Pseudopedinella

• Flagellar movement. There is some disagreement in the literature concerning the movement of the flagellum in Pseudopedinella and Pedinella. Skuja (1948) apparently could not decide whether the movement was wavy or spiral, and drew far too short waves in a rapidly moving cell. Schiller (1952) was decidedly of the opinion that flagellar movement in his species was spiral: "sie zeigt nie peitschenartige, sondern stets spiralige schraubenartige, schnelle Bewegungen". Maybe influenced by the figures of Skuja, Schiller drew flagella with (apparently) unrealistically short waves. Even Ostroff & Van Valkenburg (1978) considered that the flagellum "was rotated rapidly, circumscribing a circle of very small diameter", although they also describe the motion as a sinusoidal wave from tip to base. Only Swale (1969), in studying Pedinella hexacostata, seems to have got it right when writing: "During swimming a series of waves passes basifugally along the flagellum, the wavelength being of the order of 5-6 um. The waves appear to be in one plane only ...". In our material of Pseudopedinella elastica the wavelength was a constant 7-8 um, even when the flagellum was beating so fast that the waves could only be seen by photography, using very short exposure times. The wavelength measured from the figures of Ostroff & Van Valkenburg is also c. 7 um. Flagellar movement is definitely in one plane in Pseudopedinella, and in all its allies we have seen, viz. Pedinella spp. and Pteridomonas sp., and the waves certainly move from the base towards the tip of the flagellum. The length of the flagellum much depends on the condition of the material. Swale (1969) emphasized the role of the trailing stalk as affecting the change in swimming direction in Pedinella hexacostata, by temporary attachment in combination with sudden retraction. Ostroff & Van Valkenburg (1978) likewise thought that the trailing stalk of Pseudopedinella pyriforme might aid the rapid changes in swimming direction by serving for momentary attachment. Our observations on P. elastica indicate that this is not the case. The changes in swimming direction are mainly, if not solely, included at the flagellar end of the cell by a sideward jerk of the flagellum. The trailing stalk of P. elastica does not attach at any stage, not even temporarily. Instead, the momentum of the retracting stalk plus the cell slightly backwards, but does not seem to cause any change in swimming direction. Cells which have lost their stalk swim in smaller circles, but are able to change their swimming direction as efficiently as those with an intact trailing stalk. Thus it seems that in P. elastica the stalk mainly serves as a rudder with stabilizes swimming. The views of Schiller (1952) are not tenable: "...dass das Pseudopodium der Gattung Pseudopedinella nur in der Nebenfunktion etwa als Steuer-, in der Hauptfunktion als Fangorgan anzusehen ist", and "das erwahnte zuruckschnellen kann aus physikalischen Grunden nicht durch eine plotzliche elastische Verkurzung des Plasmodiums (sic) erfolgen, da eine solche die Zelle nur vorvartstreiben musste. Es musste das Endstuck des Pseudopodiums sich erst fest verankern, um durch eine rasche Kontraktion den Zellkorper zuruckreissen zu konnen." Schiller's further rather detailed description of the behaviour of his P. rhizopodiaca and its trailing stalk indicate that he observed cells at an advanced stage of deterioration.
