Unit of reproduction adapted for dispersal and survival in unfavorable conditions
Spores produced in a sporic life cycle.
Fresh snow partially covers rough-stalked feather-moss
(
Brachythecium rutabulum
)
, growing on a
thinned
hybrid black poplar
(
Populus x canadensis
)
. The last stage of the
moss lifecycle
is shown, where the
sporophytes
are visible before dispersion of their spores: the
calyptra
(
1
) is still attached to the
capsule
(
3
). The tops of the
gametophytes
(
2
) can be discerned as well. Inset shows the surrounding, black poplars growing on sandy
loam
on the bank of a
kolk
, with the detail area marked.
In
biology
, a
spore
is a unit of
sexual
(in fungi) or
asexual reproduction
that may be adapted for
dispersal
and for survival, often for extended periods of time, in unfavourable conditions.
[1]
Spores form part of the
life cycles
of many
plants
,
algae
,
fungi
and
protozoa
.
[2]
They were thought to have appeared as early as the mid-late
Ordovician
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]
In plants, spores are usually
haploid
and
unicellular
and are produced by
meiosis
in the
sporangium
of a
diploid
sporophyte
. In some rare cases, diploid spore is also produced in some algae, or fungi.
[6]
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
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
[
edit
]
The term
spore
derives from the
ancient Greek
word σπορ?
spora
, meaning "
seed
, sowing", related to σπ?ρο?
sporos
, "sowing", and σπε?ρειν
speirein
, "to sow".
In common parlance, the difference between a "spore" and a "
gamete
" 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 main difference between spores and seeds as
dispersal 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.
Classification of spore-producing organisms
[
edit
]
Plants
[
edit
]
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
ferns
of the order
Salviniales
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 a
megasporangium
that produces megaspores or a
microsporangium
that produces microspores. In flowering plants, these sporangia occur within the carpel and anthers, respectively.
Fungi
[
edit
]
Fungi
commonly produce spores during sexual and asexual reproduction. Spores are usually
haploid
and grow into mature haploid individuals through
mitotic
division of cells (
Urediniospores
and
Teliospores
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.
Classification of spores
[
edit
]
Spores can be classified in several ways such as by their spore producing structure, function, origin during life cycle, and mobility.
Below is a table listing the mode of classification, name, identifying characteristic, examples, and images of different spore species.
Mode of Classification
|
Name
|
Identifying Characteristic
|
Example Spore Containing Organism
|
Image
|
Spore Producing Structure
|
Sporangiospore
|
Produced by
sporangium
|
Zygomycetes
|
Sporangium of Fungi
|
Zygospores
|
Produced by
zygosporangium
|
Zygomycetes
|
Zygospores on
Rhizopus
|
Ascospores
|
Produced by
ascus
|
Ascomycetes
|
Ascospores of
Didymella Rabiei
|
Basidiospores
|
Produced by
basidium
|
Basidiomycetes
|
Typical reproductive structure of a
basidiomycete
, including the
basidiospore
and
basidium
|
Aecispores
|
Produced by
aecium
|
Rusts and Smuts
|
Aecia on foliage
|
Urediniospores
|
Produced by uredinium
|
Rusts and Smuts
|
Uredinospores
|
Teliospores
|
Produced by
teilum
|
Rusts and Smuts
|
Microscopic image of teliospores
|
Oospores
|
Produced by
oogonium
|
Oomycetes
|
Oospores of
Phytophthora agathidicida
|
Carpospores
|
Produced by carposophorophyte
|
Red Algae
|
Light microscopy of
Polysiphonia
showing a carpospores and carposporophyte inside
|
Tetraspores
|
Produced by
tetrasphorophyte
|
Red Algae
|
Tetraspores of
Polysiphonia
|
Function
|
Chalmydospore
|
Thick-walled
resting spores
of fungi produced to survive in unfavorable conditions
|
Asomycota
|
Pseudohyphae
, chlamydospores and
blastospores
of
Candida
yeast
.
