Mitosis and Cytokinesis in the Prasinophyceae. I. Mantoniella squamata (Manton and Parke)...

Mitosis and Cytokinesis in the Prasinophyceae. I. Mantoniella squamata (Manton and Parke)DesikacharyAuthor(s): Steven B. Barlow and Rose Ann CattolicoSource: American Journal of Botany, Vol. 68, No. 5 (May - Jun., 1981), pp. 606-615Published by: Botanical Society of AmericaStable URL: http://www.jstor.org/stable/2442787 .

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Amer. J. Bot. 68(5): 606-615. 1981.

MITOSIS AND CYTOKINESIS IN THE PRASINOPHYCEAE. I. MANTONIELLA SQUAMATA (MANTON AND PARKE) DESIKACHARY1

STEVEN B. BARLOW2 AND ROSE ANN CATTOLICO Department of Botany AJ-10, University of Washington, Seattle, Washington 98195

ABSTRACT

Mitosis in Mantoniella squamata (Manton and Parke) Desikachary, a small scale-covered green monad, is presented. Organelle replication precedes nuclear division and begins with the replication of the chloroplast. As the chloroplasts separate, the Golgi and flagellar apparatuses divide. The discoid microbody enlarges and becomes 'V'-shaped, with the arms extending toward depressions in the pyrenoid stalks of the chloroplasts. At prophase, microtubules produced by an amorphous microtubule organizing center enter the nucleus via polar fenestre. The nuclear membrane remains intact. As the chloroplasts migrate further apart, the spindle pole- to-pole distance increases. By metaphase, daughter-cell lobes are discernible as a cleavage furrow, which appears as early as prophase, and begins to incise the cell. A single Golgi apparatus is situated near the spindle pole; the flagellar apparatus lies adjacent to the pole. The cleavage furrow continues to constrict the cell, resulting in a narrowing isthmus containing the elongate microbody, nucleus and a rootlet system connecting the basal bodies of the daughter flagella. At telophase, no extra-nuclear microtubular systems other than the previously observed rootlet are present and the nuclei remain separated from each other. In cells undergoing multiple divisions to produce more than two daughter cells, the orientation of organelles changes some- what, with the basal bodies and the Golgi apparatus separating daughter nuclei prior to the onset of cytokinesis. The mechanics of mitosis in Mantoniella are compared with other green monads and the evolutionary implications discussed.

THE PRASINOPHYCEAE represent a complex, non-uniform algal group which has become im- portant in defining evolutionary origins and taxonomic relatedness among the green algae. Fine structure analyses have shown that genera of this class vary significantly in mitotic mechanisms. For example, a persistent spindle similar to that observed in Charophycean cells (Pickett-Heaps and Marchant, 1972) is seen in Pedinomonas minor (Pickett-Heaps and Ott, 1974), Pyramimonas (Wood and Triemer, per- sonal communication), and Nephroselmis angulata (Mattox and Stewart, 1977) whereas a collapsing mitotic apparatus and phycoplast characteristic of Chlorophycean division (Pickett-Heaps, 1975; Stewart and Mattox, 1975a) has been observed in Platymonas subcordiformis (Stewart, Mattox and Chandler, 1974). Two suggestions have been made with

1 Received for publication 19 April 1980; revision ac- cepted 20 August 1980. We would like to acknowledge the advice and assistance of Drs. Kent McDonald, R. E. Nor- ris and C. D. Sandgren for their useful discussions, and the Director of Friday Harbor Laboratory for use of the facilities; S.B. is indebted to E. O'Connor for support and encouragement during this study. This research was sup- ported by the University of Washington Graduate Re- search Fund, and NSF Grant PCM 76-2240 to Rose Ann Cattolico.

2 Present address: Department of Botany, Rutgers Uni- versity, Box 1059, Piscataway, New Jersey 08854.

respect to the evolutionary development within the Prasinophyceae: 1) mitotic mechanism might be related (Stewart and Mattox, 1975b; Mattox and Stewart, 1977) to scale/theca/wall development; that is, the phycoplast mode of division dominated as a more rigid wall-like structure evolved, and 2) organisms covered with multiple scale layers, theca, or wall have been evolutionarily derived (Norris, 1980) from a naked or single-scale layered cell.