• Chrysophytes or heliozoa. Davidson (1982) published electron micrographs of the heterotrophic flagellate Ciliophrys marina and demonstrated close phylogenetic relationship between this species, usually referred to the heliozoa, and the Pedinella group usually referred to the Chrysophyceae. At the same time Fenchel (1982) published sections of Pteridomonas (as Actinomonas), and showed its affinity to the same group. A more detailed study of Ciliophrys and Pteridomonas is now being undertaken by Patterson (in prep.). A detailed discussion of the phylogenetic relationship between the Pedinella group and the "true" heliozoa in general must await completion not only of these studies but also new information of Actinomonas (J. Larsen in prep.), and any other helioflagellates that can be obtained. The following organisms form a well-circumscribed group of closely related species: The phytoflagellates (photoflagellates sence Corliss 1983) Pedinella, Pseudopedinella and Apedinella, usually grouped as the family Pedinellaceae within the chrysophycean algae, and the non-photosythetic Ciliophrys, Pteridomonas and Actinomonas, variously treated as colourlees members of the Pedinellaceae or as belonging to the order Actinophryida of the protozoan class Heliozoa (Levine et al. 1980). Whether these organisms should be treated as plants, animals or protists is a matter of debate, but the new data have shown that in a natural classification they should be grouped together, probably in a single family Pedinellaceae. The similarities include details of flagellation, type of mitochondrial cristae, and perhaps the most crucial feature, the presence of tentacles supported internally by triads of microtubules originating at the nuclear envelope. Hibberd (1984) suggested that the Pedinella-groups sufficiently distinct from other chrysophytes to justify inclusion in a separate class of algae (order of protozoa in the zological system), while Christensen (1980) kept the Pedinella-group as the family Pedinellaceae of the order Ochromonadales. We agree with Hibberd that the members of the Pedinellaceae are sufficiently different from other chrysophytes to justify erection of a separate order, the Pedinellales, and this formally proposed below. Setting up of new classes, one for each of the Pedinellales, the Dictyochales (silicoflagellates) and the Bicosoecaceae, as done by Hibberd and others, is more controversial. If one accepts the narrow definition of the class used by Hibberd, the number of algal classes will readily increase to about twice the number proposed in a more conservative treatment such as Christensen (1966). Considering that some of the classes used by Christensen are difficult to define ultrastructurally, it may be argued that even the number in Christensen's treatment is too high. More recent examples of these difficulties may be drawn from Christensen (1980), who merged the Raphidophyceae and Xanthophyceae, from the phylogenetic position of Olisthodiscus which has been much debated (chrysophyte, raphidophyte, xanthophyte ? see discussion in Moestrup 1982), and from the cells of brown algae and chrysophytes being ulstrastructurally very similar. Hibberd's ideas are effectively devaluation of the class level. Similarly Round (1973) suggested phyla for most algal classes thus effectively devaluing the phylum level. The most satisfactory level of a class or a phylum is particularly difficult to determine when the whole range of lower eukaryotes is considered, algae, protozoan, and primitive fungi. The present strong interest in the lower eukaryotic organisms offers some hope, however, that it will eventually become possible, when many more organisms have been studied in sufficient detail (protozoans in particular), to construct a satifactory classification, i.e. a natural classification which includes all groups of lower eukaryotes. Until this goal is in sight, re- or devaluation of the larger systematic entities seems unnecessary and will only increase the number of superfluous names. (ref. ID; 7737)

1. Pseudopedinella ambigua Bourrelly, 1957 (ref. ID; 7737)
2. Pseudopedinella disciformis Schiller, 1952 (ref. ID; 7737)
3. Pseudopedinella elastica Skuja, 1948 (ref. ID; 7737)
4. Pseudopedinella erkensis Skuja, 1948 (ref. ID; 7737)
5. Pseudopedinella gallica Bourrelly, 1957 (ref. ID; 7737)
6. Pseudopedinella pyriforme Carter, 1937 (ref. ID; 7737)
7. Pseudopedinella rhizopodiaca Schiller, 1952 (ref. ID; 7737)
8. Pseudopedinella variabilis Rouchijajnen, 1968 (ref. ID; 7737)

Pseudopedinella elastica Skuja, 1948 (ref. ID; 7737)

Descriptions

• Light microscopy: The cells are externally radially symmetrical with six axes of symmetry. They are normally barrel-shaped to pyriform, approximately 9-15 um long and 10-14 um wide, and usually longer than wide. There are six oval or elogated gloden-brown chloroplasts arranged in an almost closed tier at the priphery of the cell, extending along almost its entire length. When the cultures are grown in weak light (less than 1000 lux) the chloroplasts overlap slightly. Prior to cell division the chloroplasts divide transversally to produce two tiers. In transverse optical section all chloroplasts are easily visible, and the cells are hexagonal or even six-lobed. Deviating individuals, much smaller or larger, or with an abnormal number of chloroplasts, are fairly common, especially in aging of cultures. Occasionally indications of an inner pyrenoid pointing towards the nucleus chould