PHENOTYPIC PLASTICITY IN SCENEDESMUS COMMUNIS HEGEW. (CHLOROPHYCEAE). II. EXAMPLES OF ALGAL CYCLO- AND NONCYCLOMORPHOSIS

A new dimension to the well-known phenotypic plasticity in Scenedesmus is presented. Large colony type Scenedesmus communis Hegew. reproduced in a typical fashion in batch culture in standard media but produced small colonies (SCT) resembling S. komarekii Hegew. and S. subspicatus Chod. at low cell densities in dilute media. Highest frequency of production occurred after cells had been pretreated in a concentrated (inorganic nutrients) medium. Examples of both cyclic and noncyclic behavior in the life history of Scenedesmus are presented. The noncyclic behavior resembles previous reports on induced heritable changes in flax and tobacco due to different treatment applications of fertilizers. An analysis of gross morphological features using scanning electron microscopy showed that isolated strain SCT1 was similar to S. komarekii (strain UTEX 1236) and isolated strain SCT2 was similar to S. subspicatus (strain UTEX 1358). Growth and morphological expression of the induced SCT strains were highly similar to those of the respective UTEX species. The current state of taxonomy and the implications of phenotypic plasticity on taxonomy in Scenedesmus are discussed.     

THE ULTRASTRUCTURE OF SCENEDESMUS (CHLOROPHYCEAE). I. SPECIES WITH THE “RETICULATE” OR “WARTY” TYPE OF ORNAMENTAL LAYER1

The ultrastructure of the wall layers and ornamentative features of Scenedesmus pannonicus and S. longus are described using carefully correlated freeze-etched replicas, thin sections and scanning electron micrographs. The two species arc enclosed by different types of ornamented layers, S. pannonicus by the tightly filling, “warty” layer and S. longus by the loosely fitting, “reticulate” layer, held off the coenobium by 2 types of tubular propping spikelets and rosettes. The reticulate layer has an intricate substructure, especially when studied with freeze-etching. Its inner and outer surfaces appear different, as is its attachment to the 2 types of spikelets. Whole cells of S. longus subjected to acetolysis lack the cellulose wall and cytoplasm, but all other surface features survive, including the Trilaminar Sheath (TLS); this ornamentation cannot be “pectic.” The cellulose wall and ornamentation is unaffected by boiling water alone. Boiling in 6n NaOH removes the surface ornamentation, but the TLS and wall remain; the possibility that these features contain silica is discussed. The terminal spines of both species consist of closely packed spikelets enclosed within a skin of hexagonally-packed subunits. Similar subunits are seen in the propping spikelets of S. longus, and in the rows or “combs” of laterally fused spikelets of S. pannonicus. The warty layer of S. pannonicus is tightly appressed to the TLS except close to where the cells are joined, where it is suspended free. It is composed of a layer of globular subunits, and small indentations form the warts. Single, evenly distributed warts characterize the freely suspended sections of the warty layer, and the layer that encloses young coenobia soon after they have been formed: in contrast, the warts are clumped over the surface of older and larger colonies. Some of the single warts form characteristic double rows, but these latter remain single even on older cells. The surface structure of the warty layer, TLS, and plasmalemma are revealed by the freeze-etching process.     

Effects of UV‐A Radiation on Desmodesmus armatus: Changes in Growth Rate,Pigment Content and Morphological Appearance

Laboratory cultures of Desmodesmus armatus (R. Chod. ) Hegew. were grown under different levels of photosynthetically active radiation (PAR) supplemented with 3.75 mW · cm^–2 UV‐A radiation. Growth rate was monitored daily, chlorophyl‐a concentration, total carotenoid content, cell number and the relative abundance of different coenobial forms was determined at the end of each experiment. Exposure to UV‐A radiation resulted in an increasing inhibition of growth towards higher PAR levels, reaching 100% at 400 µmol · m^–2 · s^–1. Cellular carotenoid content was higher in the presence of UV‐A radiation, on the other hand no differences were observed in cellular chlorophyll‐a concentration. UV‐A radiation also induced changes in coenobium formation with a decreasing proportion of 4‐celled coenobia and an increase in the abundance of 2‐celled and teratologic coenobia, suggesting that high intensity UV‐A radiation may influence cell cycle events or morphology development. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

