Pleomorphism inScenedesmus quadricauda (Turp) Bréb. (Chlorophyceae)

A clone ofScenedesmus quadricauda, isolated from Tjeukemeer, exhibits a high degree of morphological variation in synchronized cultures. Cells are synchronized by light-dark cycles. During the photoperiod they build up the capacity to divide. First division into 2- and 4-celled coenobia is induced, then during the second half of the photoperiod the induction of division into 8 unicells takes place. Division itself and the subsequent liberation of daughter cells occur in the dark period.By giving a definite photoperiod the formation of either coenobia or unicellular stages is determined. The formation of both coenobia and unicells is followed using a light microscope. In both cases only the pattern of cytokinesis is similar. After cytokinesis the unicells become ovoid in shape and form two spines at each pole. They are released from the parental wall as separate cells and show remarkable similarity to theChodatella-like cells described by SWALE (1967) and FOTT (1968). The coenobial cells elongate, adhere to one another and each of the two outmost cells forms two spines (SMITH, 1914).     

Zellteilung und Bildung von Innenschalen beiHantzschia amphioxys undAchnanthes coarctata

Before cytokinesis, the identically constructed chromatophores ofHantzschia amphioxys andAchnanthes coarctata are transformed into less effigurated bodies. In normal cytokinesis, the course of mitosis, chromatophore division, and cleavage furrowing are exactly synchronized. The division of the chromatophore appears as a passive process, i.e. intersection by the cleavage furrow. In inequal cell divisions before the formation of inner valves cytokinesis can take place without chromatophore division. Once chromatophore division without mitosis and cytokinesis was observed. InHantzschia there are three types of inner valve formation, inAchnanthes coarctata only two. The inner valves develop under unfavorable growth conditions, the cells possessing them, however, are not resting spores as in some other diatoms. InHantzschia, auxospore formation is suppressed under the cultural conditions used, the cells multiply intensely without diminution.

     

CONTROL OF COLONY AND UNICELL FORMATION IN A SYNCHRONIZED SCENEDESMUS12

Colony formation in a synchronized Scenedesmus could be controlled by the addition of 0.05% Na₃ citrate or 85 μM EDTA to modified Bristol's medium. No unicells were formed; only S. quadricauda-like or S. longispina-type colonies were observed in young cultures grown in that medium. A colony population could be made completely unicellular in 2 days if grown in soil-Bristol's medium and transferred daily. The pleomorphic Scenedesmus was examined in synchronized culture. When the organism was grown in a defined medium in a 15-hr light /9-hr dark cycle on a roller tube rotator at 2-6 rpm and transferred daily, synchrony of cell division and release of the products were achieved. In a synchronized culture 2 doublings/day were recorded, with most cytoplasmic cleavages and all releasing of daughter cells taking place in the dark period. In many observations with several synchronized strains of Scenedesmus, no fixed pattern of release of daughter products from mother cells or colonies was detected. Colonies or unicells had their full spine complement at the time of release. As a cell or colony aged the spines sometimes increased in thickness. Other Scenedesmus strains were examined to provide supporting data.     

PHENOTYPIC PLASTICITY IN SCENEDESMUS COMMUNIS (CHLOROPHYCEAE). I. RELATIONSHIP OF S. COMMUNIS TO S. KOMAREKII1

When scenedesmus communis Hegew. (UTEX 76) was transferred daily in dilute media and a low cell density was maintained (ca. 1000 cells · mL^?1), up to 30% unicells were produced in that population. Unlike previously described uncell-coenobium-unicell transformation with other species, these unicells never produced S. communis coenobia (large coenobium type, LCT) but rather small coenobium type (SCT) resembling S. komarekii Hegew. Growth and morphological development of the paratype strain of S. komarekii (UTEX 1236) was compared with an isolated SCT strain (SCT 76–8). SCT 76–8 never produced LCTs and grew significantly faster than UTEX 1236. Both SCT 76–8 and UTEX 1236 produced uncells at low cell densities. Coenobia formed when cell densities increased over time in batch cultures. SCT 76–8 and UTEX 1236 did not differ morphologically when viewed with the light microscope. Under scanning electron microscopy, an outer opaque layer covered an inner warty layer on unicells. The outer layer was reduced or absent in coenobia from batch cultures in stationary growth. In addition, long spikelets, not present on the walls of unicells, were prominent on coenobial walls. The spikelets of UTEX 1236 appeared smaller and more uniformly distributed than in strain 76–8. In contrast, the surface wall morphology of LCT S. communis was composed of an outer reticulate layer supported by spikelets and appeared as a pentagonal meshwork covering the cell walls. This phenotypic plasticity, as demonstrated by SEM and light microscopy, provides further evidence needed for an understanding of Scenedesmus evolution.     

