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In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many plants, algae, fungi and some protozoans.[1] A chief difference between spores and seeds as dispersal units is that spores have very little stored food resources compared with seeds.
Spores are usually haploid and unicellular and are produced by meiosis in the sporophyte. Once conditions are favorable, the spore can develop into a new organism using mitotic division, producing a multicellular gametophyte, which eventually goes on to produce gametes.
Two gametes fuse to create a new sporophyte. This cycle is known as alternation of generations, but a better term is "biological life cycle", as there may be more than one phase and so it cannot be a direct alternation. Haploid spores produced by mitosis (known as mitospores) are used by many fungi for asexual reproduction.
Spores produced in a sporic life cycle.
Spores are the units of asexual reproduction, because a single spore develops into a new organism. By contrast, gametes are the units of sexual reproduction, as two gametes need to fuse to create a new organism.
The term spore may also refer to the dormant stage of some bacteria or archaea; however these are more correctly known as endospores and are not truly spores in the sense discussed in this article. The term can also be loosely applied to some animal resting stages. Fungi that produce spores are known as sporogenous, and those that do not are asporogenous.
The term derives from the ancient Greek word sp??a ("spora"), meaning a seed.
Classification
Spores can be classified in several ways:
By spore-producing structure
In fungi and fungus-like organisms, spores are often classified by the structure in which meiosis and spore production occurs. Since fungi are often classified according to their spore-producing structures, these spores are often characteristic of a particular taxon of the fungi.
Fern sori, clusters of meiosporangia on the underside of the leaf.
In plants, microspores, and in some cases megaspores, are formed from all four products of meiosis.
In contrast, in many seed plants and heterosporous ferns, only a single product of meiosis will become a megaspore (macrospore), with the rest degenerating.
By function
- Chlamydospores: thick-walled resting spores of fungi produced to survive unfavorable conditions.
By origin during life cycle
By motility
Spores can be differentiated by whether they can move or not.
Trilete spores
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A late Silurian sporangium. Green: A spore tetrad. Blue: A spore bearing a trilete mark – the Y-shaped scar. The spores are about 30-35 µm across.
Trilete spores, formed by the dissociation of a spore tetrad, are taken as the earliest evidence of life on land,[2] dating to the mid-Ordovician (early Llanvirn, ~470 million years ago).[3]
Parlance
In common parlance, the difference between a "spore" and a "gamete" (both together called gonites) is that a spore will germinate and develop into a sporeling, while a gamete needs to combine with another gamete before developing further. However, the terms are somewhat interchangeable when referring to gametes.
A chief difference between spores and seeds as dispersal units is that spores have little food storage compared with seeds, and thus require more favorable conditions in order to successfully germinate. (This is not without its exceptions, however: many orchid seeds, although multicellular, are microscopic and lack endosperm, and spores of some fungi in the Glomeromycota commonly exceed 300µm in diameter.)[4] Seeds, therefore, are more resistant to harsh conditions and require less energy to start mitosis. Spores are produced in large numbers to increase the chance of a spore surviving in a number of notable examples.
The endospores of certain bacteria are often simply called "spores," as seen in the 2001 anthrax attacks, where the media called anthrax endospores "anthrax spores." Unlike eukaryotic spores, endospores are primarily a survival mechanism, not a reproductive method, and a bacterium only produces a single endospore.
Diaspores
In the case of spore-shedding vascular plants such as ferns, wind distribution of very light spores provides great capacity for dispersal. Also, spores are less subject to animal predation than seeds because they contain almost no food reserve; however they are more subject to fungal and bacterial predation. Their chief advantage is that, of all forms of progeny, spores require the least energy and materials to produce.
Vascular plant spores are always haploid and vascular plants are either homosporous or heterosporous. Plants that are homosporous produce spores of the same size and type. Heterosporous plants, such as spikemosses, quillworts, and some aquatic ferns produce spores of two different sizes: the larger spore in effect functioning as a "female" spore and the smaller functioning as a "male".
Under high magnification, spores can be categorized as either monolete spores or trilete spores. In monolete spores, there is a single line on the spore indicating the axis on which the mother spore was split into four along a vertical axis. In trilete spores, all four spores share a common origin and are in contact with each other, so when they separate, each spore shows three lines radiating from a center pole.
Parasitic Fungal spores
Parasitic fungal spores may be classified into internal spores, which germinate within the host, and external spores, also called environmental spores, released by the host to infest other hosts.[5]
See also
References
- ^ Spore FAQ - Aerobiology Research Laboratory
- ^ Gray, J. (1985). "The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970-1984". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (1934-1990) 309 (1138): 167–195. doi:10.1098/rstb.1985.0077, http://links.jstor.org/sici?sici=0080-4622(19850402)309%3A1138%3C167%3ATMROEL%3E2.0.CO%3B2-E. Retrieved on 26 April 2008.
- ^ Wellman, C.H., Gray, J. (2000). "The microfossil record of early land plants". Philosophical Transactions: Biological Sciences 355 (1398): 717–732. doi:10.1098/rstb.2000.0612, http://www.journals.royalsoc.ac.uk/index/2NWB35JF2C34PJHG.pdf. Retrieved on 13 February 2008.
- ^ INVAM
- ^ [1].
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