Amitosis in Three Species of Euglena

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1 Amitosis in Three Species of Euglena Gordon F. Leedale Department of Botany, The Durham Colleges in the University of Durham, England Received July 29, 1958 Amitosis is a division of the nucleus without the emergence, orientation and division of individualised chromosomes. This "direct" division of nuclei is known to occur, notably in glandular tissues of Arthropods (Conklin 1917, Kater 1940) and in the endosperm of seeds (Cutter et al. 1955), but many descriptions of an amitotic nuclear division have subsequently been shown to be wrong. Amitosis is rarely followed by cell cleavage, the process usually resulting in permanently binucleate cells. Cleavage subsequent to amitosis inevitably produces aneuploid cells and it is therefore thought that the amitotic process is degenerative and cannot be followed by a normal mitosis. Amitosis has been reported in mammalian tissues (Macklin 1916, Macmahon 1933, Clara 1935, Stough 1935) but there is no positive evidence of cytodieresis or sub sequent development of the resulting aneuploid cells. In the course of an investigation into the structure and division of the nucleus in the Euglenineae (Leedale 1958 a, 1958 b), involving 40 species drawn from 14 genera, a process of amitosis was discovered in three species of the genus Euglena. Jn the present paper this process is described and compared with previous records of amitosis for this group of flagellates. Material and methods The three species in which amitosis occurred, Euglena acus Ehrenberg, Euglena spirogyra Ehrenberg, Euglena viridis Ehrenberg, were inoculated into biphasic culture (Pringsheim 1946 a, 1946 b) from wild collections and from material from the Cambridge Culture Collection of Algae and Protozoa (Cultures 1224/1, 1224/13 and 1224/17a) and the Sammiung von Algenkulturen, Gottingen. Strains were isolated by both Professor E. G. Pringsheim and myself. The cultures were grown on racks in a north facing window at a temperature of 20 Ž. Material was fixed in methanol or Carnoy's acetic-alcohol and stained with aceto-carmine following a ferric acetate mordant, or by the Feulgen reaction. Living cells were examined by positive phase contrast and Reichert Anoptral contrast. Cytologia

2 214 G. F. Leedale Cytologia 24 Observations In biphasic cultures of Euglena acus and Euglena spirogyra, cells oc curred with two resting nuclei in the interphase position in the centre of Figs , resting nucleus in a uninucleate cell of Euglena aces. 2-4, constricted nuclei in cells of Euglena aces. 5 and 6, binucleate cells of Euglena aces. 7, cell of Euglena viridis with a constricted resting nucleus. 8, cell of Euglena viridis with two part-nuclei. (Figs. 1-8 all ~2000, aceto-carmine). the cell (Figs. 5 and 6). The chromatin was often more filamentous in appearance than in the single, elongated resting nucleus (Fig. 1). The two

3 1959 Amitosis in Three Species of Euglena 215 nuclei were usually similar in size and together had the same volume as a single resting nucleus. In addition to these binucleate cells, cells with intermediate stages between a single, oval nucleus and two separate, spherical ones were found (Figs. 2-4). Under anoptral contrast, the constricted nuclei were seen to be en closed in a single nuclear membrane, whereas each nucleus of the binucleate cell had its own membrane. The two portions of a constricted nucleus showed no change in their relative positions within the cell over a period of several hours. Figs Euglena spirogyra: Late anaphase of mitosis in a binucleate cell. 9 and 11, two focal levels of the double division figure. 10 and 12, camera lucida drawings at the same focal levels as figs. 9 and 11. (Figs. 9 and 11 aceto-carmine, Figs all ~2000). Nuclei of normal cells contained 1 to 8 (to 20+) endosomes ("nucleoli"), the commonest numbers being 4, 5 or 6 (Fig. 1). The half-nuclei contained 1 to 10 endosomes, the number usually being the same in each nucleus of a binucleate cell or in each part of a constricted nucleus (Figs. 2, 3 and 5). Rarely, one half of a constricted nucleus contained all the endosomal material (Fig. 4). In biphasic culture, 4% of the cells were binucleate or had constricted nuclei in Euglena acus, 3% in Euglena spirogyra. Binucleate cells were 15*