A clone of Scenedesmus with Chodatella-stages

A clonal culture of an organism isolated as Chodatella quadriseta Lemm. developed coenobial stages resembling Scenedesmus quadricauda (Turp.) Bréb. The sequence of these changes was followed, the population being found to consist almost entirely of unicells when young, but with a regular increase in coenobia with increasing age. Definition of the coenobia was attempted by numerical analysis of shape and size. Another clone, from a different habitat, was found to behave similarly. Some of the taxonomic implications of this form of pleomorphism are discussed.     

Hydroxyurea-induced mitotic synchronization in Allium sativum root meristems

The synchronized divisions following a treatment with hydroxyurea (HU) — an inhibitor of DNA synthesis — were studied in root meristems of Allium sativum using two methods: autoradiography of median sections and morphological labeling with a cytokinesis inhibitor. It is shown that the second wave of mitoses is heterogeneous: it is composed mostly of cells which have been synchronized in the S phase by the HU treatment, of cells coming from the quiescent center stimulated to enter DNA synthesis and of cells which were not blocked by the 23 h HU treatment (slow cycling cells). It is also shown that the cell cycle following the first synchronized division is considerably shortened by the synchronization procedure.Abbreviations QC quiescent center - HU hydroxyurea - MHQD methyl-3 hydroxy-6 quinazoline dione 2–4     

cAMP uncouples DNA synthesis and cell division in Tetrahymena

Tetrahymena cells were synchronized by a differential density labeling method. Millimolar concentrations of db-cAMP will cause a delay in the division of the synchronized cells only if added before late S period. The effective concentrations will also cause a precocious initiation of DNA synthesis during G2 just prior to cell division, suggesting that the cyclic nucleotide causes an uncoupling of the program of DNA synthesis and that of karyo- and cytokinesis.     

A STUDY OF WALL ORNAMENTATION IN CULTURES OF SCENEDESMUS

In a comparative study, six strains of Scenedesmus isolated from one small pond were examined in culture. Three of the isolates, S. denticulatus, S. dispar, and S. abundans, proved to be morphologically quite stable when studied in the laboratory. In an axenic culture of any one of the three pleomorphic strains one could find coenobia which resemble S. longispina or S. armatus. High percentages of four-celled coenobia with several ridges but with only two spines (the S. semipulcher type) were found typically in older cultures of two of the pleomorphic cultures. Long, delicate appendages (bristles) were easily detected on most coenobia in desiccated preparations of all but the S. dispar culture. The use of cultures in identifying Scenedesmus coenobia is discussed.     

Generic concept in Chlorella‐related coccoid green algae (Chlorophyta,Trebouxiophyceae)

Using a combined set of sequences of SSU and ITS regions of nuclear‐encoded ribosomal DNA, the concept of the experimental algal genus Chlorella was evaluated. Conventionally in the genus Chlorella, only coccoid, solitary algae with spherical morphology that do not possess any mucilaginous envelope were included. All Chlorella species reproduce asexually by autospores. However, phylogenetic analyses showed that within the clade of ‘true’Chlorella species (Chlorella vulgaris, C. lobophora, and C. sorokiniana), taxa with a mucilaginous envelope and colonial lifeform have also evolved. These algae, formerly designated as Dictyosphaerium, are considered as members of the genus Chlorella. In close relationship to Chlorella, five different genera were supported by the phylogenetic analyses: Micractinium (spherical cells, colonial, with bristles), Didymogenes (ellipsoidal cells, two‐celled coenobia, with or without two spines per cell), Actinastrum (ellipsoidal cells within star‐shaped coenobia), Meyerella (spherical cells, solitary, without pyrenoids), and Hegewaldia (spherical cells, colonial, with or without bristles, oogamous propagation). Based on the secondary structures of SSU and ITS rDNA sequences, molecular signatures are provided for each genus of the Chlorella clade.     

Mitosis and cytokinesis inTribonema regulare (Tribophyceae,Chrysophyta)

Summary The ultrastructure of mitosis and cytokinesis of the uninucleateTribonema regulare has been investigated by employing transmission electron microscopy. Prophase is characterized by settlement of a pair of centrioles at the presumptive poles of the spindle, metaphase by equatorial bulging of the nucleus, anaphase by non-synchronous separation of the chromosomes, and telophase by a persistent, strongly elongated, interzonal spindle. Throughout mitosis, at each pole dictyosomes are associated with the polar gaps of the nuclear envelope that otherwise remains intact. Cytokinesis does not immediately follow mitosis; from the static images it can be concluded that it is necessary for the daughter nuclei to approach each other before cytokinesis is initiated by complete division of the protoplast via plasma membrane cleavage. Afterwards, a ring of cell wall material is deposited close near the lateral wall in the plane of protoplast separation followed by a simultaneous or centripetal development of a single integral partitioning septum. Once the septum is completed, the cylindrical portion of the H-shaped segment is manufactured. The phylogenetic position ofTribonema amongst those algae, which may have evolved from unicells into filaments, is discussed.