| Talk about algae reproduction in general | 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. |
| Talk about Asexual reproduction of algae | - Mitotic division for single-celled forms ex. Diatoms.
- Fragmentation for multicellular organisms. Some multicellular algae, e.g. Sargassum reproduce asexually through fragmentation, in which fragments of the parent develop into new individuals.
-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 a colony.
- 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. |
| Talk about monobiontic haplontic sexual reproduction of algae | Spirogyra is a filamentous haploid green alga (gametophyte) which is common in freshwater habitats. When sexual reproduction begins, the individual cells of adjacent aligned filaments (of two different mating types) form protuberances which grow and fuse to form a conjugation tube connecting the opposite pair of cells.
Fertilization occurs when the condensed protoplast of cells of one filament, migrates through a conjugation tube and fuses with the protoplast of the adjacent cells of the second mating filament, forming a zygote. Each filament of protoplast moving cells is considered as a male filament, while the stationary one functions as a female filament. Sexual reproduction is done by conjugation.
The resulting zygote forms a zygospore with a tough resistant outer covering within the cells of the female filament. The zygospores break away from the parent when the filament cell walls rupture. After a period of dormancy, on return of favorable conditions, these zygotes undergo meiosis and germinate, resulting in new haploid filaments of Spirogyra |
| Talk about monobiotic diplontic cycle of algae | Fucus is a brown alga presenting a branched thallus (diploid sporophyte). At the tips of the frond, the receptacles bear many small cavities called conceptacles. Each conceptacle contains the gametangia (structures where gametes are produced) and sterile hairlike filaments called paraphyses. Some species develop male and female gametangia inside the same conceptacle, whereas other species have separate male and female conceptacles on separate thallus
The female gametangia, oogonia, produce by meiosis haploid non motile female gametes (eggs). The male gametangia, antheridia, produce by meiosis haploid biflagellated (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. The zygote develops by mitosis a mature diploid sporophyte. |
| Talk about digenetic life cycle of algae *isomorphic* | All Ulva species alternate between gametophytic and sporophytic life stages (haplodiplontic life cycle n/2n) with similar morphologies (isomorphic generation). The gametophytes are haploid and the sporophytes are diploid. Both gametes (biflagellated) are motile (zoogamy, planogamy) and the female gamete is larger than the male (anisogamous). The gametes fuse to produce a quadriflagellate zygote. The zygote swims for a short time then comes to rest, withdraws its flagella and grows into a new sporophyte. The mature sporophyte forms sporangia and produces by meiosis haploid quadriflagellate zoospores. Zoospores withdraw their flagella and develop into gametophytes. |
| Talk about digenetic cycle of algae *heteromorphic* | Laminaria exhibits an alternation of heteromorphic generations. The sporophyte plant is a large multicellular alga whereas the microscopic female and male gametophytes are composed of few cells.
The sporophyte generation (2n) develops on the surface of its 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. Female gametophytes produce eggs which remain attached to it. Male gametes fertilize the eggs; the fertilization of Laminaria is oogamous. |
| Talk in general about trigenic algae cycle | Most red algae (Rhodophyta) have a trigenetic life cycle. A typical example of trigenetic life cycle of Algae is the one of Polysiphonia and Nemalion…
The Trigenetic cycle include three successive generations |
| Talk about the first generation of red algae | The male and female gametophytes are isomorphic haploid generations and bear the gametangia. The male gametangia called spermatangia produce non motile sperm cells called spermatia. The female gametangia are called carpogonia. Each carpogonium consists of a single large cell and has 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 fertilization is trichogamy. The produced zygote goes on to develop parasitically on the female gametophyte. |
| Talk about the second generation of red algae | carposporophyte stage (parasitic on the gametophyte). The zygote, divides repeatedly by mitosis and gives birth to the carposporophyte (diploid, Sporophyte I), producing then diploid carpospores in the carposporangia. These carpospores are released and carried away by ocean currents. |
| Talk about the third generation of red algae | tetrasporophyte stage. When a carpospore lodges in a suitable location, it germinates and grows into a diploid tetrasporophyte (Sporophyte II). Tetrasporangia are formed along the branches of the tetrasporophyte. Mother cells of each tetrasporangium undergoe meiosis, giving rise to four haploid tetraspores. When tetraspores germinate, they develop into male and female gametophyte, thereby completing the life cycle.
In this life cycle, the three types of thalli (male gametophyte, female gametophyte and tetrasporophyte) all outwardly resemble one another. |