THE CELL WALL OF SCENEDESMUS QUADRICAUDA

Bisalputra, T., and T. E. Weier. (U. California, Davis.) The cell wall of Scenedesmus quadricauda. Amer. Jour. Bot. 50(10): 1011–1019. Illus. 1963.—Fine structure of the cell wall of Scenedesmus quadricauda fixed in both KMnO₄ and osmium tetroxide is described. The cell wall consists of 3 layers: the inner cellulosic layer which delimits individual cells; the outer pectic layer which binds the cells of the coenobium together; and a thin middle layer, bounded by membranes on either side, which is electron-dense in osmium-fixed material but of medium electron density in KMnO₄. The structure of the outer pectic layer is similar in both fixatives; it consists of a hexagonal network of electron-dense material on the surface, and a system of tubules or “props” which radiate out from the middle layer of the wall to support the net. The pectic layer appears in the daughter coenobia before their liberation from the parent colony.     

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).     

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.     

INTERCOLONIAL VARIABILITY IN MACROMOLECULAR COMPOSITION IN P‐STARVED AND P‐REPLETE SCENEDESMUS POPULATIONS REVEALED BY INFRARED MICROSPECTROSCOPY1

Macromolecular variability in microalgal populations subject to different nutrient environments was investigated, using the chlorophyte alga Scenedesmus quadricauda (Turpin) Bréb. as a model organism. The large size of the four‐cell coenobia in the strain used in this study (～35 μm diameter) conveniently allowed high quality spectra to be obtained from individual coenobia using a laboratory‐based Fourier transform infrared (FTIR) microscope with a conventional globar source of IR. By drawing sizable subpopulations of coenobia from two Scenedesmus cultures grown under either nutrient‐replete or P‐starved conditions, the population variability in macromolecular composition, and the effects of nutrient change upon this, could be estimated. On average, P‐starved coenobia had higher carbohydrate and lower protein absorbance compared with P‐replete coenobia. These parameters varied between coenobia with histograms of the ratio of absorbance of the largest protein and carbohydrate bands being Gaussian distributed. Distributions for the P‐replete and P‐starved subpopulations were nonoverlapping, with the difference in mean ratios for the two populations being statistically significant. Greater variance was observed in the P‐starved subpopulation. In addition, multivariate models were developed using the spectral data, which could accurately predict the nutrient status of an independent individual coenobium, based on its FTIR spectrum. Partial least squares discriminant analysis (PLS‐DA) was a better prediction method compared with soft independent modeling by class analogy (SIMCA).     

The photosynthesis of individual algal cells during the cell cycle of Scenedesmus quadricauda studied by chlorophyll fluorescence kinetic microscopy

A microscope for imaging of chlorophyll fluorescence kinetics was equipped with a chamber that allows the growth of an immobilised population of algae and their study under well-defined conditions. Single cells of the chlorococcal alga Scenedesmus quadricauda were grown and recorded for periods of whole cell cycles (up to 48 h) displaying a normal course of cell development. Heterogeneity in fluorescence yield among individual coenobia in the population and among different cells in one coenobium were analysed. Differences were observed both in the shape of Kautsky transients and in the modulation of fluorescence parameter values during the progress of the cell cycle. The extent of heterogeneity in fluorescence parameters was cell cycle dependent – in some phases of the cycle, the population was almost homogeneous, while distinct heterogeneity was observed, in particular between the protoplast division and the release of the daughter coenobia. The heterogeneity was not random but reflected developmental processes.     

Adaptation strategies of two closely related Desmodesmus armatus (green alga) strains contained different amounts of cadmium: A study with light-induced synchronized cultures of algae

During the Desmodesmus armatus cell cycle, 8-celled coenobia of 276-4d strain accumulated a much lower amounts of cadmium than unicells of B1-76 strain. Cadmium reduced growth and photosynthesis in the cells of strain B1-76, but not those of 276-4d strain. Cells of 276-4d strain revealed a higher activity of superoxide dismutase (SOD) isoforms, in particular the activity and protein content of Fe-SOD. Cu/Zn-SOD was earlier and much stronger induced by cadmium in 276-4d than in B1-76 strain, whereas Fe- and Mn-SOD activity and Fe-SOD synthesis were induced only in 276-4d strain. Cadmium did not affect the heat shock protein 70 synthesis in B1-76 strain, but significantly stimulated this process in 276-4d strain. The level of glutathione increased 30-fold during cell development of Cd-exposed 276-4d strain, while in B1-76 it increased about 12 timed. Matured cells of both strains exposed to cadmium produced comparable amounts of phytochelatins and other thiol peptides, but their production in young cells of B1-76 strain was much higher than in 276-4d strain. In conclusion, a complex of internal detoxification mechanisms appeared to be more efficient in cells of 276-4d strain than B1-76 one.     