4 216 G. F. Leedale Cytologia 24 more frequent than cells with constricted nuclei in the ratio of 3 to 2 for both species. Both cell-types were absent from wild material. In neither species did cell cleavage occur to separate the two nuclei of a binucleate cell. The binucleate condition was not connected with re production. The most interesting feature of these binucleate cells was the behaviour of the nuclei during mitosis. Prior to mitosis in uninucleate cells the nucleus moves to the cell anterior, and during mitosis endosomal material divides as a single body in Euglena spirogyra, the several endosomes of interphase having fused during prophase (Leedale 1958 b). In binucleate cells both nuclei moved to the anterior during prophase but divided separately, lying one behind the other, each small nucleus having a single dividing endosome in Euglena spirogyra (Figs. 9-12) and several dividing endosomes in Euglena acus. In Euglena spirogyra the chromosome number in each of the four late anaphase groups was between 40 and 45 (Figs. 9 and 10), about half the normal chromosome number of 86 in this species (Leedale 1958 a). Cell and organelle cleavage proceeded as in uninucleate cells, producing two daughter Fig. 13. Miscleavage after mitosis in a binucleate cell cells, each with two half-size of Euglena spirogyra. (Aceto-carmine, ~1200). nuclei. Movement of the nuclei during cleavage sometimes resulted in the inclusion of one half-nucleus in one cell and three in the other (Fig. 13). Cells with three small nuclei were seen in both Euglena acus and Euglena spirogyra. The duration of mitosis in a binucleate cell was of the same order as that in a normal cell (three to four hours (Leedale 1958a, 1959)). The nuclei in binucleate cells of Euglena viridis (also occurring only in cultured material) were more irregular in size and shape. A fragmentation of the nucleus occurred (Fig. 7), resulting in large and small part-nuclei (Fig. 8), Each portion of the nucleus contained some endosomes (Figs. 7 and 8). Subsequent mitosis was not observed in this species. No amitosis was found in wild or cultured material of 40 other species of the Euglenineae, drawn from the genera Astasia, Colacium, Cryptoglena, Distigma, Euglena, Eutreptia, Hyalophacus, Lepocinclis, Menoidium, Pera nema, Phacus, Rhabdomonas, Strombomonas and Trachelomonas.

5 1959 Amitosis in Three Species of Euglena 217 Discussion The binucleate condition in Euglena acus, Euglena spirogyra and Euglena viridis cannot have arisen (as occasionally happens in Euglena gracilis and Euglena deses) by incorrect cleavage following a normal mitosis. Such an occurrence has never been observed in living cells of these species; binucleate cells occur too frequently to be cleavage abnormalities; and, if the nuclei were the products of a true mitosis, each would be the size of a normal resting nucleus. In Euglena spirogyra it is clear that the two nuclei of a binucleate cell together constitute a single chromosome complement, the chromosome number in each half-nucleus being approximately half that of the species. There can be no doubt that amitotic splitting of the resting nucleus does occur in these three species to give two smaller nuclei. It it not certain whether the constricted nuclei are stages in a slowly occurring division, or whether they are nuclei which have become arrested after undergoing an incomplete amitosis. Accounts of amitosis in the Euglenineae have been given by Steuer (1904) for Eutreptia viridis, Dobell (1908) for Copromonas subtilis (?=Scytomonas pusilla), and Wenrich (1924) for Euglenamorpha hegneri. Although an amitotic process is now being recorded for three species of the Euglenineae, the records of amitosis by Steuer (1904) and Dobell (1908) are thought by the present author to be incorrect. These workers have called the nuclear division an amitosis because the chromosomes were not seen, the nucleus apparently dividing as a homogeneous body; the amitosis is stated to be the only form of nuclear division in the species concerned; and their descriptions refer to wild material. In the three species for which an amitotic process is recorded in the present paper, the chromatin is clearly seen in a resting state; the main form of nuclear division is mitosis; and the amitosis occurs only in biphasic culture. Leedale (1958 b) has now given a description of mitosis in Eutreptia viridis. The amitosis found by Wenrich (1924) in EuglenamorPha hegneri is a similar nuclear fragmentation to that described here for species of Euglena. He saw clearly the resting state of the constricting nuclei. Furthermore, nuclear division in green cells was by mitosis, the amitosis occurring only in degenerate cells which had lost their chromatophores. He observed no cell cleavage following amitosis but considered it probably took place. This is unlikely, the binucleate products of amitosis were probably degenerate as in Euglena viridis, and not even viable units as in Euglena acus and Euglena spirogyra. Degenerative amitosis in other Protozoa has been recorded by Kofoid (1923) and Kater (1930).