be seen in strain Z. G. H. was not able to clearly demonstrate pyrenoids in the Tv strains. The nucleus occupies a central position in the cell. The single emergent flagellum arises from a depression or pit in the anterior end of the cell. It usually measures 25-40 um, rarely up to 90 um in length. The flagellum beats in a sinusoidal wave in one plate, from the basis outwards. This flagellar movement is seen very clearly during slow movement. When the flagellum is beating fast, only the exremes of the movement can be discerned. Attached in a depression of the posterior end is a contractile trailing stalk which may measure up to 140 um in length. Following Christensen (1980) the term trailing stalk will be used for this organelle because of its similarity to the stalk in the closely related species Pedinella hexacostata. In Pedinella the stalk may function as an attachment organelle, or, when the cells are moving freely, it trails behind as in Pseudopedinella (other terms used for this organelle include pseudopodium: Carter 1937, Skuja 1948; rhizoid: Bourrelly 1957; rhizopodium: Ostroff & Van Valkenburg 1978; rhizopodial filament: Bourrelly 1968; tail-like peduncle: Swale 1969; Stiel, elastische plasmatische Schwanz: Skuja 1948). The stalk has its largest diameter about a third to a quarter of its length from the cell, where a distinct swelling can frequently be observed. The stalk possesses a number of short branches or << blebs >> , and these are usually most numerous in the thickest part, especially in not quite fresh cells. Occasionally it has large branches or << knots >>. When contracted, and in fixed cells, the stalk forms a << bead >> (cf. Ostroff & Van Valkenburg 1978), which is withdrawn into the posterior invagination. In samples fixed with Lugol's solution for Utermohl counts, the << bead >>, which is not stained by iodine, is a good characteristic of the species when its cytology has otherwise mostly deteriorated. Approximately six posterior tentacles may form a ring at the extreme hind end of the cell around the depression from which the stalk emerges. These are usually 4-6 um long, rarely reaching 8-10 um. The tentacles are very difficult to count accurately in vigorous cells, but their number seems to equal the number of chloroplasts. The presence of posterior tentacles is directly correlated with the condition of the stalk. Thus when the stalk is poorly developed or lost, tentacles are rarely detectable. A few refractive globules are usually observed in each cell, frequently near the inner surface of the chloroplast, and mainly in the anterior half of the cell. Similar globules were found by Carter (1937) and Swale (1969) in Pseudopedinella pyriforme and Pedinella hexacostata, respectively. Bourrelly (1957) seems to interpret the globules as muciferous bodies. Skuja (1948) drew similar globules in Pseudopedinella elastica and P. erkensis, but did not discuss them. (ref. ID; 7737)
  • Behavior. The cells of Pseudopedinella elastica are always swimming freely. They have never been to attach to any object. Swimming individuals with the stalk relaxed and extended rarely follow a straight course, but swim in large circles, slowly rotating around the longitudinal axis, with the limp stalk passively trailing after the cell. Occasionally, mostly upon hitting some object, for example anterior cell in the culture, less frequently apparently spontaneously, the cells suddenly contract the stalk, and with a sideward jerk of the flagellum change the direction of swimming by up to more than 180 degrees. Simultaneously, the tentacles are retracted and the flagellum starts beating so rapidly that its sinusoidal movement is obscured, and the swimming speed increases quickly. After the cell has started upon a new course, the beat of the flagellum gradually slows down, while the stalk slowly extends and relaxes. The tentacles seem to extend to their full length a little later than the stalk. This cycle, is repeated with varying frequency in different individuals. Some cells continue to circle for several turns, following approximately the same track at low speed until they set off in another direction. Others change their course at frequent intervals. (ref. ID; 7737)
  • Variability. The valiability of P. elastica in batch culture is expressed at four different levels:
    • 1. Even in a fresh preparation from an exponentially growing culture there is some variation in the size and shape of the cell, and in the length of the flagellum and the stalk. Deviating chloroplast numbers (usually 5, 7 or 8), and << senile >> cells are occasionally observed.
    • 2. When a microscopical preparation is made, the morphology and behaviour start to change almost immediately. If thoroughly cleaned glassware is not used, there is great risk that natural (<< normal >>) morphology/behaviour will not be observed. There is a gradual reduction in swimming speed until the cells are caught between the slide and the cover glass. Quite early on the cells shorten and become wider than long. At this stage, one side of the cell usually becomes larger than the other and the cells become pomiform. Then the chloroplasts partly separate from the cell margin by large vesicles. The stalk develops more "hairiness", some times even branches, and eventually "blebs" which slough off. Part or all of the stalk is discarded and the tentacles are withdrawn. The flagellum becomes shorter. Finally, the flagellum is discarded and the cell membrane bursts.
    • 3. A batch culture develops in a similar way after the exponential growth phase. In senescent cultures rather small pomiform cells with short flagella and pseudopodia are mostly found. The number of deviating cells with atypical chloroplast numbers increase, and so does the number of incompletely divided cells.
    • 4. There are slight differences between the strains even when cultured under identical conditions. Tv32 differed from Tv6 in its slightly more elongated cell shape, a longer flagellum, and more rapid swimming. Tv39 was slightly larger and slower than Tv32, and showed a greater frequency of irregular elongated cells than the other strains, but a rather short flagellum like Tv6. (ref. ID; 7737)
• Electron microscopy: The flagellar apparatus, the anterior tentacles. As in other examined species of the Pedinellaceae the single emergent flagellum is associated with an extra basal body (centriole). The pair is attached to the nuclear envelope in a small pocket in the nucleus. The basal bodies are at a slight angle to each other, but this probably varies. There are no signs of flagellar roots, but very thin bands emanating from the basal body to the nuclear envelope may represent a vestigial cross-banded root (rhizoplast) as suggested by Hibbert (1976) in an unidentified species of Pseudopedinella. Prior to cell division two new centorioles, form, one on each side of the old pair. During division each daughetr cell will therefore probably receive one from the old pair and one newly formed centriole. Cell division has, however, not been examined ultrastructurally in any member of the Pedinellaceae. It is therefore not known whether in the daughter cell that receives the "non-functional basal body" this or the newly formed centriole will grow into a functional flagellum. The transitional helix typical of Chrysophyceae, Xanthophyceae and Eustigmatophyceae is absent, but the zone contains the other structural feature characteristic of heterokont protists, the transverse partition with its central thickening. Dense bands which link the peripheral pairs of microtubules with the flagellar membrane are not in the plane of the transition plate, but a slight distance further away from the cell. These findings agree well with Hibberd's micrograph of Pseudopedinella sp. (Hibberd 1979), and Pedinella hexacostata is probably similar (Swale 1969). One of the distinguishing characters between Pedinella and Pseudopedinella is often considered to be the presence of anterior tentacles in the former genus. It is therefore interisting that cells of Pseudopedinella elastica may occasionally form short tentacles, which can be so long that they become visible also under the light microscope. Each tentacle is supported internally by microtubules arranged regularly in a triad. The microtubules are interconnected by very thin dense plates, to form a triangle. The triads represent one of the main diagnostic features of the Pedinella-group and their presence is known as well as in the three phototrophic members, Apedinella (Throndsen 1971), Pseudopedinella (Ostroff & Van Valkenburg 1978) and Pedinella (Swale 1969), as in the three heterotrophic genera Pteridomonas (Patterson 1984), Ciliophrys (Davidson 1982) and Actinomonas (Larsen in prep.). They have apparently not been found in any other group of protists. The triads are attached at one end to dense material on the nuclear envelope, while the other extends into tentacles (axopodia) in most genera. The finding of tentacles in Pseudopedinella is not entirely surprising, as some authors, using light microscopy, have reported the occasional appearance of tentacle-like structures in P. erkensis (Skuja 1948) and P. elastica (Javornicky 1967). Throndsen found c.24 triads in the anterior end of Apedinella, but no tentacles. We have evidence, however, that Apedinella may under certain circumstances form anterior tentacles (Moestrup & Thomsen unpubl. obs.). These are long and slender and when present not difficult to detect under high power light microscopy. Their presence has been confirmed by electron microscopy (unpubl. obs.). The flagellum of Pseudopedinella elastica is of a characteristic appearance. After leaving the anterior depression of the cell, one side extends into a unilateral wing supported in the longitudinal direction by an electron dense rods. Flagellar rods are characteristic features of euglenoid and dinoflagellate flagella. They are usually striated (cross-banded) but the details vary greatly. In the Pedinella-group, a flagellar rod occurs in species of Pseudopedinella (Ostroff & Van Valkenburg 1978; pres. paper), Pedinella hexacostata (Swale 1969), Apedinella (Throndsen 1971), and Actinomonas (Larsen in prep.). It was not reported in Ciliophrys marina (Davidson 1982). In Pseudopedinella elastica we have occasionally seen an indistinct