- Sporogenesis
Sporogenesis is the production of
spore s inbiology . The term is also used to refer to the process ofreproduction via spores. Reproductive spores are formed in many eukaryotic organisms, such asplant s,algae and fungi, during their normal reproductive life cycle. Dormant spores are formed, for example by certain fungi and algae, primarily in response to unfavorable growing conditions. Most eukaryotic spores arehaploid and form through cell division, though some types arediploid ordikaryon s and form through cell fusion.Reproduction via spores
Reproductive spores are generally the result of cell division, most commonly
meiosis (e.g. in plantsporophyte s). Sporic meiosis is needed to complete the sexual life cycle of the organisms using it.In some cases, sporogenesis occurs via
mitosis (e.g. in some fungi and algae). Mitotic sporogenesis is a form ofasexual reproduction . Examples are the conidial fungi "Aspergillus " and "Penicillium ", for which mitospore formation appears to be the primary mode of reproduction. Other fungi, such asascomycete s, utilize both mitotic and meiotic spores. The red alga "Polysiphonia " alternates between mitotic and meiotic sporogenesis and both processes are required to complete its complex reproductive life cycle.In the case of dormant spores in eukaryotes, sporogenesis often occurs as a result of
fertilization orkaryogamy forming a diploid spore equivalent to azygote . Therefore, zygospores are the result ofsexual reproduction .Reproduction via spores involves the spreading of the spores by water or air. Algae and some fungi (
chytrids ) often use motilezoospore s that can swim to new locations before developing into sessile organisms. Airborne spores are obvious in fungi, for example when they are released frompuffball s. Other fungi have more active spore dispersal mechanisms. For example, the fungus "Pilobolus " can shoot its sporangia towards light. Plant spores designed for dispersal are also referred to asdiaspore s. Plant spores are most obvious in the reproduction offern s andmoss es. However, they also exist inflowering plant s where they develop hidden inside the flower. For example, thepollen grain s of flowering plants develop out of microspores produced in theanther s.Reproductive spores grow into multicellular haploid individuals or
sporeling s. Inheterosporous organisms, two types of spores exist: microspores give rise to males and megaspores to females. Inhomosporous organisms, all spores look alike and grow into individuals carrying reproductive parts of both genders.Formation of reproductive spores
Sporogenesis occurs in reproductive structures termed sporangia. The process involves sporogenous cells (sporocytes, also called spore mother cells) undergoing cell division to give rise to spores.
Meiospore formation
In meiotic sporogenesis, a diploid spore mother cell within the sporangium undergoes meiosis, producing a tetrad of haploid spores. In organisms that are
heterosporous , two types of spores occur: Microsporangia producemale microspores, and megasporangia producefemale megaspores. In megasporogenesis, often three of the four spores degenerate after meiosis, whereas in microsporogenesis all four microspores survive.In
gymnosperm s, such asconifer s, microspores are produced through meiosis from microsporocytes in microstrobili or male cones. Inflowering plant s, microspores are produced in theanther s of flowers. Each anther contains fourpollen sac s, which contain the microsporocytes. After meiosis, each microspore undergoes mitotic cell division, giving rise to multicellularpollen grain s (six nuclei in gymnosperms, three nuclei in flowering plants).Megasporogenesis occurs in megastrobili in conifers (for example a pine cone) and inside the
ovule in the flowers of flowering plants. A megasporocyte inside a megasporangium or ovule undergoes meiosis, producing four megaspores. Only one is a functional megaspore whereas the others stay dysfunctional or degenerate. The megaspore undergoes several mitotic divisions to develop into a femalegametophyte (for example the seven-cell/eight-nucleiembryo sac in flowering plants).Mitospore formation
Some fungi and algae produce
mitospore s through mitotic cell division within a sporangium. In fungi, such mitospores are referred to asconidia .Formation of dormant spores
Some algae, fungi and bacteria form
resting spore s designed to survive unfavorable conditions. Typically, changes in the environment from favorable to unfavorable growing conditions will trigger a switch fromasexual reproduction tosexual reproduction in these organisms. The resulting spores are protected through the formation of a thickcell wall and can withstand harsh conditions such as drought or extreme temperatures. Examples arechlamydospore s,teliospore s,zygospore s, and myxospores.Chlamydospore and teliospore formation
Chlamydospore s are generally multicellular, asexual structures.Teliospore s are a form of chlamydospore produced through the fusion of cells orhyphae where the nuclei of the fused cells stay separate. These nuclei undergokaryogamy and meiosis upon germination of the spore.Zygospore, oospore and auxospore formation
Zygospore s are formed in certain fungi (zygomycota , for example "Rhizopus ") and some algae (for example "Chlamydomonas "). The zygospore forms through the isogamic fusion of two cells (motile single cells in "Chlamydomonas") or sexualconjugation between two hyphae (in zygomycota).Plasmogamy is followed bykaryogamy , therefore zygospores are diploid (zygote s). They will undergo zygotic meiosis upon germinating.In
oomycete s, the zygote forms through the fertilization of an egg cell with a sperm nucleus and enters a resting stage as a diploid, thick-walledoospore . The germinating oospore undergoes mitosis and gives rise to diploid hyphae which reproduce asexually via mitotic zoospores as long as conditions are favorable.In
diatom s, fertilization gives rise to a zygote termedauxospore . Besides sexual reproduction and as a resting stage, the function of an auxospore is the restoration of the original cell size, as diatoms get progressively smaller during mitotic cell division. Auxospores divide by mitosis.Endospore formation
The term sporogenesis can also refer to
endospore formation inbacteria , which allows the cells to survive unfavorable conditions. Endospores are not reproductive structures and their formation does not require cell fusion or division. Instead, they form through the production of an encapsulating spore coat within the spore-forming cell.Bibliography
*S.S. Mader (2007): "Biology", 9th edition, McGraw Hill Companies, New York, ISBN 978-0-07-246463-4
*P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): "Biology of Plants", 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2
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