To date, mitotic information on the Prasi- nophyceae is available for a thecate cell (Stew- art et al., 1974), a naked cell (Pickett-Heaps and Ott, 1974) and for those genera which pos- sess two or more scale layers (Pearson and Norris, 1975; Mattox and Stewart, 1977; Wood and Triemer, pers. commun.). In this communication we wish to report upon the mitotic events which occur in Mantoniella squamata. This biflagellate organism is covered by a single layer of spiderweb-shaped scales. The interphase morphology of Mantoniella has been presented elsewhere (Manton and Parke, 1960; Barlow and Cattolico, 1980).

MATERIALS AND METHODS-Cell culture and sampling-Unialgal cultures of Manton- iella squamata (listed as Micromonas squamata) were obtained from the Indiana Culture Collection. This algal strain is presently available from the Culture Collection of Algae,

606



May-June, 1981] BARLOW AND CATTOLICO-MITOSIS IN MANTONIELLA 607

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Fig. 1-6. 1. Preprophase. Cells enlarge and become globose. 2. Preprophase. Organelle division begins. 3. Prophase. Organelle duplication is completed and daughter flagella move apart. Spindle microtubules form at amorphous MTOCs and enter the nucleus via fenestre. 4. Metaphase. Spindle pole-to-pole distance increases as chromosomes align at the metaphase plate. The basal bodies lie laterally adjacent to the poles. The microbody elongates and extends into depressions in the pyrenoid stalk. 5. Anaphase. Pole-to-pole distance reaches a maximum and chromosome-to-pole distance decreases. 6. Telophase/cytokinesis. Microbody division is completed. Chromosome-to-pole distance continues to decrease. The nuclear region becomes increasingly bilobed and the cleavage furrow deeply incises the cell. Nuclear division and cytokinesis are completed simultaneously.

University of Texas, #UTEX 990 (Starr, 1978). Cultures were maintained on a 12-hr light:12-hr dark cycle at 20 C with 5,940 Lux of lateral illumination. Details of medium preparation and culture maintenance have been presented elsewhere (Barlow and Cattolico, 1980).

The generation time of a Mantoniella culture maintained on a 12-hr light: 12-hr dark cycle was 24 hr. Cell growth was exponential until cultures reached a density of 4-5 x 106 cells/ ml after which cultures entered the linear- growth phase. Dividing cells were observed most frequently during the first 5 hr of the dark cycle. For this reason, culture material fixed for electron microscopic examination was sam- pled during this time period. It should be noted that several attempts were made to enhance synchronous cell division by light:dark cycle manipulation (Pearson and Norris, 1975). No

increase in cell synchronization was ever observed under any experimental conditions tested.

Electron microscopy-Aliquots of exponen- tially growing cells were fixed for transmission (TEM) and scanning electron microscopic (SEM) examination by the simultaneous ad- dition of glutaraldehyde and osmium tetroxide. The final concentration of these reagents was 1.5% and 0.35% respectively. Technical details of electron microscopy have been published (Barlow and Cattolico, 1980). The observations presented in this report result from ana- lyzing random thin sections as well as the serial sections obtained from 22 cells at various stages of division.