cross-banding of the flagellar rod, somewhat like that illustrated in Pseudopedinella pyriforme by Ostroff & Van Valkenburg (1978), but we have not obtained any reliable measurements of the periodicty of the cross-banding. The rod in Apedinella spinifera appears to be more distinctly cross-banded (Throndsen 1971), but this should be confirmed by high-power electron microscopy. The flagellum in members of the Pedinellaceae carry flagellar hairs of the usual heterokont type. They are divided into three distinct regions. Flagellar hairs in the Pedinella-group are usually considered to be arranged in a single row opposite the flagellar wing (Swale 1969; Ostroff & Van Valkenburg 1978; Throndsen 1971). The evidence for this arrangement is not entirely convicing, and the hairs on Pseudopedinella elastica are at least partly in two rows. Our attempts to examine this arrangement further have failed, but a distinct bilateral arrangement of the flagellar hairs occurs both in Pteriomonas, which lacks a distinct flagellar rod (Patterson in prep.) and in Actinomonas, which possesses one (Larsen pers. comm.). (ref. ID; 7737)
  • Nucleus, mitochondria, chloroplasts, microbodies. The nucleus is attached to all size chloroplasts, with a prominent connection in the pyrenoid region, though further connections occur elsewhere. As in other chrysophytes the connection is mediated by the outer membrane of the nuclear envelope, which continues around the chloroplast. The perinuclear space between the two nuclear membranes commonly shows profiles of presumptive flagellar hairs. Similar profiles occur elsewhere in the cytoplasm in ribosome-covered cisternae, which in some cases can be seen to be connected to the nuclear envelop. Profiles of mitochondria are commonly seen in the narrow triangular area between two adjacent pyrenoids and the nucleus. Whether these profiles represent separate entities is not known, and it is equally possible that they belong to a single mitochodrial reticulum, as shown for certain prasinophytes, green algae and cryptophytes (McFadden & Wetherbee 1982; Santore & Greenwood 1977). This has apparently not been examined in any heterokont alga. The mitochondrial cristae are tubular and contain rather unusual inclusions, seen at higher magnification as one or more fibrils. Such fibrils are common in brown algae, from primitive forms such as Pilayella (Markey & Wilce 1976) to the most advanced as Fucus (Brawley et al. 1976; Pollock & Cassel 1977). They have apparently not been reported in any other groups of algae. The intracristal fibrils are almost certainly present also in Pseudopedinella pyriforme examined by Ostroff & Van Valkenburg (1978), and they were clearly illustrated in the heterotrophic species Ciliophrys marina (Davidson 1982) and Pteridomonas sp. (Patterson 1984). They have apparently not been noted in typical chrysophytes, but in Phaeosaccion, a marine benthic alga now considered to be a chrysophyte shows intracristal filaments. Similar structures may be present in haptophytes. The pyrenoids of Pseudopedinella elastica are prominent, but usually not easily seen under the light microscope. When the type species of Pseudopedinella was described, Carter (1937) specifically noted the absence of pyrenoids in this species. Nevertheless one of her drawings, that there is not doubt that the type species possesses pyrenoids too. The pyrenoids of Pseudopedinella elastica are similar to those of Apedinella spinifera. In Pedinella hexacostata pyrenoids are absent. The pyrenoid core of Pseudopedinella and Apedinella is penetrated by invaginations lined by the two inner membranes of the chloroplast envelope. This is not an uncommon type, and examples may be drawn from chrysophytes (Ostroff & Van Valkenburg 1978), green algae including prasinophytes (Ettl & Moestrup 1980; Norris et al. 1980), and the dinoflagellate Heterocapsa triquetra (Dodge & Crawford 1971). The cloroplast proper contains three-thylakoid lamellar, usually 12-20 arranged longitudinally. There is a single girdle-lamella, which loses one thylakoid near the end of the chloroplast to consist of only two thylakoids. Terminations of the thylakoids often show a dense granule as noted in Chrysamoeba radians by Hibberd (1971). Other notable features of the chloroplast include abrupt discontinuities of the innermost thylakoid lamellae in the region of the pyrenoid, giving rise to transverse channels through the chloroplast. Microbodies appear scattered in the cytoplasm and are commonly associated with mitochondria. (ref. ID; 7737)