Structure and chemical composition of endodermal and rhizodermal/hypodermal walls of several species

CWM, isolated cell wall material
ECW, isolated endodermal cell walls
G, guaiacyl monomer
H, p-hydroxyphenyl monomer
HCW, isolated hypodermal cell walls
RHCW, isolated rhizodermal and hypodermal cell walls
S, syringyl monomer
XV, isolated xylem vessels

Endodermal cell walls of the three dicotyledoneous species Pisum sativum L., Cicer arietinum L. and Ricinus communis L. were isolated enzymatically and analysed for the occurrence of the biopolymers lignin and suberin. From P. sativum, endodermal cell walls in their primary state of development (Casparian strips) were isolated. Related to the dry weight, these isolates contained equal amounts of suberin (2·5%) and lignin (2·7%). In contrast, the endodermal cell walls of C. arietinum and R. communis, which were nearly exclusively in their secondary state of development, contained significantly higher proportions of suberin (10–20%) and only traces of lignin (1–2%). The results of the chemical analyses were supported by a microscopic investigation of Sudan III-stained root cross-sections, showing a Casparian strip restricted to the radial walls of the endodermis of P. sativum and well-pronounced red suberin lamellae in C. arietinum and R. communis roots. Compared with recently investigated monocotyledoneous species, higher amounts of suberin by one order of magnitude were detected with the secondary state of development of dicotyledoneous species. Furthermore, the carbohydrate and protein contents of primary (Clivia miniata Reg. and Monstera deliciosa Liebm.), secondary (C. arietinum and R. communis) and tertiary endodermal cell walls (Allium cepa L. and Iris germanica L.) were determined. The relative carbohydrate content of secondary endodermal cell walls was low (14–20%) compared with the content of primary (42–50%) and tertiary endodermal cell walls (60%), whereas the protein content of isolated endodermal cell walls was high in primary (13%) and secondary (8%) and low in tertiary endodermal cell walls (0·9–2%). The results presented here indicate that the quantitative chemical composition of primary, secondary, and tertiary endodermal cell walls varies significantly. Finally, cell wall proteins are described as an additional important constituent of endodermal cell walls, with the highest concentrations occurring in primary (Casparian strips) and secondary endodermal cell walls.     

Characterization of a green alga isolated from infected human external tissue

A green alga was isolated from infected external human tissue. The alga consisted of spherical non‐flagellate unicells, 3–13 µm in diameter, surrounded by a thick outer wall of variable thickness and containing a single bowl‐shaped chloroplast without a pyrenoid. Asexual reproduction occurred through multiple autospores produced within the parental cell wall. 18S rRNA and 16S rRNA gene‐sequence analyses indicated that the alga was closely related to ‘Chlorella’saccharophila. d ‐glucose considerably enhanced both phototrophic and heterotrophic growth. Compared with two other strains of ‘C.’saccharophila, the organism in the present study grew more rapidly at temperatures greater than 30°C in darkness, but it could not grow at 37°C in light or darkness. The results suggest that this strain may not normally invade tissues, but becomes established and grows on previously infected tissues of external body extremities where the temperature is somewhat lower than normal body temperature.     

ANATOMY OF MACROZAMIA COMMUNIS LATERAL ROOTS AND ROOT NODULES FORMED IN VITRO STUDIED WITH LIGHT AND SCANNING ELECTRON MICROSCOPY

The anatomy of Macrozamia communis L. Johnson lateral roots and nodules was studied following axenic culture in light and darkness. Pointed lateral roots from dark cultures had an open apical organization similar to that of other cycads and gymnosperms. A distinct protoderm-derived epidermis was not observed. At the apex, the dermis was formed by the outer root capcortical cell layer. Subapically, the outer cortex formed the dermis. No evidence of an algal zone was observed in these roots. The stele was bounded by a distinct endodermis and contained an exarch, diarch xylem. Apogeotropic nodules which developed at the root-shoot junction in darkness, branched dichotomously and had rounded tips covered by tangentially-enlarged root cap cells. The root cap was reduced to a few cell layers and was confined to the extreme nodule apex. The central region of the apical meristem was enlarged, and meristematic cells contained differentiated amyloplasts. A presumptive algal zone was present in some but not all nodules and divided the cortex into inner and outer regions. Stelar anatomy was similar to that observed in pointed, dark-grown lateral roots, except that there was greater xylem differentiation. Nodules which developed in the light were similar to dark-formed nodules, except that root cap cells were radially enlarged and extended over the flanks of the nodule forming a persistent root cap. The heteromorphic lateral roots of M. communis formed a developmental continuum not a heterorhizic root system.     