RESULTS-Interphase morphology-De- tails of the interphase morphology of Manton-



608 AMERICAN JOURNAL OF BOTANY [Vol. 68

iella have been discussed elsewhere (Barlow and Cattolico, 1980) and only a brief review is noted below. The organism is reniform in shape (Fig. 7) with two flagella inserted laterally into a groove or pit on the convex side of the cell. The long flagellum is three and one- half to four times the length of the cell whereas the second, reduced flagellum is only 1 ,um in length. This smaller flagellum is not readily visible by light microscopy. Two rootlet systems are observed in the cell. Both are associated with the basal body of the long flagellum. The proximal rootlet, composed of four microtubules arranged in a '3-over-i' configura- tion, extends from the proximal tip of the long flagellum alongside the nucleus and terminates at the posterior end of the nucleus. The two- membered distal rootlet extends partially around the circumference of the cell perpendicular to the long axis. The chloroplast lies opposite to the flagellar apparatus and lines the convex side of the cell. This organelle contains a central pyrenoid enclosed by a large ovoid starch shell. The Golgi apparatus is situated in the anterior lobe of the cell whereas the nucleus is located in the posterior lobe. Sand- wiched between the chloroplast and the nucleus is a small discoid microbody. A single 'U'-shaped mitochondrion is located within a depression in the chloroplast. The entire cell, including the flagellar apparatus, is covered by a single layer of overlapping spiderweb-shaped scales.

Preprophase-Interphase cells are normally reniform in shape but in preparation for division they become globose (Fig. 1) and enlarge to a diameter of 3.5-4.5 ,um. A longitudinal furrow which first appears as a crease in the base of the starch shell (Fig. 2, 8) bisects the pyrenoid while a simultaneous infolding of the outer chloroplast membrane divides the stigma (Fig. 9). The furrow continues to bisect the chloroplast until chloroplast replication is completed. Later in division, within some cells, a vesicle, which functions as part of the cleavage

furrow, forms between the daughter chloroplasts (Fig. 10).

The mitochondrion elongates and subse- quently divides by simple fission. The deep- ening cleavage furrow interjects itself between the newly formed daughter mitochondria. The exact timing for the completion of this cleavage event is not presently known.

The Golgi cisternae divide and the division products (Fig. 12) migrate to either side of the centrally located basal bodies. No intermediate stages of Golgi-apparatus replication were observed.

The basal bodies replicate to form a linear sequence along the transverse axis of the cell. In a fortuitous sequence of serial sections (Fig. 11), it was possible to observe in cross-section three of the four basal bodies immediately after replication. The parallel orientation of the basal bodies does not persist. The basal bodies of the long flagella reorient themselves so that they lie almost perpendicular to their respective stubby flagella. The four-membered proximal rootlet associated with each flagella apparatus appear to be continuous with one another. These rootlets elongate as the basal bodies migrate farther apart during spindle maturation. The stubby flagella exit the cell perpendicular to the cell body and parallel to each other. One result of this orientation is that the stubby flagella are always perpendicular to the long axis of the spindle.

Throughout preprophase and subsequent division stages, cells retain their flagella (Fig. 13) and are motile.

Prophase-Except for the microbody and possibly the mitochondrion, organelle replication in Mantoniella is completed before the onset of nuclear division.

The first sign of impending nuclear division is the appearance of numerous microtubules in the cytoplasm (Fig. 14). These presumptive spindle microtubules radiate throughout the cell cytoplasm from an amorphous electron- opaque region adjacent to the proximal tip of

Fig. 7-11. 7. Interphase. Longitudinal section of an interphase cell showing the distribution of major organelles: Chloroplast (C), Golgi apparatus (G), long flagellum (LF), short flagellum (SF), mitochondrion (M), nucleus (N), pyrenoid (P) with its associated starch shell, microbody (mb), microtubular proximal rootlet (arrow), electron-dense region (arrowheads). x25,920. Bar = 1 ,um. Fig. 8-13. Preprophase. 8. Cleavage of the chloroplast begins with the invagination of the pyrenoid (arrowheads). x59,000. Bar = 0.5 ,um. 9. The stigma, having doubled in size, begins to divide. x52,600. Bar = 0.5 ,um. 10. The preprophase cell is globose in shape with a cleavage furrow (arrowhead) forming between daughter chloroplasts. Presence of the nucleolus (nc) suggests the nuclear division cycle has not yet started. x26,720. Bar = 1 ,um. 11. The replicated flagellar apparatus. Note the presence of the '3-over-i' rootlet (arrowhead) already forming adjacent to the daughter long flagellum (lf); proximal rootlet (pr); distal rootlet (dr); x 80,000. Bar = 1 Aum.




