiedonly in zooids 2 and 4.

Mature taeniocyst. Mature taeniocysts (approx. ll-2/im X 2-2/an) were found inprotrusions of the chute above the theca (Fig. 14). Cross-sections through a taeniocystanchored within its chute revealed differences in the chute wall at various levels (Figs19—22). Proximally, the taeniocyst emerged from an anteriorly thickened sheath wallcontaining chute organelles and microtubules (Figs 14, 18, 19). Two bands of longi-tudinally oriented microtubules were parallel to a broad striated fibre that came to apoint above the apex of the taeniocyst (Figs 18, 22). A pair of longitudinally orienteddense rods (approx. 80 nm in diameter) were parallel to the inner band ofmicrotubules (Figs 19-22). The body of the taeniocyst was filled with a lamellarstructure, which was irregularly folded centrally, and concentrically compactedperipherally (Figs 19, 20). Concentric cortical lamellae also surrounded the double-walled neck region that was continuous with the body (Figs 14, 21). The head, whichcapped the neck, resembled a stopper occluding the central opening in the neck of thetaeniocyst (Fig. 22).

DISCUSSION

The results of our ultrastructural study on the development and maturation ofthe nematocyst-taeniocyst complex in Polykrikos confirmed those of Greuet &Hovasse (1977) and Greuet (1972) in showing that there are separate origins butsynchronous development and tandem arrangement for two distinct organelles. We

Fig. 18. Longitudinal section of anterior chute wall showing microtubules (mt) and chuteorganelles (•). X42800. Inset: cross-section of chute organelle showing spherical centresurrounded by dense band. X523OO.

Fig. 19. Cross-section through body of taeniocyst (T) in chute with thickened anteriorwall containing a chute organelle (•), two bands of microtubules (mt) and the longitudinalstriated fibre (sf). X19000.

Fig. 20. Cross-section through body of taeniocyst (T) in chute at higher level than Fig.19. Note longitudinal striated fibre (sf) associated with outer row of microtubules and twosmall fibres (arrows) associated with inner row of microtubules. Body of taeniocyst con-tains membranous lamellae in medulla and cortex. X18 500.

Fig. 21. Cross-section through chute at level of neck of taeniocyst (T). Note striated fibre(sf) and microtubules (mt) in anterior wall of chute and concentric lamellae around double-wall (arrows) of taeniocyst neck. X25 100.

Fig. 22. Oblique-section through head (h) of taeniocyst (T) and vacuole (va) at tip ofchute with membranous terminal containing the striated fibre (sf) and microtubules.X17 100.

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258 Jf. A. Westfall, P. C. Bradbury andjf. W. Townsend

have characterized the morphogenesis and maturation of these organelles in detail andshown for the first time their location in a specialized chute with a putative triggerorganelle for discharge (Figs 23, 24). Mature dinoflagellate nematocysts arespecialized cell organelles, like those of coelenterates (Westfall, 1966) and myxo-sporidians (Lorn, 1969), and like them they differ from microsporidian spores (Vavra,19760,6).

Comparative cnidogenesis

Cnidogenesis in Polykrikos, like cnidogenesis in other organisms, begins with aproliferation of Golgi complexes and endoplasmic reticulum. Cnidogenesis in otherorganisms, however, is easy to recognize because it takes place in small cells whoseonly or major function is to form a nematocyst. Polykrikos is a colony of cells (zooids)that carry out the simultaneous functions of a whole animal. Light microscopistsdisagreed about the existence of Golgi complexes in Polykrikos (Chatton & Grasse,1929; Hovasse, 1951), but the electron microscope reveals that Golgi complexes arealways present and play an important role in forming the paired organelles in Poly-krikos, as in other cnidogenic species. The nematocyst and taeniocyst lie withinseparate membranes; in addition, a Golgi-derived vacuole shared by both organellessupplies each organelle with substances necessary to grow and at the same timeprovides a mechanism for articulation of the nematocyst-taeniocyst complex. The

Fig. 23. Diagram of probable sequence of formation of the nematocyst-taeniocyst com-plex in Polykrikos kofoidi. A. Primordium; B, early differentiation; c, advanced dif-ferentiation; D, mature complex.

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Nematocyst-taeniocyst complex 259

nematocyst is a larger and visibly more complex organelle than the taeniocyst and hasa much more elaborate pattern of morphogenesis. Even so, continuity is maintainedbetween the two organelles throughout their development as well as in their finallocation before they are discharged through the chute.

