What does Heterokontophyta mean

heterokontophyta key features,heterokontophyta classification and heterokontophyta characteristics. clasificacion taxonomica heterokontophyta
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Heterokontophyta related to the Xanthophyceae and Phaeophyceae. PHAEOTHAMNIOPHYCEAE The cytology of these three classes is similar (Fig. 20.1). The cells have two membranes of chloro- Recent nucleotide sequencing has uncovered an plast endoplasmic reticulum with the outer evolutionary line of golden-brown algae not membr ane of chloroplast E.R. continuous with related to other golden-brown algae (Bailey et al., the outer membrane of the nuclear envelope. 1998). These algae have been placed in the class The chloroplasts have a ring-shaped genophore Phaeothamniophyceae, a class that is most closely and girdle lamellae. The flagella are inserted Fig. 20.1 A filament and zoospore of Phaeothamnion polychrysis. Also included is the fine structure of a zoospore.HETEROKONTOPHYTA, PHAEOTHAMNIOPHYCEAE 425 Fig. 20.2 Some algae classified in the Phaeothamniophyceae. lat erally into the motile cells. The anterior tinsel diameter (Fig. 20.2). The cells divide to form four flagellum has tripartite hairs that lack lateral autospores containing two chloroplasts. filaments. The posterior flagellum lacks hairs. New daughter cells are formed by eleutheroschi- REFERENCES sis (parent cell wall is completely cast off and Andersen, R. A., Potter, D., Bidigare, R. R., Latasa, M., new daughter cell walls are formed). Vesicles Rowar, K., and O’Kelly, C. J. (1998). Characterization under the plasma membrane appear similar to and phylogenetic position of the enigmatic golden the physodes that occur in the Phaeophyceae. The alga Phaeothamnion confervicola: ultrastructure, pig- Phaeothamniophyceae is the only class of algae ment composition and partial SSU rDNA sequence. J. where fucoxanthin and heteroxanthin occur Phycol. 34:286–98. together. Endogenous siliceous cysts (statospores) Bailey, J. C., Bidigare, R. R., Christensen, S. J., and are not produced by these algae. Andersen, R. A. (1998). Phaeothamniophyceae classis Phaeothamnion is a filamentous brown alga nova: a new lineage of chromophytes based upon that produces zoospores that settle to produce photosynthetic pigments, rbcL sequence analysis and new filaments (Fig. 20.1) (Andersen et al., 1998). ultrastructure. Protist 149:245–63. Tetrachrysis occurs in environments such as peat Dop, A. J., Kosterman, Y., and van Oers, F. (1980). ponds and has cells embedded in a common Coccoid and palmelloid benthic Chrysophyceae mucilage (Dop et al., 1980) (Fig. 20.2). Tetrasporopsis from the Netherlands. Acta Bot. Neerl. 29:87–102. is a colonial freshwater alga that consists of a Entwisle, T. J., and Andersen, R. A. (1990). A re- brown, gelatinous, bladdery sac (Entwisle and examination of Tetrasporopsis (Chrysophyceae) and a description of Dermatochrysis gen. nov. Andersen, 1990) (Fig. 20.2). Phaeoschizochlamys (Chrysophyceae): a monostromatic algae lacking occurs among detritus or suspended between cells walls. Phycologia 29:263–74. other freshwater algae. The cells occur in mucilage of different shapes up to 0.5 mm inChapter 21 Heterokontophyta outer membrane of the nuclear envelope. The stor- PHAEOPHYCEAE age product is laminarin. There are no unicellular or colonial organisms in the order, and the algae The Phaeophyceae, or brown algae, derive their are basically filamentous, pseudoparenchymatous, characteristic color from the large amounts of the or parenchymatous. They are found almost exclu- carotenoid fucoxanthin in their chloroplasts as sively in the marine habitat, there being only four well as from any phaeophycean tannins that might genera containing freshwater species, that is, be present. The chloroplasts also have chlorophylls Heribaudiella, Pleurocladia, Bodanella,and Sphacelaria a, c ,and c . There are two membranes of chloro- (Fig. 21.1) (Schloesser and Blum, 1980). A number of 1 2 plast E.R., which are usually continuous with the marine forms penetrate into brackish water, where Fig. 21.1 Some freshwater brown algae. (a) Pleurocladia lacustris. (b) Sphacelaria lacustris. (c) Heribaudiella fluviatilis. (H) Hair; (P) pleurilocular (a) sporangia; (U) unilocular sporangia. ((b) after Schloesser and Blum, 1980.) (c) (b)HETEROKONTOPHYTA, PHAEOPHYCEAE 427 they often form an important part of the salt Prymnesiophyceae, Bacillariophyceae, and marsh flora. These brackish water plants have Xanthophyceae, which are closely related to the almost totally lost the ability to reproduce sexu- Phaeophyceae. The main difference lies in the ally, and propagate by vegetative means only. Most large amounts of extracellular polysaccharides of the Phaeophyceae grow in the intertidal belt surrounding the protoplast. and the upper littoral region. They dominate these regions in colder waters, particularly in the Cell walls Northern Hemisphere, where the number of Phaeophycean cell walls are generally composed phaeophycean species is less than that of the of at least two layers, with cellulose making up Rhodophyceae, but the number of phaeophycean the main structural skeleton (Kloareg and plants is much greater. In the tropics, the only Quatrano, 1988). The amorphous component of place where large numbers of Phaeophyceae are the cell wall is made up of alginic acid and found isthe Sargasso Sea of the Atlantic. fucoidin, whereas the mucilage and cuticle are The Phaeophyceae probably evolved from an composed primarily of alginic acid (Evans and organism in the Phaeothamniophyceae, which Holligan, 1972a; Vreeland, 1972). Alginic acid is have motile cells similar to those in the basically made up of -1,4 linked mannuronic Phaeophyceae, but lack the characteristic unilocu- acid units that have a variable amount of lar and plurilocular sporangia of the Phaeophyceae guluronic acid