|
Parasitic Fungal Spore
|
Internal Spores
|
Germinate within a host
|
|
A parasitic pink fungi on a
Lichen
tree
|
External (Environmental) spores
|
Spores released by the host to infest other hosts
[7]
|
|
Origin During Life Cycle
|
Meiospores
|
Microspores
|
Produced sexually through
meiosis
, and give rise to a male
gametophyte
|
Pollen
in seed plants
|
In
plants
,
microspores
, and in some cases megaspores, are formed from all four products of meiosis.
|
Megaspores
(macrospores)
|
Produced sexually through
meiosis
, and give rise to a female
gametophyte
|
Ovule
in seed plants
|
In contrast, in many
seed plants
and heterosporous
ferns
, only a single product of meiosis will become a
megaspore
(macrospore), with the rest degenerating.
|
Mitospores
|
Produced asexually though
mitosis
|
Ascomycetes
|
Ascomycete containing mitospores
|
Mobility
|
Zoospores
|
Mobile through
flagella
|
Some algae and fungi
|
Microscopic image of a Zoospore
|
Aplanospores
|
Immobile, however still produce flagella
|
|
|
Autospores
|
Immobile spores that
do not
produce flagella
|
|
Autospores of a strain of
Jenufa aeroterrestrica
|
Ballistospores
|
Forcibly discharged from the fungal
fruiting body
due to internal force (such as built up pressure)
|
Basidiospores and/or part of the genus
Pilobus
|
Ballistospore mechanism of dispersal from fungi
|
Stratismospores
|
Forcibly discharged from the fungal
fruting body
due to external force (such as raindrops or passing animals)
|
Puffballs
|
Puff Balls containing Stratismospores
|
External anatomy
[
edit
]
Fossil trilete spores (blue) and a spore tetrad (green) of
Late Silurian
origin
Tricolpate pollen of
Ricinus
Under high
magnification
, 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. In
monolete spores
, there is a single narrow line (laesura) on the spore.
[8]
Indicating the prior contact of two spores that eventually separated.
[3]
In
trilete 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 a
colpus
.
[8]
The number of colpi distinguishes major groups of plants.
Eudicots
have
tricolpate
spores (i.e. spores with three colpi).
[9]
Spore tetrads and trilete spores
[
edit
]
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 modern
cryptogamic
plants first appeared in the fossil record at the end of the Ordovician period.
[12]
Dispersal
[
edit
]
Spores being ejected by fungi.
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.
[13]
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
.
[14]
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 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-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
.
[16]
Origin
[
edit
]
Spores have been found in
microfossils
dating back to the mid-late
Ordovician
period.
[17]
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 as
embryophytes
, that allowed for plants to easily disperse while adapting to their non-aquatic environment.
[17]
[18]
This is particularly supported by the observation of a thick spore wall in
cryptospores
. These spore walls would have protected potential offspring from novel weather elements.
[17]
The second more recent hypothesis is that spores were an early predecessor of land plants and formed during errors in the
meiosis
of
algae
, a hypothesized early ancestor of land plants.
[19]
Whether spores arose before or after land plants, their contributions to topics in fields like
paleontology
and plant
phylogenetics
have been useful.
[19]
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.
[17]
Both cryptospores and modern spores have diverse morphology that indicate possible environmental conditions of earlier periods of Earth and evolutionary relationships of plant species.
[17]
[19]
[18]
Gallery
[
edit
]
-
Spores of the
moss
Bartramia ithyphylla
. (microscopic view, 400x)
-
Dehisced fern sporangia. (microscopic view, no spores are visible)
-
Spores and elaters from a horsetail. (
Equisetum
, microscopic view)
-
Fossil plant spores (
Scylaspora
) from Silurian deposits of Sweden.
-
Fruit mold with spores and distinguishable cellular growth. (2000x)
-
Spore clusters, formed inside sporangia of the slime mold
Reticularia olivacea
, from pine forests of eastern
Ukraine
.
-
Internal surface of the
peridium
of the slime mold
Tubifera dudkae
with spores.
See also
[
edit
]
References
[
edit
]
- ^
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"Spores and Their Significance"
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ISBN
978-1-68367-058-2
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13 December
2023
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Myxospores consist of several cells, which are transformed to shell valves, nematocyst-like polar capsules with coiled extrudible polar filaments and amoeboid infective germs.
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