Cnidogenesis in coelenterates and Myxospora involves the formation of a long tubethat extends from the capsule anlage into the cytoplasm (Westfall, 1966; Lorn,1969). This external tube subsequently becomes the coiled filament within thenematocyst. The filament in Polykrikos condenses from a homogeneous matrixwithin the capsule. There is no external tube connected with its development, norare there intracapsular Golgi complexes or rough endoplasmic reticulum. In Micro-spora the coiled filament within the spore is formed with contributions from therough endoplasmic reticulum and Golgi complex (Vdvra, 1976a,6). Comparison ofcnidogenesis in the Microspora with that of other groups is complicated by the factthat the spore is a nematocyst cell and the polar filament is not separated from therest of the cell (for a review, see Loubes & Maurand, 1976). Similarities incnidogenesis in coelenterates and Myxospora has served to link the two groups andsever the Myxospora from the Microspora. Cnidogenesis in Polykrikos produces anematocyst that visibly resembles the coelenterate nematocyst but the stages of itsdifferentiation are dissimilar.

•Transverseflagellum

Longitudinalflagellum

Fig. 24. Diagram illustrating the position of the nematocyst-taeniocyst complex withina cytoplasmic chute overlying the flagellar apparatus.

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260 J. A. Westfall, P. C. Bradbury andjf. W. Townsend

Functional morphology

Little is known about the biology of Polykrikos and the function, of its nemato-cyst-taeniocyst complex. Earlier investigators always assumed that in Polykrikos thenematocyst were useful in feeding, but the discharge of nematocysts from a livingintact Polykrikos has never been reported. The discharge of nematocysts upon theirexposure to sea water when the dinoflagellate was ruptured or crushed has beendescribed by several investigators (Faur6-Fremiet, 1913; Chatton, 1914; Kofoid &Swezy, 1921). Chatton (1914) illustrated the exploded nematocyst with its filamentstill attached to the everted anterior chamber, which led Kofoid & Swezy (1921) tosuggest that eversion of the slender tube had occurred as in coelenterate nemato-cysts.

The discovery of the chute, apparently a permanent site for release of thetaeniocysts and nematocysts, still does not explain their function. Probably the chutehas been seen earlier. Hovasse (1951) reports that Chatton observed the taeniocyst ina fissure in the annulus and at first thought it was being expelled. Then he thoughtthat the fissure might be the mouth, and that the taeniocyst was helping capture food.Our observations of the taeniocyst projecting above the theca in association with a longstriated fibre suggested a trigger mechanism for discharge of the taeniocyst. Thechute, which was just below the annulus, may be the fissure that Chatton observed.The cytostome of other dinoflagellates is located in the sulcus near the origin of thetransverse flagellum; no cytostome has yet been observed in Polykrikos. Perhaps thechute serves as a cytostome after ejection of the taeniocyst—nematocyst complex. Thethin membranes of the protruding chute are the only part of the surface not underlainby the other complex structures of the theca. The chute described here, however, ismuch too small to take in the large prey organisms reported in the food vacuoles ofPolykrikos (Bovier-Lapierre, 1888; Kofoid & Swezy, 1921; Hovasse, 1951). A singlelarge food vacuole crammed with semidigested material in our specimens testified thatparticulate food was ingested somewhere on the surface of the organism. We speculatethat the taeniocyst uncoils as a ribbon that entangles the food organism and thenematocyst everts its filament to anchor the prey.

Electron microscopy has confirmed that two complex and distinct organelles areformed in intimate association, although their purposes and function remain to bediscovered. For 30 years (1914-1944) the nematocysts of Polykrikos were cited asexamples of organelles originating from cyclic autonomous replication. The partiallydifferentiated nematocyst would supposedly induce the formation of what is nowcalled a taeniocyst. After the nematocyst had matured and separated from thetaeniocyst, the taeniocyst would then mature into a nematocyst that would induce theformation of another taeniocyst. In 1944 Chatton, provided with a wealth of sectionsand whole mounts of Polykrikos by Hovasse, repudiated his earlier theory because hecould discover no unequivocal intermediate stage between a nematocyst and ataeniocyst. And now over 30 years later electron microscopy has provided the detailsof morphogenesis in two distinct organelles that come to lie in tandem in a specializedchannel for extrusion.