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each stubby flagellum. By late prophase (Fig. 3), these spindle microtubules have entered the nuclear area (Fig. 15) via polar fenestre. The nuclear envelope appears otherwise intact and persists throughout division. The nucleolus disperses. The basal bodies are situated lateral to each spindle pole.

Metaphase-By metaphase, the dividing nucleus is one and one-half times the size observed in the interphase cell. It was difficult to discern distinct chromosomes in these dividing cells. However, dark-staining chromatin could be observed in some favorably stained sections. Distribution of organelles (Fig. 4) was the major criterion by which a metaphase cell was identified.

The microbody is situated between the two daughter chloroplasts adjacent to the stalks of the pyrenoid. During late prophase and early metaphase, the microbody doubles in size. The stalks of the pyrenoids, normally flattened at their tips in interphase cells, become invagi- nated as the microbody begins to cleave. The lobes of the 'V'-shaped microbody extend into

-these pyrenoid invaginations (Fig. 14, 15), but never penetrate the pyrenoid stalk itself. Cleavage of the microbody is not completed until late anaphase or telophase.

The Golgi apparatuses are located (Fig. 16) near the poles of the spindle at either end of the cell. Microtubules no longer radiate into the cytoplasm around the Golgi.

The basal bodies of the stubby flagella are oriented perpendicular to the spindle axis and lie within a depression of the nuclear envelope. The poles of the spindle are adjacent to the proximal tip of these basal bodies. The long flagella, which emerge from the cell at an acute angle, are oriented almost perpendicularly to their respective stubby flagella. As daughter flagella apparatuses become further separated from each other during nuclear elongation, the proximal 3-over- I rootlet microtubules concurrently elongate. We were unable to deter- mine the exact number and orientation of these extra-nuclear microtubules. There could be either one continuous 3-over-I system or two distinct sets, one from each flagellar apparatus,

which overlap at the equatorial region of the nucleus. It was not possible to differentiate between these two possibilities from our longitudinal sections. It is clear in either case, that an extra-nuclear set of microtubules extends between the basal bodies parallel to the axis of the spindle (Fig. 17).

Anaphase-The cell becomes increasingly bilobed as daughter chloroplasts separate from each other (Fig. 5, 17). The forming daughter cells are joined by a narrowing isthmus in the posterior end of the cell which contains the dividing nucleus and microbody. The widely separated lobes of the microbody remain attached to each other by a thinning isthmus. Spindle pole-to-pole distance continues to increase but the chromosome-to-pole distance begins to shorten. The cleavage furrow is most strongly developed between the chloroplasts on the side of the cell opposite the flagella. The flagellar groove region is elongate and flattened.

Telophase/cytokinesis-Telophase (Fig. 6, 18) was rarely observed in section and may be of extremely short duration. No dark-staining chromatin plate is seen within the forming daughter nuclei. Although cell bodies rotate 1800 relative to one another in Pedinomonas (Pickett-Heaps and Ott, 1974), we did not observe any rotation of the two cell lobes in Mantoniella at any stage of mitosis. As cytokinesis progresses, the lobes of the daughter cells remain attached to one another by a deeply incised isthmus. The four-membered proximal rootlet from each flagellar apparatus are present in the isthmus between daughter cells. The cleavage furrow which bisected most of the cell contents in the earlier stages of nuclear division has now bisected the microbody and is impinging upon the nuclear area. This cleavage furrow results from the coalescing of vesicles derived from the Golgi apparatus and the endoplasmic reticulum. Those vesicles origi- nating from the Golgi contain spiderweb scales. As the nuclear isthmus decreases in size, the connection between daughter cells becomes increasingly tenuous. The cleavage

Fig. 12-15. 12. Golgi division products have migrated apart. x26,000. Bar =1 ,um. 13. SEM of a prophase cell. The cleavage furrow is already forming. One of the short flagella (arrow) and both of the daughter long flagella can be observed. x21,000. Bar =1 ,um. 14, 15. Prophase. Sequential sections. Microtubules (t), which emanate from an amorphous MTOC (arrow) adjacent to the basal bodies (bb), radiate into the cell cytoplasm but can also be observed within the nucleus (N). The arms of the dividing microbody (mb) extend into depressions (arrowhead) of the pyrenoid stalk. x34,800. Bar = 1 gm.




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19 Fig. 19. Cytokinesis in a multiply-dividing cell. Darkly stained chromatin can be observed within both nuclei (N).

The other complete set of organelles is not in this plane of section. Note the central location of the Golgi apparatus (G) compared with its peripheral location in cells undergoing simple fission (c.f. Fig. 16-18). x30,000. Bar = 1 Am.

furrow now bisects the four-membered microtubule rootlet.

In cells which undergo multiple divisions to form more than two daughter cells, cleavage is postponed and no indication of a cleavage furrow is seen even at telophase. When this event occurs, cellular organelles, particularly the flagellar apparatuses, maintain the sepa- ration between daughter nuclei. We never observed more than two nuclei in any given cell, although gigantic cells with multiple sets of organelles were not unusual. Nuclear division in these cells appears to be sequential and not

multipolar. Organelle orientation at telophase in multiply-dividing cells is quite different from that observed in simply-dividing cells. For example, the Golgi apparatuses are not situated at either end of the cell, separated by the dividing nucleus. Instead, the nuclei occupy a peripheral position with the Golgi apparatuses in the center of the isthmus (Fig. 19). This orientation may occur as a result of an in- creased requirement for scale and membrane material which would be necessary to suc- cessfully complete a cytokinesis resulting in the formation of more than two daughter cells.

Fig. 16-18. 16. Metaphase. Both poles of the spindle (arrowheads) can be seen in glancing section. The Golgi apparatuses are situated behind the poles of the spindle; the basal bodies are laterally adjacent. The approximate locations and orientations of other cell organelles not contained in this section are noted: long flagella (large arrows), short flagella (diamonds), microbody (small arrows). x39,600. Bar = 1 jum. 17. Early anaphase, showing one of the two chromatin masses (ch) pierced by microtubules (t). Note the longitudinal glancing section of the microtubular rootlet (arrowheads) which extends between daughter long flagella. x 46,000. Bar = 1 Am. 18. Late anaphase/telophase. A few microtubules (t) are visible in one lobe of the nucleus. The well-developed cleavage furrow (cf) has impinged upon the nuclear region. x30,750. Bar = I ,um.




DISCUSSION-Early studies of the fine structure of Mantoniella (Manton and Parke, 1960) suggest that nuclear replication is completed before the onset of cell cleavage. The data presented in our study clearly demonstrate a different sequence: the division cleavage furrow initially appears between daughter chloroplasts prior to mitosis. However, cytokinesis may be postponed in lieu of further organelle division in those cells which undergo multiple divisions to form three or more daughters.

Regardless of the number of cells produced per mother cell, daughter nuclei remain spatially separate from one another. The separa- tion is effected by the persistence of the nuclear spindle and the presence of the deeply en- croaching cleavage furrow which appears by metaphase or early anaphase. Daughter nuclei in multiply-dividing cells are maintained at opposite ends of the organism by the interposition of cytoplasmic organelles, particularly the flagellar apparatus. From these observations it is difficult to interpret the electron micrograph presented by Manton and Parke (1960, fig. 20) in which a Mantoniella cell shows two closely appressed daughter nuclei with duplicated basal bodies equatorially situated with respect to the nucleus. We have never seen an analogous arrangement in any of our own material, and suggest that this micrograph represents a cell undergoing multiple division.

Unlike the thecate Platymonas (Stewart et al., 1974) which displays a phycoplastic mode of division, Mantoniella is more similar to the multi-scale layered Pyramimonas (Wood and Triemer, pers. commun.) and the naked Pe- dinomonas (Pickett-Heaps and Ott, 1974) in that a persistent spindle is maintained during mitosis. This observation adds support to the suggestions that the transition from a naked or scaled cell to a thecate or walled cell may be evolutionarily paralleled by a change in mitotic mechanism.

