Algal Growth Forms Some Divisions such as the
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Algal Growth Forms • Some Divisions, such as the Euglenophyta, Bacillariophyta, and Dinoflagellata consist entirely of unicellular forms. • Other Divisions, especially the Phaeophyta and Rhodophyta consist almost entirely of multicellular forms (except in the spore stage of the life cycle.
(A) Levels of Complexity within the Chlorophyta and other Divisions (1) A Unicellular Form • Motile unicell • 2 apical flagella • Single large chloroplast with one or more pyrenoids and starch granules. • An "eyespot" region within the chloroplast at the anterior end allows the organism to seek out areas of appropriate light intensity. Chlamydomonas
(2) Colonial Forms • Motile colony consisting of a hollow spherical network of interconnected, flagellate, Chlamydomonas-like cells Volvox • Non-motile colonies occur if the cells lose their flagella or never develop them. (3) Filamentous Forms • This results if cells are held tightly by a middle lamella and if all the cells divide transversely.
If occasional cells undergo longitudinal division, the filament, branches. One end of the filament serves as attachment to a holdfast. (4) Membranous or thallose forms • This results if the orientation of cell division is controlled precisely such that all new walls occur in only two planes. The result is a sheet of cells that become more extensive but remain thin.
Thallose Ulva
Parenchymatoius
(5) Parenchymatous Forms • This results when the cell division occurs regularly in three-dimension. All cells are interconnected by plasmodesmata, and a true parenchyma tissue is formed. (6) Coenocytic or siphonous Forms • This results if karyokinesis occurs without cytokinesis, and a giant multinucleate cells results.
Mode of Reproduction in Algae 1. Asexual reproduction (a) In unicellular algae, the organism reproduces by means of cell division. It is just a cell cycle; mitosis and cytokinesis constitute reproduction. • The repeated bipartitions occur to form new individual like the parental cell. This is also called binary fission.
(b) Non-coenobic colonial, filamentous and other multicellular algae reproduce by various types of fragmentation, the fragments have the capacity into developing new individual. This is also called vegetative reproduction. © In addition to fragments, algae also reproduce by forming spores. (d) Many non-motile green, yellow and brown algae produce flagella agents of asexual reproduction known as zoospores, the name implying that their motility is an animal-like trait.
2. Sexual reproduction • affords the opportunity for exhange and formation of new combinations of genetic materials. • Sexual reproduction at the cellular level is meant the union of cells, plasmogamy – union of their nuclei, then karyogamy – the association of their chromosome and genes and meiosis.
Sexual Reproduction The algae have evolved many variations in sexual reproduction such as different types of gametes, different means of gamete transfer, and different locations of fertilization. The process of gamete formation is called gametogenesis. The relative form of the two fusing gametes defines two categories of sexual reproduction -- isogamy and heterogamy.
Isogamy • Isogamy is the form of sexual reproduction in which the gametes produced are identical in shape, size and motility. There is no structural distinction between "male" and "female" gametes. • Pairs of isogametes align themselves with their flagellar poles touching and after several seconds, the motile gametes fuse to form a single, non-motile, diploid zygote.
Life Cycle of Chlamydomonas
Isogametes less commonly, may be non-motile structures. A specific example exhibiting non-motile isogametes is the reproductive process known as conjugation, which occurs in the filamentous green alga, Spirogyra sp.
Conjugation in Spirogyra is a filamentous green algae � Sexual reproduction through conjugation � The Process � 1. 2. 3. 4. 5. two Spirogyra line up form conjugation tube + gamete swims through tube and fuses with – gamete zygote forms thick wall and falls from parent zygote divides to become new Spirogyra
Heterogamy In heterogamy, two different types of gametes are produced. The male gamete, the sperm cell, is typically very small, highly motile and is produced in very large numbers. The female gamete, the egg cell, is much larger and non-motile.
Reproduction of Oedogonium �Oedogonium is a filamentous green algae �Reproduce through self-fertilization �The process: male gametangia (antheridium) produce sperm above female gametangia (oogonium) that produce eggs sperm are released from antheridium and swim to oogonium and fertilize egg diploid zygote forms a resting spore which buds off meiosis results in 4 haploid zoospores each zoospore lands and divides one of the cells becomes a holdfast (root-like structure) the rest grow into an adult Oedogonium
Alternation of Generations in Ulva
Alternation of Generations in Ulva �Ulva is leaf-like (seaweed) � Alternation of generations means to alternate between a haploid form and a diploid form in an ongoing lifecycle The Process: �Diploid phase = sporophyte houses reproductive cells called sporangia produce zoospores through meiosis zoospores divide by mitosis, form motile spores which settle on rocks and divide to form mature, haploid gametophytes �Haploid phase = gametophyte house gametangia produce + and – gametes which fuse to form zygote diploid zygote settles on rock and divides into sporophyte
�Concept 1: Land plants evolved from green algae �Researchers have identified green algae called charophyceans as the closest relatives of land plants Morphological and Biochemical Evidence • Many characteristics of land plants - Also appear in a variety of algal clades
Charophyta
There are only four genera of Charophyta. They are generally found in freshwater, anchored by rhizoids in sandy or muddy substrates. The Charales, have large, macroscopic, thalli growing up to 120 cm long, they are branched, multicellular, and use chlorophyll to photosynthesize. They may be called stoneworts, because the plants can become encrusted in lime (calcium carbonate) after some time. The "stem" is actually a central stalk consisting of giant, multinucleated cells. They are unique in having a whorl of small branchlets at each node in the stipe. This has no true leaves only branchlets
Whorls of 6 -8 branchlets arising from the nodes along the stem
The main "stem" is more than one cell wide. There is one long central cell that extends throughout the whole internodal region. This cell is surrounded by several pericentral cells that do not extend the entire internodal region. All of the growth originates from an apical cell at the end of the "stem".
Reproduction in Chara sp. is primarily sexual. There is a limited amount of fragmentation, particularly at the rhizoidal region. Sexual reproduction is oogamous and the plants may be monoecious or dioecious. The gametangia develop in the axis at the nodes.
The reproductive structure are the antheridium and the oogonium which are protected by a layer of sterile cells. In the antheridium, it has shield cells surrounding the structure, and the antheridial filaments. Each cell in the antheridial filaments produce a single sperm cell by mitosis. Nitella sp. oogania and antheridia
The oogonium is also surrounded by sterile cells but inside there is only a single, large egg cell. Fertilization is internal and the zygote eventually germinates by producing a small filament out through a split in the outer cells.
Charophyta This is a small group of organisms that has defied classification over the decades. They have been placed as a class of the green algae, as separate division of the algae, as group of bryophytes, and as a separate group of organisms of their own. They seem to transcend the taxonomic boundaries of both the algae and the bryophytes.
�Systematists � Are currently debating the boundaries of the plant kingdom Defining the Plant Kingdom Viridiplantae Streptophyta Plantae Red algae Chlorophytes Ancestral alga Charophyceans Embryophytes
There are four key traits that land plants share only with charophyceans • Rose-shaped complexes for cellulose synthesis 30 nm
Peroxisome enzymes Structure of flagellated sperm Formation of a phragmoplast
�In charophyceans �A layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out �The accumulation of traits that facilitated survival on land �May have opened the way to its colonization by plants
Derived Traits of Plants �Five key traits appear in nearly all land plants but are absent in the charophyceans Apical meristems Alternation of generations Walled spores produced in sporangia Multicellular gametangia Multicellular dependent embryos
Chara
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