LAGERHEIMIA HINDAKII IS THE UNICELLULAR STAGE OF A SCENEDESMUS1

Extensive variability in spine number and position was emphasized in the protologue for the spiny unicell Lagerheimia hindakii Hegew. et A. Schmidt. Using an axenic clone established from culture Hindk 1975/95, which provided the type material of that organism, we initiated a morphogenetic study. While in the unicellular stage, three short spines were usually formed at each cell pole. A colonial morph was first observed during the initial isolation procedure after unicells were dispersed on firm agar and allowed to grow. We propose that L. hindakii is a Scenedesmus, close to S. subspicatus R. Chod. This organism is unlike most representatives of the genus Scenedesmus because colonies do not readily appear in dilute media; they also fail to form in Bristol's liquid medium unless there is an organic supplement, e.g. glucose. Hindk 1975/95 may be more closely related to truly unicellular taxa than to other Scenedesmus.     

Seed Epidermis Development and Histochemistry in Solanum melongena L. and S. violaceum Ort.

Structure, development and histochemistry of the seed epidermiswere studied inSolanum melongena L. andS. violaceum Ort. usinglight and scanning electron microscopy. The epidermal cellsat the endosperm mother cell stage of ovule development hadthickened outer periclinal walls, consisting of two layers,a thin inner layer, and a thick outer layer. The latter whichstained positively for pectic substances became further thickenedduring the course of seed development; more so inS. melongena.The inner layer of the outer periclinal wall also was thickenedby depositions of cellulose but remained comparatively thin.The development of the inner periclinal and anticlinal wallstook place by the uneven deposition of concentric layers. Thesesecondary wall thickenings which appeared as pyramids in transversesection stained for cellulose, lignin and pectin. Further unevensecondary thickenings near the outer part of the anticlinalwalls resulted in the formation of projections which were hair-or ribbon-like in appearance. InS. melongena, these projectionsprogressed only a short distance from the anticlinal wall. InS.violaceum, on the other hand, they grew much longer formingstriations on the inside of the outer periclinal wall. InS.melongena, partial removal of the outer periclinal wall by enzymeetching exposed to surface view a beaded appearance of the cellboundaries. Complete erosion of the outer periclinal wall revealedthe hair-like projections of the underlying anticlinal walls.InS. violaceum, enzyme treatment exposed the striations whichformed bridge-like structures over the curves in the anticlinalwalls. Solanum melongena ; Solanum violaceum; seed epidermis; seed structure; seed development; cell wall histochemistry; cell wall projections; cell wall striations     

ULTRASTRUCTURE AND DEVELOPMENT OF THE MICROSPORANGIUM OF PSEUDOTSUGA MENZIESII (MIRB.) FRANCO. I. DORMANT STAGES

The dormant (mid-November to mid-February) microsporangia of Pseudotsuga menziesii (Douglas-fir) contain pollen mother cells (PMC's) in diffuse diplotene, surrounded by 1–2 layers of tapetal cells and 3–4 layers of microsporangial wall cells. At the beginning of dormancy, PMC's are large and their walls are lysed. The cell walls contain a thick layer of loosely-arranged fibrils which are produced in large vesicles in the PMC cytoplasm and are secreted across the plasma membrane. PMC's contain several layers of rough ER. The inner tangential and the radial walls of the tapetal cells are lysed. During dormancy the PMC's form many new autophagic vacuoles, the chromatin consists of a network of fine threads comprised of medium-sized granules of uniform size and the nucleoli split. The outer tapetal wall is thick and becomes encrusted by an irregular lipid layer. The tapetal cytoplasm is similar to the PMC cytoplasm but is devoid of amyloplasts. The tapetal cytoplasm shows secretory activity at the beginning of dormancy and again near the end of dormancy. The later secretory activity results in the deposition of a spongy material, especially along the radial and inner walls of the tapetal cells. Tapetal cells contain 1–2 large nuclei which show prominent and irregular clumps of chromatin. Subcellular developmental changes occur in the dormant microsporangia of Pseudotsuga in much the same manner as has been reported for Pinus.     

