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Spores

For other uses, see Spore (disambiguation).


In biology, a spore is a unit of asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavorable conditions. By contrast, gametes are units of sexual reproduction. Spores form part of the life cycles of many plants, algae, fungi and protozoa.[1] In bacteria, spores are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions.

Spores are usually haploid and unicellular and are produced by meiosis in the sporangium of a diploid sporophyte. Under favourable conditions 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 form a zygote which develops into a new sporophyte. This cycle is known as alternation of generations.

The spores of seed plants, however, are produced internally and the megaspores, formed within the ovules and the microspores are involved in the formation of more complex structures that form the dispersal units, the seeds and pollen grains.

Definition

The term spore derives from the ancient Greek word σπορά spora, meaning "seed, sowing," related to σπόρος sporos, "sowing," and σπείρειν speirein, "to sow."

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 to form a zygote before developing further.

The chief difference between spores and seeds as dispersal units is that spores are unicellular, while seeds contain within them a multicellular gametophyte that produces a developing embryo, the multicellular sporophyte of the next generation. Spores germinate to give rise to haploid gametophytes, while seeds germinate to give rise to diploid sporophytes.

Classification of spore-producing organisms

Vascular plant spores are always haploid. Vascular plants are either homosporous (or isosporous) or heterosporous. Plants that are homosporous produce spores of the same size and type. Heterosporous plants, such as seed plants, spikemosses, quillworts, and some aquatic ferns produce spores of two different sizes: the larger spore (megaspore) in effect functioning as a "female" spore and the smaller (microspore) functioning as a "male".

Classification of spores

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.

By function

  • Chlamydospores: thick-walled resting spores of fungi produced to survive unfavorable conditions.
  • 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.[2]

By origin during life cycle

By mobility

Spores can be differentiated by whether they can move or not.

  • Zoospores: mobile spores that move by means of one or more flagella, and can be found in some algae and fungi.
  • Aplanospores: immobile spores that may nevertheless potentially grow flagella.
  • Autospores: immobile spores that cannot develop flagella.
  • Ballistospores: spores that are actively discharged from the body of the fungal fruiting body. Most basidiospores are also ballistospores, and another notable example is spores of Pilobolus.
  • Statismospores: spores that are not actively discharged from the fungal fruiting body. Examples are puffballs.

Anatomy

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.

Spore tetrads and trilete spores

Envelope-enclosed spore tetrads are taken as the earliest evidence of plant life on land, Individual trilete spores resembling those of modern cryptogamic plants first appeared in the fossil record at the end of the Ordovician period.[5]


Dispersal

File:Fungus spore ejection.ogg

In fungi, both asexual and sexual spores or sporangiospores of many fungal species are actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as travelling through the air over long distances. Many fungi thereby possess specialized mechanical and physiological mechanisms as well as spore-surface structures, such as hydrophobins, for spore ejection. These mechanisms include, for example, forcible discharge of ascospores enabled by the structure of the ascus and accumulation of osmolytes in the fluids of the ascus that lead to explosive discharge of the ascospores into the air.[6] The forcible discharge of single spores termed ballistospores involves formation of a small drop of water (Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000 g.[7] Other fungi rely on alternative mechanisms for spore release, such as external mechanical forces, exemplified by puffballs. Attracting insects, such as flies, to fruiting structures, by virtue of their having lively colours and a putrid odour, for dispersal of fungal spores is yet another strategy, most prominently used by the stinkhorns.

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.

In the spikemoss Selaginella lepidophylla, dispersal is achieved in part by an unusual type of diaspore, a tumbleweed.[8]

Gallery

See also

Fungi portal

References

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