6 218 G. F. Leedale Cytologia 24 In some euglenoid species, the chromosomes at telophase consist of pairs of chromatids which will be the daughter chromosomes of the next mitosis (Leedale 1958 b). This is so in Euglena acus, but the pairs of chromatids must remain closely associated in the resting nucleus, the mitosis of binucleate cells showing that sister chromatids are always in the same half-nucleus. In Euglena spirogyra the chromatids are not seen until the metaphase of the mitosis during which they segregate (Leedale 1958 b). The binucleate cell is genetically identical with the normal, uninucleate one. In Euglena acus and Euglena spirogyra the binucleate cell is a viable and healthy unit. The nuclear fragmentation may have the function of in creasing the nuclear surface (see Conklin 1917, Kater 1940). In Euglena viridis the binucleate cell is degenerate, the number of amitoses increasing with the age of the culture. In the rare case of miscleavage following mitosis in a binucleate cell, individuals are formed having either half or one-and-a-half times the normal chromosome complement. Cells with one half-nucleus and three half-nuclei have been isolated into culture and the life-history and behaviour of the clones are being studied. The cells are viable and capable of multiplication by mitosis and cytodieresis, retaining the cytological characteristics of the species. If these species are highly polyploid (a possibility suggested by their high chromosome numbers (Leedale 1958 a)), the loss or gain of genomic material may make little difference to the individual cell. Summary Amitosis is recorded for Euglena acus, Euglena spirogyra and Euglena viridis, occurring only in biphasic culture. The process is a nuclear frag mentation not connected with reproduction, the product being a binucleate cell. The binucleate cell is capable of subsequent mitosis in Euglena acus and Euglena spirogyra, the half-nuclei dividing simultaneously. Cell cleavage following this mitosis produces two binucleate cells. Rare miscleavage results in cells with one half-nucleus (approximately half the specific chromosome complement) and with three half-nuclei (one-and-a-half times the specific chromosome complement). These types of cells are viable and their progeny retain the cytological characteristics of the species. References Clara, M Untersuchungen fiber Wachstum und Regeneration der Nierenepithelien. Z. Anat. Entw. Gesch. 104: Conklin, E. G Mitosis and amitosis. Biol. Bull. 33: Cutter, V. M., Wilson K. S. and Freeman, B Nuclear behaviour and cell formation in the developing endosperm of Cocos nucifera. Amer. J. Bot. 42: Dobell, C. C The structure and life-history of Copromonas subtilis, nov. gen, et nov. spec.: a contribution to our knowledge of the Flagellata. Quart. J. micr. Sci. 52:

7 1959 Amitosis in Three Species of Euglena 219 Kater, J. McA The question of amitotic division in flagellates. Northwest Sci. 4: Amitosis. Bot. Rev. 6: Kofoid, C. A The life cycle of the Protozoa, Science 57: Leedale, G. F. 1958a. Mitosis and chromosome numbers in the Euglenineae. Nature 181: b. Nuclear structure and mitosis in the Euglenineae. Arch. Mikrobiol. 32: The time-scale of mitosis in the Euglenineae. Arch. Mikrobiol. 32: Macklin, C. C Amitosis in cells growing in vitro. Biol. Bull. 30: Macmahon, H. E Ober die physiologische und pathologische Teilung von Kern und Zelle an Leberepithelien. Jb. Morph. Mikro. Anat. 32: Pringsheim, E. G. 1946a. 'Pure Cultures of Algae.' Cambridge Univ. Press b. The biphasic or soil-water culture method for growing algae and flagellata. J. Ecol. 33: Steuer, A. 1904, Uber eine Euglenoide (Eutreptia) aus dem Canale Grande von Triest. Arch. Protistenk. 3: Stough, H. B Further studies in modified mitosis. J. Morph. 58: Wenrich, D. H Studies on Euglenamorpha hegneri n. g., n. sp., a euglenoid flagel late found in tadpoles. Biol. Bull. 47: