Algae Reproduction PDF

 Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 3: REPRODUCTION OF ALGAE (Kingdom of Protista) Algae (Protists) comprise a large heterogenous assemblage of photoautotrophic organisms which vary vastly in size, shape and...

 Plant Reproduction – Lebanese University, Faculty of Sciences CHAPTER 3: REPRODUCTION OF ALGAE (Kingdom of Protista) Algae (Protists) comprise a large heterogenous assemblage of photoautotrophic organisms which vary vastly in size, shape and mode of life. Algae show morphological, cytological, physiological differences. The reproductive systems of algae also show similar diversity. Algae reproduce in astoundingly diverse ways; some reproduce asexually, others use sexual reproduction, and many use both mode of reproduction. 1. ASEXUAL REPRODUCTION Asexual reproduction may be by: - Mitotic division for single-celled forms - Fragmentation for multicellular organisms. Some multicellular algae, e.g. Sargassum reproduce asexually through fragmentation, in which fragments of the parent develop into new individuals. Figure 14: Fragmentation - Budding, a process similar to fragmentation, where the parent organism divides into two unequal parts. New individuals develop as buds on the outer surface of the parent organism. The buds may break off and live independently or they may remain attached, forming colony. Figure 15: Budding - Spore formation. Many algae produce spores asexually by mitosis. If these spores are flagellated and motile, they are called zoospores or planospores. The majority of algae produce zoospores except red algae. Many algae carry out both sexual and asexual reproduction. This is well demonstrated in the life cycle of the alga Chlamydomonas. The mature alga is a single haploid cell - that is, it contains only one set of chromosomes. During asexual reproduction the algae absorbs its flagellum. The algal cell then undergoes mitosis, 2 to 3 times. Four to eight daughter cells are created that emerge from the enclosing parent cell as spores. The spores develop into mature haploid cells that are genetically identical to the parent cell. 13  Plant Reproduction – Lebanese University, Faculty of Sciences 2. SEXUAL REPRODUCTION 2.1. Monobiontic Haplontic life cycle (monogenetic) 2.1.1. Chlamydomonas Recall that Chlamydomonas is one of many algae that incorporate both sexual and asexual modes of reproduction. The mature alga is a single haploid cell. During the sexual phase haploid cells divide mitotically to produce either “plus” or “minus” motile gametes of the same size. A (+) gamete and a (-) gamete come into contact with one another, shed their cell walls, and fuse to form a diploid zygote (planogamy isogamous). This resting stage of a zygote is called a zygospore which withstands bad environmental conditions. When conditions become favorable, the zygospore undergoes meiosis and produce 4 haploid motile tetraspores called zoospores. The thick wall opens and the living zoospores emerge. Figure 16: Chlamydomonas life cycle 2.1.2. Spirogyra Spirogyra is a filamentous green alga which is common in freshwater habitats. Spirogyra lacks a motile variant at all stages of its life history; i.e. no motile gametes (ova or sperm), no zoospores etc. Sexual reproduction is done by a process called conjugation. Certain filaments in a loose parallel bundle of Spirogyra assume the female and others the male role. The cells of adjacent filaments develop bumps which grow towards one another and eventually fuse to form a conjugation tube between the cells. This type of reproduction is called cystogamy. Meanwhile the contents of each cell have detached themselves from their respective cell walls and have formed a round ball. Fertilization occurs when the contents of a (-) male cell moves through the tube and fuses to the contents of a (+) female cell. This fertilization takes place between somatic cells without differentiation of sexual cells, so it is a somatogamy. The resulting zygote forms a thick walled spore (zygospore) with a tough resistant outer covering within the chambers of the female filament (+) that breaks away from the parent. After a dormant period, these zygotes undergo meiosis and germinate, resulting in new haploid filaments of Spirogyra. Meiosis occurs immediately after karyogamy, it is a monobiontic haplontic cycle. 14  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 17: Conjugation tube Figure 18: Spirogyra zygote formation 2.2. Monobiontic Diplontic life cycle (monogenetic): Fucus vesiculosis Diplontic life cycles (gametic meiosis) are fairly unique among life history of algae. The genus Fucus vesiculosus is characterized by a dichotomous thallus composed of flat frond with paired air bladders. These air bladders help keep the brown algae afloat. Sporophytes are either homothallic (monoecious: Fucus vesiculosis monoica) or heterothallic (dioecious: Fucus vesiculosus dioica) depending on species. 15  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 19: Fucus vesiculosus morphology The life cycle contains a reduced haploid gametophyte and a well shown diploid sporophyte (monobiontic diplontic life cycle). Both forms are distinct one from the other with the sporophyte being the visible form (heteromorphic). At the tip of each frond a receptacle is formed containing conceptacles, it is within these conceptacles that sporangia, i.e. male and female gametangia, are formed. Conceptacles bear also filaments of sterile cells called paraphyses. The female gametangia, oogonia, use meiosis to produce haploid non motile female gametes (eggs). The male gametangia, antheridia, follow a similar procedure to produce haploid motile male gametes (sperm). Eggs and sperm are released simultaneously into water; the eggs release a pheromone that attracts the sperm (chemotaxis). Fertilization occurs externally (in water), by oogamy. The fertilized egg settles to the substrate where it becomes attached after just a few hours. The zygote germinates immediately. By mitosis and differentiation the zygote develops into a mature diploid plant. 16  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 20: Diplontic life cycle of a monoecious Fucus 2.3. Haplodiplontic life cycle (digenetic). Haplodiplontic (diplobiontic) life cycles include both a sporophyte (diploid generation) and a gametophyte (haploid generation), characteristic feature of what is known as alternation of generations. In some species (e.g. Ulva lactuca) the two mature forms of algae, alternating between diploid and haploid individuals, are identical in appearance; such generations are called isomorphic. When the gametophyte and the sporophyte are distinct in appearance (e.g. Laminaria), generations are termed heteromorphic. 17  Plant Reproduction – Lebanese University, Faculty of Sciences 2.3.1. Isomorphic haplodiplontic life cycle: Ulva All Ulva species alternate between gametophytic and sporophytic life stages (haplodiplontic life cycle n/2n) with similar morphologies (isomorphic). The gametophytes are haploid and the sporophytes are diploid. The gametophytes produce biflagellate haploid gametes through mitosis, and the sporophytes produce quadriflagellate haploid zoospores through meiosis. Both gametes are motile (zoogamy, planogamy) and the female is a little bit larger (anisogamous). Reproductive activities occur near the margins of Ulva fronds, with the fertile portions turning slightly brown. Figure 21: Ulva lactuca life cycle (isomorphic haplodiplontic) 2.3.2. Heteromorphic haplodiplontic life cycle: Laminaria Laminaria exhibits not only an alternation of generations but also heteromorphy. The sporophyte plant is a large multicelled alga whereas the microscopic female and male gametophytes are only one cell or a few cells in size. The sporophyte generation (2n) develops on the surface of the blades sori containing sporangia and paraphyses; sporangia produce by meiosis motile flagellated zoospores (n ch), which develop into male and female gametophytes (n ch). Male gametophytes release motile male gametes and female gametophytes produce eggs, which remain attached to the female gametophytes. Male gametes fertilize the eggs; the fertilization of Laminaria is oogamous. 18  Plant Reproduction – Lebanese University, Faculty of Sciences Figure 22: Laminaria life cycle (heteromorphic haplodiplontic). (Unilocular sporangia are called so because they are not divided by cross walls or locules) 2.4. Triplobiontic life cycle (Trigenetic): Polysiphonia, Antithamnion, Nemalion Most red algae (Rhodophyta) have a trigenetic life cycle. A typical example of trigenetic life cycle of Algae is the one of Polysiphonia (Rhodophyta). Other red algae display the same pattern of life cycle, e.g. Antithamnion, Nemalion… The Trigenetic cycle include three successive generations:  The first generation : the gametophyte stage. The gametophytes male and female are isomorphic gametophytes presenting n chromosomes. The male sex structures called spermatangia produce non motile sperm cells called spermatia. The female sex structures are egg-like cells called carpogonia. Each carpogonium consists of a single large cell with a long tubular hair-like extension the trichogyne that basically acts as a receptor of drifting spermatia. Since reproductive organs of Rhodophyta are non motile, the spermatia are carried to the trichogyne of carpogonium by water currents. If a spermatium should brush against a trichogyne, it may become attached, and plasmogamy occurs. Then the nucleus of the spermatium migrates to the egg nucleus and fuses with it, forming a zygote. This type of 19  Plant Reproduction – Lebanese University, Faculty of Sciences fertilization is trichogamy. The produced zygote goes on to develop parasitically on the female gametophyte.  The second generation : the carposporophyte stage (parasitic on the gametophyte). The zygote, divides repeatedly by mitosis and gives birth to the carposporophyte with 2n chromosomes, producing then the carpospores (2n chromosomes) in the carposporangia. These carpospores are released and carried away by ocean currents.  The third generation : the tetrasporophyte stage. When a carpospore lodges in a suitable location, it germinates and grows into a tetrasporophyte. Tetrasporangia are formed along the branches of the tetrasporophyte. Each tetrasporangium undergoes meiosis, giving rise to four haploid tetraspores. When tetraspores germinate, they develop into male and female gametophyte, thereby completing the life cycle. In Polysiphonia, the three types of thalli (male gametophyte, female gametophyte and tetrasporophyte) all outwardly resemble one another. Figure 23: Trigenetic life cycle N.B.  Gametangium (pl. gametangia) any cell or structure in which gametes are produced (equivalent to both “gametocyste” and “gametangia” in french)  Sporangium (pl. sporangia) a structure in which spores are produced; it may be either unicellular or multicellular (equivalent to both “sporocyste” and “sporangia” in french) 20