units attached through C-1 and C-4 (Bailey et al., 1998). linkages (Fig. 1.7). Fucoidin is composed primarily of -1,2 linked sulfated-fucose units, with a lesser amount of -1,4 linked sulfated-fucose units (Fig. Cell structure 1.7). The relative quantities of alginic acid and fucoidin vary between species, different parts of The cell structure (Figs. 21.2, 21.3) is in many the plant, and different environments. ways similar to that of the Chrysophyceae, Calcification of the wall occurs only in some Fig. 21.2 Left: Diagrammatic representation of a male gamete of Ectocarpus siliculosus showing the distribution of cellular organelles. Right: Transmission electron micrograph of a thin section of a male gamete of E. siliculosus. (af) Anterior flagellum; (c) chloroplast; (e) eyespot; (fh) flagellar hairs (present along entire length, for clarity only shown on part of the flagellum); (fs) proximal swelling of the posterior flagellum; (g) Golgi apparatus; (li) lipid body; (m) mitochondrion; (mb) microbody; (mt) microtubules; (n) nucleus; (p) pyrenoid; (pf) posterior flagellum; (v1) physode; (v2) storage granule; (v3) vesicles with cell wall or adhesive material. (From Maier, 1997a.)428 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.3 Diagram of a hypothetical brown algal cell. (ce) Chloroplast envelope; (cen) centrioles; (cer) chloroplast teriorly directed whiplash flagellum (Fig. 21.2) endoplasmic reticulum; (d) dictyosome; (er) endoplasmic (Bouck, 1969; Loiseaux and West, 1970). The reticulum; (f) DNA fibrils; (m) mitochondrion; (ne) nuclear Fucales (Fig. 21.44) are an exception to this, with envelope; (nu) nucleolus; (p) pyrenoid; (ps) pyrenoid sac; the posterior flagellum of the spermatozoid being (v) vacuole. (From Bouck, 1965.) longer than the anterior flagellum. The posterior flagellum usually has a swelling near the base, species of Padina where calcium carbonate is and this swelling fits into a depression of the cell deposited as needle-shaped crystals of aragonite immediately above the eyespot. The eyespot con- in concentric bands on the surface of the fan-like sists of 40 to 80 lipid globules arranged in a single thallus (Borowitzka et al., 1974). layer between the outermost band of the thy- The parenchymatous Phaeophyceae have plas- lakoids and the chloroplast envelope. The eyespot modesmata or pores between most of the cells. (stigma) acts as a concave mirror focusing light These pores are bounded by the plasmalemma, onto the flagellar swelling, which is the photore- and protoplasm is continuous from one cell to the ceptor site for phototaxis in brown-algal flagellate next through them. In the Laminariales, Fucales, cells (Kawai et al., 1996). Light at 420 and 460 nm and Dictyotales, the pores are grouped in pri- is most effective in phototaxis in the brown algae, mary pit areas, whereas in the more primitive and is probably detected by a flavin-like substance parenchymatous Phaeophyceae the plasmodes- in the flagellar swelling of the posterior flagellum mata are scattered in the cell wall (Bisalputra, (Kawai et al., 1991). 1966; Bourne and Cole, 1968; Cole, 1970). Chloroplasts and photosynthesis Flagella and eyespot The chloroplasts of the Phaeophyceae have three Generally the motile cells of the Phaeophyceae thylakoids per band and are surrounded by the (always zoospores or gametes, as there are no chloroplast envelope and two membranes of motile vegetative cells) have a long anterior tinsel chloroplast E.R. (Figs. 21.3, 21.4). The outer mem- flagellum with tripartite hairs and a shorter pos- brane of the chloroplast E.R. is generally continu-HETEROKONTOPHYTA, PHAEOPHYCEAE 429 ous with the outer membrane of the nuclear enve- spermatozoids, and/or zoospores (Bourne and lope in the Ectocarpales but appears to be discon- Cole, 1968; Evans, 1968; Bisalputra et al., 1971). A tinuous in the Dictyotales, Laminariales, and pyrenoid in the Phaeophyceae is usually a stalk- Fucales. Membrane-bounded tubules are common like structure set off from the main body of the in the area between the chloroplast E.R. and chloroplast and containing a granular substance chloroplast envelope where the latter two are not not traversed by thylakoids. Surrounding the closely appressed (Bouck, 1965; Evans, 1968). pyrenoid but outside the chloroplast E.R. is a Microfibrils of DNA occur in the plastids, and membrane-bounded sac that presumably con- in Sphacelaria sp. there is a ring-shaped geno - tains the reserve product. The long-term storage phore inside the outermost band of thyla - product is laminarin (Fig. 1.7), a -1,3 linked koids (Bisalputra and Burton, 1969). The DNA glucan. The sugar alcohol, D-mannitol (Fig. 1.7) is, microfibrils are both linear and circular and are however, the accumulation product (up to 25% of attached to the thylakoid membranes. The plas- the dry weight of some Laminaria species in the tids contain chlorophylls a, c , and c , with the autumn) of photosynthesis. 1 2 major carotenoid being fucoxanthin. In a number of brown algae, the mannitol con- All the phaeophycean orders have representa- centration in the cell increases or decreases as the tives with pyrenoids (Figs. 21.3, 21.4) (Chi, 1971), salinity of the surrounding medium increases or but their presence, even in one species, can vary decreases (Reed et al., 1985). This osmoregulatory according to the stage of the plant. If the species mechanism prevents the cells from bursting in is one that has pyrenoids only in some stages, then hypotonic media or shrinking in hypertonic a pyrenoid is usually present in the eggs and/or media. The increase in mannitol concentration sporelings but absent in the macroscopic phase, occurs in the dark as well as in the light, showing that photosynthesis is not involved in the process. The brown algae are unique among the algae in having uptake of inorganic carbon, and therefore photosynthetic carbon fixation, stimu- lated by blue light (Forster and Dring, 1994). Most of the Phaeophyceae live in the littoral zone where they receive a generous amount of light. Photosynthesis is usually limited by the supply of inorganic carbon in this environment. The Phaeophyceae have evolved a mechanism to increase the amount of inorganic carbon uptake, but only when the cells are illuminated by blue light, thereby conserving the energy required for the process when the cells are in the dark. The only brown algae that do not have this mechanism are the fucoids, which appear to have evolved a separated carbon-concentrating mechanism. Phlorotannins and physodes Phlorotannins (phaeophycean tannins) are stored in physodes (Fig. 21.5) in the cytoplasm of many brown algae. Phlorotannins are formed by Golgi in the perinuclear area of the cell by poly- Fig. 21.4 A transmission electron micrograph of a portion merization of phloroglucinol (1,3,5-tri-hydroxy- of a cell of Scytosiphon lomentaria. (ES) Eyespot; (g) Golgi; benzene) (Fig. 21.6) through the acetate-malonate (P) pyrenoid. (From Nagasato and Motomura, 2002.) pathway (Schoenwaelder and Clayton, 2000; Pavia430 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.5 Physodes in zygotes of Hormosira banksii. (a) Bright-field microscopy of sectioned zygotes showing at high levels in brown algae of the temperate accumulation of physodes around the nucleus (arrow). and tropical Atlantic, whereas levels are low (b)Transmission electron micrograph showing physodes (less than 2% of dry mass) in the tropical Pacific around the nucleus. (From Schoenwaelder and Clayton, 2000.) and Indo-Pacific (Targett and Arnold, 1998). In temperate areas, the fucoids (Fucales) have high concentrations of phlorotannins whereas kelps (Laminariales) have low concentrations of phlorotannins. The same species of brown alga will have a higher phlorotannin content when deprived of nitrogen (Van Alstyne and Pelletreau, 2000). Phlorotannins have been postulated to func- tion in (1) deterring grazing by herbivores, (2) absorbing ultraviolet radiation, and (3) serving as a component of cell walls (Henry and Van Alstyne, 2004). The effect of phlorotannins as a chemical defense against herbivores has been best studied. Phlorotannins deter feeding by inhibiting the gl ycosidases of gastropods (Shibata et al., 2002). Fig. 21.6 Chemical structure of phloroglucinal, the basic Phlorotannins of high molecular weight inhibit building block of polyphenols and the chemical structure of herbivory more than phlorotannins of low the polyphenol procyanidin. molecular w eight (Target and Arnold, 1998; Kubanek et al., 2004). There is variation among et al., 2003). The phlorotannins in the physodes herbivores in the effectiveness of phlorotannins against grazing. Herbivores with digestive systems appear as a colorless, highly refractive acidic fluid that stains red with vanillin and hydrochloric containing surfactants (wetting agents) or high acid. The tannins are non-glycosidic (do not con- pH are able to tolerate phlorotannins better. tain sugars), bind proteins, have strong reducing Phlorotannins are not normally secreted outside the cell (Shibata et al., 2002). It is necessary for the action, and are astringent to the taste. They are readily oxidized in air, resulting in the formation cells to be damaged before the phlorotannins are of a black pigment, phycophaein, giving dried released. brown algae their characteristic black color. Phlorotannins are always present in brown algae and thus are a “constitutive” chemical The phlorotannin content of brown algae varies from 1% to 15% of dry mass. Phlorotannins occur defense. Stimulation of phlorotannin synthesis byHETEROKONTOPHYTA, PHAEOPHYCEAE 431 grazing would be “inducible” chemical defense, zygote (Bell, 1997). Although meiotic divisions which does not occur in most brown algae (Luder have been considered the rule in the unilocular and Clayton, 2004). Synthesis of phlorotannins sporangium, a disturbingly large number of inves- may be induced by the growth regulator jasmonic tigations have not found this to be the case. In acid as it is in higher plants (Arnold et al., 2001). these investigations there is a “direct” type of life Eggs of the Phaeophyceae contain phenolic history, with no meiosis or fusion occurring. More vesicles just under the plasma membrane that are research is needed in this area to clarify the situa- discharged outside the cells by exocytosis after fer- tion. In the phaeophycean life cycle, a plethys- tilization. It has been postulated that the dis- mothallus is a filamentous stage (or one charge of these phenolic vesicles has a toxic effect composed of compacted filaments) that can mul- on spermatozoids and acts as a polyspermy block tiply itself by spores (usually zoospores from before the primary wall is secreted (Clayton and plurilocular sporangia) (Papenfuss, 1951). Ashburner, 1994). These peripheral phenolic vesi- The thallus of many Phaeophyceae is relatively cles are distinguishable from physodes, which large and complex with a number of different also contain phenolic compounds, but which are types of growth that include: (1) diffuse, with significantly larger and tend to be localized most of the cells of the plant capable of cell divi- around the egg nucleus. sion (Ectocarpus, Fig. 21.17 and Petalonia, Fig. 21.20); (2) apical, with a single cell at the apex giving rise to the cells beneath (Dictyota, Fig. 21.11 and Life history Sphacelaria, Fig. 21.13); (3) trichothallic, where a cell divides to form a hair above and a thallus The unilocular sporangium (Fig. 21.7(b))is gener- below (Cutleria, Fig. 21.14 and Dermarestia, Fig. ally considered to be the site of meiosis, the hap- 21.15); (4) promeristem, with a non-dividing loid zoospores that are released forming the apical cell controlling a large number of smaller gametophyte generation. The gametophyte then meristematic, dividing promeristematic cells produces the gametes, which fuse to form the beneath it (Fucus, Fig. 21.41); (5) intercalary, with Fig. 21.7 Ectocarpus fasciculatus, plurilocular (a) and unilocular (b) sporangia. (From Dixon et al., 2000.)432 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.8 The chemical structure of some brown algal pheromones with the names of the algae that secrete the open-chain hydrocarbons containing at least one pheromones. (Modified from Pohnert and Boland, 2002.) double bond), most of them incorporating a five- or seven-membered ring structure. All of the attractions in the brown algae identified so far a zone of meristematic cells forming tissue above are highly volatile and hydrophobic. Very small and below the meristem (Laminaria, Figs. 21.24, amounts of the pheromones are released, only 0.6 21.23); (6) meristoderm, with a layer of usually fmol per cell per hour from Ectocarpus siliculosus peripheral cells dividing periclinally (parallel to gametes. However, the non-polar nature of the the surface of the thallus) to form a tissue below pheromones contrasts strongly with the highly the meristoderm (usually cortex) and occasionally polar nature of water, making the pheromones anticlinally (perpendicular to the surface of the easily recognizable to the responding cells. thallus) to add more cells to the meristoderm Perception of the pheromone starts with the (Fucus, Fig. 21.41). simple partition from water of the non-polar Investigations on the sexual hormones of the pheromone into the lipid plasma membrane of brown algae in the 1970s and 1980s by Müller the gamete that contains the macromolecular (Fig. 21.9) and his associates represent a major receptors (Pohnert and Boland, 2002). The sub- advancement in the knowledge of the brown stances have a very strong tendency to leave the algae (Müller, 1982; Maier and Müller, 1986). A aqueous solution and escape into the air. This sexual hormone (sirenine, pheromone) is a dif- characteristic helps to avoid the buildup of fusible substance that coordinates cellular activi- chronic concentrations which would decrease the ties during sexual reproduction. Two types of efficiency of the gradients around the female biological effects mediated by sexual hormones cells. The attractants probably do not function occur in the brown algae: (1) the explosive dis- more than 0.5 mm away from the female cells, charge of spermatozoids from antheridia, and (2) and thus attraction is clearly a short-distance the attraction of male gametes by female gametes phenomenon (Müller, 1982). The sexual hormones or eggs. All sexual hormones in the brown algae include ectocarpene from Ectocarpus (Müller et al., are unsaturated hydrocarbons (have at least one 1971), desmarestene from Demarestia aculeata double or triple bond) (Fig. 21.8) (Müller et al., (Müller et al., 1982), lamoxirene from Laminaria 1982). With the exception of the Fucus sperm (Müller et al., 1985a,b), multifidene from Cutleria attractant, all of the sexual hormones in the multifida (Jaenicke et al., 1974), dictyopterene C brown algae are C to C olefins (unsaturated from Dictyota dichotoma (Müller et al., 1981), and 8 11HETEROKONTOPHYTA, PHAEOPHYCEAE 433 Fig. 21.10 Choristocarpus tenellus showing the uniseriate Fig. 21.9 Dieter G. Muller Born January 24, 1935, in filament and a propagule with an apical cell. Stuttgart, Germany. From 1956 to 1961, Dr. Muller studied at the Universitat Tubingen; from 1961 to 1963, he was a Postdoctoral Fellow at the University of Pennsylvania; from Within the Phaeophyceae, the Fucales are a 1964 to 1973, he was Wissenschaftlicher Mitarbeiter at the sister group to the remainder of the algae in the Max-Planck-Institut fur Zuchtungsforschung at Koln- class. The early divergence of the Fucales is con- Vogelsang. In 1973, he received his Habilation at the sistent with the presence of a proboscis in the University of Koln; and since 1973, he has been professor in spermatozoids of Vaucheria and Fucus (de Reviers the Fakultat für Biologie at the Universitat Konstanz. In 1964, and Rousseau, 1999). Fossils similar to Cutleria Dr. Muller received strong support from Professor J. Straub, occur in 25 million-year-old Miocene deposits director of the Max-Planck-Institut, Koln, to work out in while fossils similar to algae in the Laminariales detail the life cycle of Ectocarpus siliculosus. After evidence for and Fucales occur in 16–20 million year old a sexual hormone was discovered, a cooperation scheme was established with the Institut für Biochemie at the Miocene deposits. University of Koln. These two events led to the The orders considered here are presented in an characterization of the sexual hormones of the brown algae. evolutionary sequence with the Dictyotales and Sphacelariales being the most ancient and the Ectocarpales and the Laminariales the most recent fucoserratene from Fucus serratus and F. vesiculosus (de Reviers and Rousseau, 1999; Draisma et al., (Müller and Jaenicke, 1973) (Fig. 21.9). 2001). Order 1 Dictyotales: growth by an apical cell; Classification meiosis occurring in the production of four to eight non-motile spores; The Phaeophyceae are an ancient lineage, originat- oogamous sexual reproduction. ing between 150 (Medlin et al., 1997) and 200 mil- Order 2 Sphacelariales: growth by an apical lion years ago (Lim et al., 1986). The Xanthophyceae cell; daughter cells divided and Phaeothamniophyceae are the closest known longitudinally to give a polysiphonous sister taxa to the Phaeophyceae. The first true structure; isogamous sexual brown alga was probably similar to the extant reproduction. Choristocarpus tenellus (Fig. 21.10) with creeping fila- Order 3 Cutleriales: trichothallic growth ments, apical growth, and an isomorphic life his- forming a fan-like thallus in at least one tory (de Reviers and Rousseau, 1999; Draisma et al., generation; anisogamous sexual 2001). reproduction.434 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Order 4 Desmarestiales: trichothallic growth divides and redivides in vertical and horizontal forming axial cells; oogamous sexual planes into between 650 and 1500 compartments reproduction. (Williams, 1904). The content of each locule Order 5 Ectocarpales: thallus consisting of becomes a pear-shaped sperm with a single, later- filaments or filaments compacted ally inserted, tinsel flagellum and an anterior eye- together; reproduction isogamous or spot (Phillips and Clayton, 1993). Although there anisogamous. is only one emergent flagellum, a second basal Order 6 Laminariales: diploid thallus body is present (Manton, 1959), indicating a parenchymatous resulting from an derivation from a biflagellate ancestor. The intercalary meristem between the stipe mature sperms are set free by dissolution of the and blade; reproduction oogamous. walls of the antheridium. The male sorus is sur- Order 7 Fucales: growth primarily by a rounded by elongated sterile cells that are promeristem; gametophyte reduced to regarded as undeveloped antheridia. The egg egg and sperm; oogamous sexual secretes the pheromone dictyotene (Fig. 21.8) reproduction. (Pohnert and Boland, 2002) that attracts the sperm. The sperm fertilizes the egg to produce Dictyotales the zygote that germinates into the sporophyte; This order has organisms that grow by means of unfertilized eggs can germinate partheno - an apical cell or by a marginal row of apical cells. genetically but seldom develop normally and There is an isomorphic alternation of erect, flat- soon abort. The sporophytes produce haploid tened, parenchymatous thalli. A distinctive char- aplanospores (tetraspores) on the surface of the acter of this order is the modification of the thallus. The tetrasporangia occur singly or in unilocular sporangia to produce four to eight small groups. The naked tetraspores are released large aplanospores. Sexual reproduction is ooga- by gelatinization of the apex of the sporangium, mous. The Dictyotales are common in warmer and soon after liberation the large motionless waters throughout the world. spores secrete a cellulose wall and develop into Dictyota dichotoma has a single apical cell that the gametophytes. forms the flattened annual thallus (Fig. 21.11). The In D. dichotoma, the gametes are released at reg- mature thallus consists of three layers: a middle ular intervals. This was first noticed by Williams layer composed of large cells with few or no (1905) in Great Britain, where the gametes are chloroplasts, surrounded on both sides by a layer released fortnightly. Müller (1962) showed that of small cells densely packed with chloroplasts. moonlight is the synchronizing factor for the Gametophytes form sex organs in pro jecting sori. release. When he grew the alga in natural light, Gametogenesis can be artificially induced by gametes were released every 14 to 15 days. If the exposure of the gametophytes to blue light alga was grown under artificial conditions with a (Kumke, 1973). An oogonium develops from a sur- 14 hours light : 10 hours dark cycle, then few face cell that divides into a stalk cell and the oogo- gametes were released, and there was no syn- nium proper. Each oogonium produces a single chronony. If the artificially lighted cultures had egg, which is liberated through the gelatinized the lights left on all night, then 10 days later a apex of the wall. There are usually 25 to 50 oogo- burst of gametes was released. The lights being nia in a sorus with sterile oogonia at the margin. left on all night simulated moonlight. The deep-brown color of the female sori contrasts Species of Dictyota produce terpenoids, such with the white glistening spots that comprise the as pachydictyol and (6R)-6-hydroxydichotoma-3, male sori. The male sori can be recognized early in 14-diene-1,17,dial (Fig. 21.12), that inhibit grazing their development by the disintegration of the of Dictyota by herbivorous fish, amphipods, and chloroplasts in the cells. Like the oogonia, the sea urchins (Schmitt et al., 1998; Pereira et al., antheridia develop from surface cells. These cells 2000). enlarge and divide horizontally into a stalk cell The only calcified genus in the Phaeophyceae, and a primary spermatogenous cell. This cell Padina, is in the Dictyotales.HETEROKONTOPHYTA, PHAEOPHYCEAE 435 Fig. 21.11 The life cycle of Dictyota dichotoma. (Adapted from Thuret and Bornet, 1878; Williams, 1898; Taylor, 1960.)436 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.12 The chemical structure of two secondary metabolites secreted by Dictyota spp. that act as antifoulants. Sphacelariales sporophyte. Meiosis occurs in the production of This order is characterized by an apical meristem- zoospores in the unilocular sporangia (Clint, atic cell that divides transversely to produce the 1927). Over 200 zoospores are released through daughter cells. The algae produce distinctive veg- an apical pore in the unilocular sporangium etative propagules. Another less precise charac- (Papenfuss, 1934). The zoospores germinate to teristic is blackening of the cell walls when presumabl y form gametophytes that are similar treated with bleaching liquid (Draisma et al., to the sporophytes. The gametophytes produce 2002). plurilocular gametangia of one type, which Sphacelaria (Fig. 21.13) grows attached to rocks release isogamous gametes. Ectocarpene (Fig. or other algae and has one or more freely 21.8) is used as a pheromone in gamete attraction branched shoots arising from a discoid holdfast. (Pohnert and Boland, 2002). The fusion of gametes The apical cell undergoes transverse divisions, takes place while they are motile and produces a with subsequent longitudinal septation of the quadriflagellate zygote that may continue moving daughter cells to produce a polysiphonous struc- for several hours. The life cycle of another species, ture. Although the maturing axes and branches S. furcigera, involves anisogamy and unisexual undergo septation into smaller and smaller cells, gametophytes, which are somewhat smaller than they do not enlarge; thus the diameter of the fila- the sporophytes (van den Hoek and Flinterman, ment is essentially the same from the base to the 1968). The life cycle is controlled by temperature apex. Older axes, though, may become corticated and photoperiod. by downward-growing filaments. The erect axes are usually abundantly branched, usually in a reg- Cutleriales ular, distichous manner. This order contains only two genera, Cutleria and Asexual reproduction is by means of propag- Zanardinia. The genera show an alternation of gen- ula (Fig. 21.13), which are specialized branchlets erations that is heteromorphic in Cutleria and iso- of distinctive form that are produced throughout morphic in Zanardinia.Thethallusisflattened, the vegetative parts of the plants. They are formed blade-like, or disc-like, with entirely or partially tri- much more frequently than sporangia or game - chothallic growth. The sporophytes produce only tangia. Each propagule has an apical cell and unilocular sporangia, whereas the gametophytes usually two to three protuberances. After falling are heterothallic and markedly anisogamous. from the parent plant and contacting a suit- Cutleria is a warm-water plant of the Northern able substrate, the propagule develops into a Hemisphere that may be closely related to new plant. Propagula are formed only at tempera- Saccorhiza in the Laminariales (Rousseau et al., tures above 12 °C and under daylight conditions 1997). The gametophyte is an erect, flattened longer than 12 hours (Colijn and van den Hoek, blade with numerous dichotomies (Fig. 21.14). 1971). Growth is trichothallic at the base of many erect In S. bipinnata, the sporophyte forms both uniseriate hairs at the upper margin of the blade. unilocular and plurilocular sporangia terminally The cells that are cut off below the hairs con- on branches (Fig. 21.13). The plurilocular sporan- tribute to the thallus. The innermost of these cells gia produce zoospores that re-form the parent gradually enlarge to form the medulla, whereasHETEROKONTOPHYTA, PHAEOPHYCEAE 437 Fig. 21.13 The life cycle of Sphacelaria (S. cirrhosa and S. bipinnata). (Adapted from Savaugeau, 1900–14; Papenfuss, 1934; Taylor, 1957.)438 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.14 The life cycle of Cutleria multifida. (Adapted from Kuckuck, 1899; Savaugeau, 1899.)HETEROKONTOPHYTA, PHAEOPHYCEAE 439 the outer ones undergo divisions to form the dermal cell divides into one to six stalk cells and cortex. The gametophytes are heterothallic, with a single terminal unilocular sporangium. In the sex organs developing in clusters on the sur- the unilocular sporangium 8, 16, or 32 large, face of the thallus. A superficial epidermal cell pyriform, haploid zoospores are formed, each may develop directly into a male gametangium, or with several chloroplasts. The zoospores escape it may develop into a branched hair that bears sev- through a large apical pore in the sporangial wall, eral gametangia. The male gametangium consists swarm for 10 to 90 minutes, then settle down, of a stalk cell on which there are 20 or more tiers round up, and secrete a wall. The zoospores then of cells, each tier composed of eight cells. The pro- divide to form the gametophyte. toplast of each cell forms a biflagellate male Although germlings from zygotes and from gamete, which escapes through a pore in the zoospores of the Aglaozonia sporophytes have not gametangial wall to the outside. Female gametan- been grown to maturity in culture, they have been gia develop similarly to the male, but with a grown to a sufficiently advanced stage to show smaller number of larger cells. Female gametan- that the two stages are alternate generations of gia are four to seven tiers high, with only four each other. In Europe, the sporophyte is perennial cells in each tier. Free-swimming male gametes and fruits in winter or spring, whereas the game- are pyriform, with a single reddish chloroplast at tophyte is a spring annual that disappears during the place of flagella insertion. Free-swimming the summer. female gametes are also pyriform, but are much Fossils of a plant similar in structure to larger and have a dozen or so chloroplasts. The Cutleria, called Limnophycus paradoxa, have been female gametes release a highly volatile, low- described from the Miocene (25 million years old) molecular-weight compound, multifidene (Fig. deposits in Germany. 21.8) (Pohnert and Boland, 2002) that attracts the male gametes (Müller, 1974). When the gametes Desmarestiales fuse, the male gametes are actively swimming In this order there is an alternation of a large while the female are sluggish or immobile. Fusion macroscopic sporophyte with a small filamentous of the two nuclei follows within a few hours, and gametophyte (Fig. 21.15). The gametophyte forms the zygote begins to develop into the sporophyte oogonia and sperm, with the result that repro- within a day. Unfertilized female gametes develop duction is oogamous. Growth of the sporophyte is parthenogenetically into gametophytes. trichothallic, and the main axis is corticated The zygote germinates to produce the sporo- by downward-growing cells. In this treatment, phyte, which was first described as a separate the organisms sometimes considered in the genus, Aglaozonia. At first, growth is trichothallic Sporochnales are placed in the Desmarestiales, as and vertically upward into a columnar structure. suggested by Russell and Fletcher (1975). Upward growth ceases when the plant is about 10 Sporophytes of Desmarestia may reach a length days old, and all further growth is laterally out- of 2 to 3 m and occur primarily in the sublittoral ward from the base of the column. Repeated cell region in colder waters of both the Northern and division at the base of the column forms a flat, the Southern Hemisphere. The plants (Figs. 21.15, disc-like tissue that expands laterally as a result of 21.16) have trichothallic growth from an inter- division and redivision of the marginal cells. The calary meristem composed of flattened cells that sporophyte is homologous to a minute erect thal- cut off cells to the terminal hair above, and cells lus subtended by an enlarged fertile holdfast. The to the thallus below. The cells of the terminal hair disc-like portion of the thallus is several cells continually wear away, and each of these cells usu- thick, and the outer cells are differentiated into ally bears one or two unbranched laterals. The an epidermis-like layer. The holdfast is attached to cells produced below the meristem cut off two the substratum by numerous multicellular rhi- opposite laterals in one plane, which lengthen by zoids growing from the ventral epidermal cells. means of a basal meristem. Cortication of the The unilocular sporangia are formed in sori on thallus begins by the basal cells of the lateral cut- the dorsal surface of the sporpohyte. A single epi- ting off a number of cells that gradually form a440 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES Fig. 21.15 The life cycle of Desmarestia. (Adapted from Schreiber, 1932; Chapman and Burrows, 1971.)HETEROKONTOPHYTA, PHAEOPHYCEAE 441 unilocular sporangium. The zoospores have an eye- spot and a single chloroplast. Released zoospores settle, lose their flagella, and round up within 24 hours. The cell contents move out into a germ tube immediately after settling. The sporelings produce female and male gametophytes in roughly similar numbers, the male gametophytes having small cells and being less densely pigmented than the female gametophytes. Gametophytes grow vegeta- tively only in red light, with differentiation of oogonia and antheridia occurring under blue or white light (Müller and Lüthe, 1981). About 11 days to 3 weeks after germination of spores, conical antheridia and lateral oogonia appear on the gametophytes. The tubular oogonia dehisce api- cally to liberate the egg, which usually adheres to the gelatinized aperture. A single spermatozoid is released from each antheridium through a narrow apical aperture. The freshly released eggs secrete three volatile chemicals that cause the antheridia to burst and attract free spermatozoids to the eggs. The three sexual hormones are des- marestene, ectocarpene, and viridene (Fig. 21.8). Desmarestene is the most potent of the three Fig. 21.16 Desmarestia. (a)–(d) Cortication of an axial cell (A) by corticating hyphae (CH). (e) Partial section of a sexual hormones (Müller et al., 1982). Sporophyte mature thallus. (B,C) Laterals; (M) meristoderm. development begins with the production of a tube from one side of the zygote and a lightly pig- mented rhizoid from the opposite pole. The initial complete one-layered envelope around the elon- tube goes on to produce an oppositely branched gating axial cell that produced the lateral. The uniseriate filament, which forms a trichothallic cells of this one-layered envelope soon undergo meristem below most of the lateral branches. periclinal division, with the outer layer that is Cortication of the thallus begins, as has already produced behaving as a meristem. This meristem been described, to produce a mature sporophyte then produces the cells of the cortex to the inside, and complete the life cycle. Gametophytes of with the result that the axial cells and laterals are Desmarestia (Andersen, 1982) and the Laminariales progressively buried. In addition to this primary have several features in common, including clus- growth, the cells of the inner cortex are capable ters of unicellular antheridia, vertically elongated of secondary growth, enlarging to form hyphae intercalary oogonia, attachment of the extruded that push their way downward between the cells egg to the oogonial apex, and growth of the young of the cortex. Trumpet hyphal cells occur in the sporophyte upon the oogonial apex. medulla. These cells have perforate end walls with Some of the species of Desmarestia accumulate callose and probably function in conduction of large amounts of malic acid, lowering the pH of nutrients as do the sieve filaments in the the vacuolar sap to as low as 2. In collecting sea- Laminariales (Moe and Silva, 1981). weeds, the species of Desmarestia should be kept D. aculeata (Fig. 21.15) produces unilocular spo- separate because some of their cells will rupture, rangia on the sporophytes in the winter by the tan- releasing acid and killing the other seaweeds. gential division of a surface cell of the cortex. In the Antarctic waters, members of the Meiosis apparently occurs in the production of a Desmarestiales provide the bulk of the biomass of few biflagellate zoospores formed in each small benthic seaweeds. They are perennial, covering442 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES large areas of water to depths of about 40 m. The Desmarestiales and Laminariales (Draisma et al., largest and most abundant species (D. anceps and 2001). D. menziesii) form thickets, but not the protective Four of the families in the Ectocarpales will be canopy characteristic of many kelps. The Antarctic considered here. possesses the only cold-water flora without Family 1 Ectocarpaceae: plants with free- Laminariales, although in sub-Antarctic waters filamentous construction with no there are vast stands of kelps (Macrocystis and adherence of filaments to each other. Lessonia) (Moe and Silva, 1977). Family 2 Ralfsiaceae: algae with a basal layer The Desmarestiales and the Laminariales have supporting erect filaments that are a number of similar development characteristics compacted together to form a