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This is contribution number 78-236-j from the Kansas Agricultural Experiment Station. Wethank Julia Dewey for collecting the specimens used in this study, David E. Sims for excellenttechnical assistance, Mallory Rooks Hoover for the drawings, Drs Robert D. Klemm and F. J. R.Taylor for helpful suggestions on the manuscript, and USPHS grant NS-10264 for financial supportof the electron microscope.

REFERENCES

BOVIER-LAPIERRE, E. (1888). Nouvelles observations sur le Pfiridiniens appartenant au genrePolykrikos. C. r. Seanc. Soc. Biol. 40, 579-581.

BUTSCHLI, O. (1873). Einiges iiber Infusorien. Arch, mikrosk. Anat. EntwMech. 9, 657-678.CHATTON, E. (1914). Les cnidocystes du P6ridinien Polykrikos Schwartzi Butschli. Archs Zool.

exp.gen. 54, 157-194.CHATTON, E. & GRASSE, P. P. (1929). Le chondriome, le vacuome, les vesicules osmiophiles, le

parabasal, les trichocystes et les cnidocystes du Dinoflagelle' Polykrikos Schwartzi Butschli. C. r.Seanc. Soc. Biol. 100, 281-285.

CHATTON, E. & HOVASSE, R. (1944). Sur les premiers stades de la cnidogenfese chez le P6ridinienPolykrikos Schwartzi, Leurs rapports avec les dictyosomes. C. r. hebd. Seanc. Acad. Sci., Paris218, 60-62.

DUNLAP, H. L. (1966). Oogenesis in Ctenophora. Ph.D. thesis, University of Washington, Seattle.FAUR£-FREMIET, E. (1913). Sur les ne'matocystes et les trichocystes desPolykrikos. Bull. Soc. zool.

Fr. 38, 289-290.GREUET, C. (1972). La nature trichocystaire du cnidoplaste dans le complexe cnidoplaste

n6matocyste de Polykrikos schwartzi Butschli. C. r. hebd. Seanc. Acad. Sci., Paris 275,1239-1242.

GREUET, C. &HOVASSE, R. (1977). A proposde la genese des ne'matocystes dePo/yAn'AossdiuiarlziButschli. Protistologica 13, 145-149.

HOVASSE, R. (1951). Contribution a l'e'tude de la cnidogfinese chez les P6ridiniens. I. Cnidogdnesecyclique chez Polykrikos Schwartzi Butschli. Archs Zool. exp. gen. 87, 299—334.

HOVASSE, R. (1965). Trichocystes, corps trichocystoides, cnidocystes et colloblastes. Protoplasma-tologia m / F , 1-57.

KOFOID, C. A. & SWEZY, O. (1921). The free-living unarmored dinoflagellata. Univ. Calif. Mem.5, 1-562.

LOM, J. (1969). Notes on the ultrastructure and sporoblast development in fish parasitizing myxo-sporidian of the genus Sphaervmyxa. Z. Zellforsch. mikrosk. Anat. 97, 416-437.

LOUBES, C. & MAURAND, J. (1976). Etude ultrastructurale de Pleistophora debaisieuxi Jirovec,1943 (Microsporida): son transfert dans le genre Tuzetia Maurand, Fize, Michel et Fenwick,1971 et remarques sur la structure et la genese du filament polaire. Protistologica 12, 577-591.

MOREY-GAINES, G. & RUSE, R. H. (1980). Encystment and reproduction of the predatorydinoflagellate, Polykrikos kofoidi Chatton (Gymnodiniales). Phycologia 19, 230-236.

VAVRA, J. (1976a). Structure of the microsporida. In Comparative Pathobiology, vol. 1, Biology ofthe Microsporida (ed. L. A. Bulla & T. C. Cheng), pp. 1-85. New York, London: Plenum.

VAVRA, J. (19766). Development of the microsporida. In Comparative Pathobiology, vol. 1, Biologyof the Microsporida (ed. L. A. Bulla&T. C. Cheng), pp. 87-109. New York, London: Plenum.

WESTFALL, J. A. (1966). The differentiation of nematocysts and associated structures in thecnidaria. Z. Zellforsch. mikrosk. Anat. 75, 381-403.

(Received 1 September 1982-Accepted, in revised form, 12 April 1983)

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