  • The posterior end. In the posterior part to the cell, the large Golgi apparatus is located below the nucleus. In longitudinal sections it appears as two separate dictyosomes, but transverse sections show that these belongs to one large ring-shaped structure. The Golgi apparatus encircles a posterior vacuole system, centrally located under the nucleus. It consists of 12-15 flattened cisternae, often somewhat swollen. Tubules resembling flagellar hairs occur within the cisternae, recalling the situation described by Bouck (1971) in Ochromonas danica. Flagellar hairs of Ochromonas were described as being formed in the ER system and transported to the exterior via the Golgi system. Lateral hair-like elements were claimed in Ochromonas to be added to the flagellar hairs while the hairs passed through the Golgi apparatus. Whether this component is present also in Pseudopedinella is not known, and the same question applies to most other heterokonts in which pofiles of flagellar hairs are visible in both the ER and the Golgi-system. The posterior vacuole system is directly connected to the trailing stalk. The latter is bounded by the membrane, of which the innermost is continous with the membrane of the large central vacuole in the vacuole system. The outermost membrane is continuous with the cell membrane. The posterior vacuole system opens into the stalk through a narrow duct. This is surrounded by what in sections appears as electron-dense plates. A number of fibrillar-like structures extend at right angles from the plates, to pass along the cell membrane. Several components are visible in each plate. A somewhat similar structure -without any fibrillar-like components- has been illustrated in species of Pyramimonas (Prasinophyceae) (Manton 1966; Moestrup & Thomsen 1974). It is associated with the duct connecting the scale reservoir to the exterior. In Pyramimonas the structure was suggested to function as a sphincter, where the action might control scale liberation. In Pseudopedinella the "sphincter" and the fibrils more likely function as a skeleton stabilising the hind region which must be subjected to considerable stress during swimming. There is no indication that the "sphincter" contracts to close the duct when the stalk is shed. Above it has been described how the stalk may function as a rudder and as a brake system, whose contraction stops the swimming cell. Fine structural details of the stalk indicate, however, that some kind of transportation may also take place through the stalk. The inner membrane of the stalk folds into the lumen to form numerous tubular structures, and 40 vesicles appear to be abstricted from the outer membrane. Fibrils which might present contractile proteins have not been observed in the stalk, but the stalk cavity is filled with apparently mucilaginous material. The same type of material continues into the large posterior vacuole. One may envisage a transport system by which material from the Golgi apparatus passes through the large vacuole to the stalk, and that some regulation of the contents of the stalk occurs through the lining membranes. This may be a means of continuous regeneration of the stalk, to compensate for the material lost by sloughing. The region between the Golgi apparatus and the large vacuole is apparently one of considerable activity. Numerous small vesicles are present, some of which are coated on the outside with a layer of dense material ("coated vesicles"). Similar coat material from a number of the organisms has been shown to contain one major protein ('clathrin"), and to be implicated in intracellular translocations (Nevorotin 1980). In Pseudopedinella the coating may sometimes be seen to consist of separate electron-dense units, arranged in a single layer. While some coated vesicles are in contract with the large posterior vacuole others associate with cisternae of the Golgi apparatus. Membranous structure are often visible in the posterior vacuole and the associated system of coated vesicles. These in some case represent infoldings of the membrane into the lumen. Occasionally the large vacuole contains stacks of membranous material, whose fate is unknown. On more structure should be mentioned. Profiles of microtubules extend from the nuclear envelope, pass through the space between the Golgi apparatus and the posterior vacuole system and terminate near the plasmalemma. The microtubules may be seen along the periphery of the large posterior vacuole, or they travel through this vacuole in a centrally located cytoplasmic strand. Most intriguingly, however, the microtubules are in groups of three, exactly as in the rudimentary tentacles of the front end of the cell. Although this has yet to be confirmed by electron microscopy, it appears likely that the triads support the posterior tentacles observed in the light microscope. This unexpected finding is difficult to assess, but one may speculate that Pseudopedinella is derived from an organism with triad-supported tentacles extending regulary in all directions, as in Actinomonas. Some of these have been retained in the hind end, and a few as rudiments around the flagellum. (ref. ID; 7737)