Details of the mitotic events which occur in Mantoniella resemble those which occur in Nephroselmis (Mattox and Stewart, 1977) more closely than any other prasinophyte. In both Nephroselmis and Mantoniella, chromosome migration is a result of pole-to-pole elongation of the spindle, followed in late anaphase and telophase by a shortening of the chromosome-to-pole distance. During division, the nuclear envelope of Nephroselmis remains intact except for small occasional gaps which Mattox and Stewart (1977) suggest represent a fragmentation of the nuclear membrane. We see similar gaps in the nuclear membrane of Mantoniella in both interphase and dividing cells and suggest that these disconti- nuities may be fixation anomalies.

Spindle fibers in Mantoniella are organized by an amorphous microtubule organizing center (MTOC) adjacent to the proximal tip of the basal bodies. In Nephroselmis, the spindle fibers radiate from a broad flat pole which is intimately associated with the rhizoplast. The basal bodies of this organism are attached to the rhizoplast. A suggestion has been made (Molnar et al., 1974; Stewart and Mattox, 1975b) that the rhizoplast might serve as a spindle microtubule nucleating source. Rhizoplast dissolution may occur concurrently with spindle formation in Nephroselmis, for this organelle becomes only "faintly visible" during metaphase. Although rhizoplasts have not been observed in Mantoniella, there is an amorphous electron-dense region which extends alongside the nucleus from the flagellar apparatus to the pyrenoid. This region may be analogous to the rhizoplast seen in Nephrosel- mis, and thus serve a similar function in spindle microtubule formation.

The relationship of the microbody to the pyrenoid in Mantoniella is rather interesting. Electron-dense material has been observed extending from the basal bodies to the pyrenoid, indicating that the microbody in Man- toniella may be linked to the flagellar apparatus. During prophase or metaphase, the stalks of the daughter pyrenoids become in- vaginated and protuberances of the microbody extend into each of these depressions. This association of the microbody with the pyrenoid suggests that the microbody may play an active but undetermined role in cell division. The fate of the microbody in Mantoniella during division differs from that observed in Nephrosel- mis. Although in both organisms the microbody is associated with the chloroplast pyrenoid, the lobate microbody of Nephro- selmis divides in prophase, whereas microbody elongation and division in Mantoniella is analogous spatially and temporally to nuclear elongation and division.

Mitotic mechanism, combined with scale vestiture, has been used as a two-pronged an- alytical tool in defining evolutionary relatedness among the green algae. For example, small diamond-shaped scales have been shown to occur on the zoospores of the multicellular algae Trichosarcina and Pseudendoclonium (Mattox and Stewart, 1973). This observation has given support to the hypothesis of Pickett- Heaps and Ott (1974) and Stewart and Mattox (1975a) that these more advanced green algal forms have been derived from an asymmetri- cal, naked or scaly ancestral flagellate such as Pedinomonas rather than a symmetrical, walled, Chlamydomonas-like cell. This proposal received further experimental support in




that Pseudendoclonium and Trichosarcina uti- lize a mode of division (Mattox and Stewart, 1974) quite distinct from that observed in the phycoplastic Chlamydomonas. Elucidation of the phylogenetic relatedness among members of the Prasinophyceae and determination of their possible evolutionary role to more advanced algal forms is complex. However, some clues are emerging. For example, the similarity between the stellate outer scales of Nephro- selmis angulata and those fusing to form the theca in Platymonas suggests (Manton and Parke, 1965; Mattox and Stewart, 1977) an evolutionary link between these two genera. This linkage is particularly intriguing in that Ne- phroselmis division is effected by a persistent spindle whereas Platymonas utilizes a phycoplastic mode of division.