tissue. (Tan and Druehl, 1996) that include (1) vegetative Family 3 Scytosiphonaceae: parenchymatous development of the gametophyte in red light; thalli with mostly diffuse growth. (2) requirement of white or blue light for develop- Family 4 Splachnidiaceae: plants with ment of antheridia and oogonia; (3) existence of trichothallic growth and unilocular spermatozoid-releasing and -attracting factors sporangia formed in conceptacles. secreted by eggs; (4) unusually long and flexible hind flagella; (5) lack of eyespots in spermato- zoids; and (6) formation of sexual organs by the Ectocarpaceae gametophyte, representing an exhaustive and These organisms have free-filamentous construc- almost lethal effort for the gametophytes. tion with no adherence of the filaments to each other. Ectocarpus is the prevalent genus, and is Ectocarpales composed of freely branched, uniseriate filaments These algae consist of filaments or of filaments differentiated into prostrate and erect systems. compacted together. In the order it is possible to The prostrate parts are rhizoid-like and often pen- see the gradual morphological evolution from a etrate the substrate. Growth can be diffuse or filamentous structure to pseudoparenchymatous more or less clearly trichothallic, with intercalary (haplostichous) complex structures of compac - cell divisions confined to certain areas of the tedfilaments (from the Ectocarpaceae to the filaments. Some workers divide up the family into Ralfsiaceae, and Splachnidiaceae). Along another different genera on the basis of cytology and mor- line, the filamentous thallus has evolved by the phology, whereas others consider that the family division of the filament into true parenchyma- contains the single genus Ectocarpus (Russell and tous (polystichous) thalli (from the Ectocarpaceae Garbary, 1978). to the Scytosiphonaceae). Most of the algae in the The life cycle of E. siliculosus (Fig. 21.17) can be order are heterotrichous, with the thallus con- taken as representative of the family (Papenfuss, sisting of two different parts: (1) the prostrate 1935). The haploid and diploid phases are both creeping disc that functions as a holdfast, and (2) filamentous, but the diploid filaments have the erect filamentous, bulbous, or foliose stage. In longer cells than the haploid filaments. The some of the algae, both systems are evident diploid plants produce unilocular and plurilocu- (Scytosiphon, Fig. 21.19), whereas in others the lar sporangia either on the same plant or on sep- erect stage is reduced to filaments of a few cells arate plants. These sporangia discharge their and the thallus is crustose (Ralfsia, Fig. 21.18), zoospores between 0600 and 1200 hours. The and in yet others the erect stage is predominant mother cell of a unilocular sporangium can be dis- with the prostrate system reduced to a small hold- tinguished from a branch initial by the spherical fast (Petalonia, Fig. 21.20). Even within the same shape and large nucleus of the mother cell. The alga, there can be a stage that consists of only a cell is initially vacuolate, but the physodes and thin crust, whereas the alternate stage has a well- vacuoles are soon extruded from the cell and developed erect stage. become lodged in the wall (Loiseaux, 1973). The Nucleic-acid sequencing studies have shown a chloroplasts and nuclei of the unilocular spo- strong relationship between the Ectocarpales, rangium divide in regular sequence, with theHETEROKONTOPHYTA, PHAEOPHYCEAE 443 chloroplasts next to the wall and the nuclei in the the parent. The germ tube of the sporeling arises center of the cell (Knight, 1929). The nuclei divide from the narrow, anterior flagellated end of the meiotically. A chloroplast then becomes associ- zoospore, which is always oriented toward the ated with a nucleus, and a zoospore is delimited light. The plurilocular organs on the haploid fila- around it. A small perforation occurs at the apex ments are smaller than those on the diploid fila- of the unilocular sporangium, and up to 32 hap- ments, and produce either zoospores or gametes. loid zoospores ooze out of the sporangium in a The motile gametes are all of the same size but gelatinous matrix. The perforation is small, and differ physiologically. The female gametes settle zoospores are relatively large, being twice the down about 5 minutes after liberation and size of gametes and zoospores from plurilocular secrete a sexual hormone called ectocarpene all- sporangia. The zoospores initially swim in a cis-1-(cycloheptadien-2 ,5 -yl)-1-butene (Fig. 21.8) straight pattern, then display circling movements (Müller et al., 1971). Male gametes (Fig. 21.17) as they explore appropriate surfaces for settling (Maier, 1997a,b) move very rapidly (269 mper (Iken et al., 2001). The zoospores prefer to settle on second) in a straight lineinopenseawaterwhen a hydrophobic surface, preferably one with a no female gametes are around (Müller, 1978). The microbial film. The zoospores germinate within 2 motile male gametes (which can remain motile to 3 hours to produce haploid filaments. for up to 8 hours) swim in circular paths on The plurilocular organs (Fig. 21.17) are mod- encountering ectocarpene, the diameter of the ified lateral branches that are divided into as circular path decreasing in response to increasing many as 660 cubical cells, each containing a ectocarpene concentration (Müller, 1982). As soon motile cell. The plurilocular sporangia on the as the female gamete is reached, a firm contact is diploid filamentsproducezoosporesthatremain established between the apical part of the front motile for 3 to 5 hours, settle, and within 2 to 5 flagellum of the male gamete and the plasma hours germinate to produce diploid filaments like membrane of the female gamete. The posterior Fig. 21.17 The life cycle of Ectocarpus siliculosis.

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