Monograph of the moss genus Macrodictyum (Broth.) E.H.Hegew. (Dicranaceae)

Abstract

Macrodictyum is a small acrocarpous moss genus of three species (M. latifolium (E.B.Bartram) M.J.Price, M. proliferum (Mitt.) E.H.Hegew., M. wrightii (Sull.) E.H.Hegew.). It grows in tufts and mats on trees or rocks at 450–1500 m in the Neotropics. The genus has a restricted distribution within the Neotropics and is currently known from only a few localities in Cuba, the Dominican Republic, and southeastern Brazil. Macrodictyum is characterized by four main features: 1) appressed, non-crispate, ovate–oblong, lingulate to ligulate or linear leaves; 2) large, rhomboidal, thin-walled cells in the upper leaf; 3) large, irregular, multicellular apical teeth; and 4) guide cells that are ventral at and above mid-leaf. These features are atypical of the Dicranaceae. However, Macrodictyum shares pseudautoicous sexuality, sheathing perichaetial leaves, and peristome structure with the large pantropical Dicranaceous genus Holomitrium Brid. In these two genera the peristome consists of 16 single, triangular, entire or weakly divided, fenestrate, striate teeth that are papillose throughout on both the inner and outer surfaces. Macrodictyum is retained in the Dicranoideae (Dicranaceae), close to Holomitrium, based on its pseudautoicous sexuality, sheathing perichaetial leaves, and peristome structure.     

SCENEDESMUS WALL ORNAMENTATION. I. SCENEDESMUS PARISIENSIS CULTURES1

Four strains of Scenedesmus parisiensis Chodat were studied in xenic and axenic culture in 3 media as well as in cultures incubated in sterile vessels in nature. Organized coenobia are usually produced but these may have merely short spines, spines and serrate edges, or lack wall ornamentation. Because the serrate edge is either not formed or cannot be readily detected in most cases, it is not a satisfactory morphological feature for delimiting this species. In laboratory studies it is noted that S. parisiensis might be confused with S. denticulatus rather than S. brasilien-sis. Inasmuch as both xenic and axenic cultures of the -f strains produced similar results, S. parisiensis can be readily characterized.     

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.     

PLOIDAL CHANGES IN CLONAL CULTURES OF SPIROGYRA COMMUNIS AND IMPLICATIONS FOR SPECIES DEFINITION

A clonal culture of Spirogyra filaments of initially uniform width produced filaments of three additional significantly different widths. Group I filaments of the original clone were 30.9 ± 0.7 μm wide (mean ± SD, N = 50). Group I filaments produced Group II filaments (22.0 ± 1.1 μm) through vegetative growth and sexual reproduction. Zygospores from homothallic Group I filaments produced germlings representative of Groups I and II; zygospores from homothallic Group II filaments produced germlings representative of Group II only. Germlings of Groups III (27.7 ± 1.0 μm) and IV (44.9 ± 0.8 μm) were produced in the cross of I × II. Viable zygospores from homothallic Group III filaments were obtained. Cells of Group IV filaments were initially binucleate and did not conjugate. Of the six intergroup crosses possible, four resulted in conjugation-tube formation only; two crosses yielded zygospores (I × II and III × IV). Germlings from the successful cross of Groups III and IV produced filaments of all four groups. Chromosome counts were: Group I (24), Group II (12), Group III (18), and Group IV (24, one nucleus). Relative nuclear fluorescence values of mithramycin-stained DNA were (mean ± SD, N ≥ 30): Group I (11.1 ± 1.4), Group II (5.7 ± 0.7), Group III (8.8 ± 1.3), and Group IV (10.0 ± 0.9, one nucleus). Cytologically, Group II appears to be a diploid (2x), Group I a tetraploid (4x), and Group III a triploid (3x). Systematically, Groups I, II, and III key out to Spirogyra singularis, S. communis, and S. fragilis, respectively, using Transeau's mongraph of the family Zygnemataceae. These species are interpreted to represent a species complex of S. communis (whose name has priority) with the ancestral haploid (x = 6) missing.