Spores produced in a sporic life cycle.Fresh snow partially covers rough-stalked feather-moss(Brachythecium rutabulum), growing on athinned hybrid black poplar(Populus x canadensis). The last stage of themoss lifecycle is shown, where thesporophytes are visible before dispersion of their spores: thecalyptra (1) is still attached to thecapsule (3). The tops of thegametophytes (2) can be discerned as well. Inset shows the surrounding, black poplars growing on sandyloam on the bank of akolk, with the detail area marked.
Inbiology, aspore is a unit ofsexual (in fungi) orasexual reproduction that may be adapted fordispersal and for survival, often for extended periods of time, in unfavourable conditions.[1] Spores form part of thelife cycles of manyplants,algae,fungi andprotozoa.[2] They were thought to have appeared as early as the mid-lateOrdovician period as an adaptation of early land plants.[3]
Bacterial spores are not part of a sexual cycle, but are resistant structures used for survival under unfavourable conditions.[4]Myxozoan spores release amoeboid infectious germs ("amoebulae") into their hosts for parasitic infection, but also reproduce within the hosts through the pairing of two nuclei within the plasmodium, which develops from the amoebula.[5]
The spores ofseed plants 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, theseeds andpollen grains.
The termspore derives from theancient Greek word σποράspora, meaning "seed, sowing", related to σπόροςsporos, "sowing", and σπείρεινspeirein, "to sow".
In common parlance, the difference between a "spore" and a "gamete" is that a spore will germinate and develop into asporeling, while a gamete needs to combine with another gamete to form a zygote before developing further.
The main difference between spores and seeds asdispersal units is that spores are unicellular, the first cell of a gametophyte, while seeds contain within them a developing embryo (the multicellular sporophyte of the next generation), produced by the fusion of the male gamete of the pollen tube with the female gamete formed by the megagametophyte within the ovule. Spores germinate to give rise to haploid gametophytes, while seeds germinate to give rise to diploid sporophytes.
Vascular plant spores are alwayshaploid. Vascular plants are eitherhomosporous (or isosporous) orheterosporous. Plants that are homosporous produce spores of the same size and type.
Heterosporous plants, such asseed plants,spikemosses,quillworts, andferns of the orderSalviniales 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". Such plants typically give rise to the two kind of spores from within separate sporangia, either amegasporangium that produces megaspores or amicrosporangium that produces microspores. In flowering plants, these sporangia occur within the carpel and anthers, respectively.
Fungi commonly produce spores during sexual and asexual reproduction. Spores are usuallyhaploid and grow into mature haploid individuals throughmitotic division of cells (Urediniospores andTeliospores among rusts are dikaryotic).Dikaryotic cells result from the fusion of two haploid gamete cells. Among sporogenic dikaryotic cells, karyogamy (the fusion of the two haploid nuclei) occurs to produce a diploid cell. Diploid cells undergo meiosis to produce haploid spores.[citation needed]
In contrast, in manyseed plants and heterosporousferns, only a single product of meiosis will become amegaspore (macrospore), with the rest degenerating.
Fossil trilete spores (blue) and a spore tetrad (green) ofLate Silurian originTricolpate pollen ofRicinus
Under highmagnification, spores often have complex patterns or ornamentation on their exterior surfaces. A specialized terminology has been developed to describe features of such patterns. Some markings represent apertures, places where the tough outer coat of the spore can be penetrated when germination occurs. Spores can be categorized based on the position and number of these markings and apertures.Alete spores show no lines. Inmonolete spores, there is a single narrow line (laesura) on the spore.[8] Indicating the prior contact of two spores that eventually separated.[3] Intrilete spores, each spore shows three narrow lines radiating from a center pole.[8] This shows that four spores shared a common origin and were initially in contact with each other forming a tetrahedron.[3] A wider aperture in the shape of a groove may be termed acolpus.[8] The number of colpi distinguishes major groups of plants.Eudicots havetricolpate spores (i.e. spores with three colpi).[9]
Envelope-enclosed spore tetrads are taken as the earliest evidence of plant life on land,[10] dating from the mid-Ordovician (early Llanvirn, ~470 million years ago), a period from which no macrofossils have yet been recovered.[11]Individual trilete spores resembling those of moderncryptogamic plants first appeared in the fossil record at the end of the Ordovician period.[12]
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 ashydrophobins, for spore ejection. These mechanisms include, for example, forcible discharge of ascospores enabled by the structure of the ascus and accumulation ofosmolytes in the fluids of the ascus that lead to explosive discharge of the ascospores into the air.[13]
The forcible discharge of single spores termedballistospores 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 10,000g.[14] Other fungi rely on alternative mechanisms for spore release, such as external mechanical forces, exemplified bypuffballs. 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 thestinkhorns.
In Common Smoothcap moss (Atrichum undulatum), the vibration of sporophyte has been shown to be an important mechanism for spore release.[15]
In the case of spore-sheddingvascular 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.
Spores have been found inmicrofossils dating back to the mid-lateOrdovician period.[3] Two hypothesized initial functions of spores relate to whether they appeared before or after land plants. The heavily studied hypothesis is that spores were an adaptation of early land plant species, such asembryophytes, that allowed for plants to easily disperse while adapting to their non-aquatic environment.[3][17] This is particularly supported by the observation of a thick spore wall incryptospores. These spore walls would have protected potential offspring from novel weather elements.[3] The second more recent hypothesis is that spores were an early predecessor of land plants and formed during errors in themeiosis ofalgae, a hypothesized early ancestor of land plants.[18]
Whether spores arose before or after land plants, their contributions to topics in fields likepaleontology and plantphylogenetics have been useful.[18] The spores found in microfossils, also known as cryptospores, are well preserved due to the fixed material they are in as well as how abundant and widespread they were during their respective time periods. These microfossils are especially helpful when studying the early periods of earth as macrofossils such as plants are not common nor well preserved.[3] Both cryptospores and modern spores have diverse morphology that indicate possible environmental conditions of earlier periods of Earth and evolutionary relationships of plant species.[3][18][17]
^Abel-Santos, Ernesto (2012).Bacterial spores: current research and applications. Norfolk: Caister academic press.ISBN978-1-908230-00-3.
^Ivan Fiala (10 July 2008)."Myxozoa".Tree of Life Web Project. Archived fromthe original on 16 February 2015. Retrieved14 January 2014.Myxospores consist of several cells, which are transformed to shell valves, nematocyst-like polar capsules with coiled extrudible polar filaments and amoeboid infective germs.
^Johansson, Lönnell, Sundberg and Hylander (2014) Release thresholds for moss spores: the importance of turbulence and sporophyte length. Journal of Ecology, n/a-n/a.
.jpg)
.png)