Defining the relatedness of Mantoniella (Manton and Parke) Desikachary (1972), Dol- ichomastix Manton (1977) and Nephroselmis gilva3 Parke and Rayns (1964) to other prasinophyte genera and to the higher green algal forms is more difficult. The paucity of ultrastructural information on these three genera precludes comparative morphological analy- sis. It is known, however, that the flagella of these three monads are covered by a single layer of flat, circular scales. To date, no similar scale vestiture has been observed on zoospores or on the flagellar structure of any other scaly green flagellate. This observation suggests that these genera represent another distinct evolutionary sequence and thus adds further support to the hypothesis of Pickett-Heaps and Ott (1974) that the Prasinophyceae represent an "ancient assemblage of several distinctly different but related taxonomic groups."

LITERATURE CITED

BARLOW, S. B., AND R. A. CATTOLICO. 1980. Fine structure of the scale-covered green flagellate Mantoniella squamata (Manton and Parke) Desikachary. Br. Phy- col. J. 15: 32 1-333.

DESIKACHARY, T. V. 1972. Notes on Volvocales-I. Curr. Sci. 41: 445-447.

MANTON, I. 1977. Dolichomastix (Prasinophyceae) from arctic Canada, Alaska and South America: a new ge- nus of flagellates with scaly flagella. Phycologia 16: 427-438.

, AND M. PARKE. 1960. Further observations on a small green flagellate with special reference to possible relatives of Chromulina pusilla Butcher. J. Mar. Biol. Ass. U.K. 39: 275-298.

, AND . 1965. Observations on the fine structure of two species of Platymonas with special reference to flagellar scales and the mode of origin of the theca. J. Mar. Biol. Ass. U.K. 45: 743-754.

MATTOX, K. R., AND K. D. STEWART. 1973. Observa- tions on the zoospores of Pseudendoclonium basiliense and Trichosarcina polymorpha. Can. J. Bot. 51: 1425-1430.

, AND . 1974. A comparative study of cell division in Pseudendoclonium basiliense and Tricho- sarcina polymorpha (Chlorophyceae). J. Phycol. 10: 449-456.

, AND . 1977. Cell division in the scaly green flagellate Heteromastix angulata and its bearing on the origin of the Chlorophyceae. Amer. J. Bot. 64: 931-945.

MOESTRUP, 0., AND H. ETTL. 1979. A light and electron microscopical study of Nephroselmis olivacea (Pra- sinophyceae). Opera Bot. 49: 1-40.

NORRIS, R. E. 1980. Prasiniophyceae. Dev. Mar. Biol. 2: 85-147.

PARKE, M., AND D. G. RAYNS. 1964. Studies on marine flagellates. VII. Nephroselmis gilva sp. nov. and some allied forms. J. Mar. Biol. Ass. U.K. 44: 209-217.

PEARSON, B. R., AND R. E. NORRIS. 1975. Fine structure of cell division in Pyramimonas parkeae Norris and Pearson (Chlorophyta, Prasinophyceae). J. Phycol. 11: 113-124.

PICKETT-HEAPS, J. D. 1975. Green algae: reproduction and evolution in selected genera. Sinauer Associates Inc., Sunderland, Mass.

, AND H. J. MARCHANT. 1972. The phylogeny of the green algae: a new proposal. Cytobios 6: 255-264.

, AND D. W. OTT. 1974. Ultrastructural morphology and cell division in Pedinomonas. Cytobios 11: 41-58.

STARR, R. C. 1978. The culture collection of algae at the University of Texas at Austin. J. Phycol. 14(S): 47- 100.

STEWART, K. D., AND K. R. MATTOX. 1975a. Compar- ative cytology, evolution and classification of the green algae with some consideration of the origin of other organisms with chlorophylls a and b. Bot. Rev. 41: 104-135.

, AND . 1975b. Some aspects of mitosis in primitive green algae: phylogeny and function. Bio- systems 7: 310-315.

9 , AND C. D. CHANDLER. 1974. Mitosis and cytokinesis I in Platymonas subcordiformis, a scaly green monad. J. Phycol. 10: 65-79.

3 Comparison of Nephroselmis gilva and the type species N. olivacea suggests that the former must be tax- onomically reassigned (Moestrup and Ettl, 1979; Moe- strup, pers. commun.).



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