Different types Spores of Fungi
Different types Spores of Fungi
Spores are
minute reproductive or dispersal units produced by fungi. They are usually
single-celled or multicellular structures capable of developing into a new
fungal thallus under favourable conditions. Fungal spores are extremely diverse
in their origin, structure, method of formation, mobility, pigmentation and
function. In general, they are grouped into asexual spores, which are produced
without nuclear fusion and mainly help in rapid multiplication and dispersal,
and sexual spores, which are formed after plasmogamy, karyogamy and meiosis and
contribute to genetic variation.
Asexual Spores
Asexual spores,
also called mitospores, are formed by mitotic division and do not involve
fusion of compatible nuclei. They are especially important for rapid
colonization when environmental conditions are favourable. Depending on whether
they are formed inside a spore-bearing sac or externally on a hypha, asexual
spores are broadly described as sporangiospores or conidia.
1. Sporangiospores
Sporangiospores
are asexual spores produced endogenously inside a sac-like structure called a
sporangium. The sporangium is usually borne at the tip of a specialized hypha
called a sporangiophore. When mature, the sporangial wall breaks or dissolves
and releases the spores. A common example is Rhizopus stolonifer, the black bread mould, in which numerous
non-motile sporangiospores are produced inside spherical sporangia.
2. Zoospores
Zoospores are
motile, usually naked asexual spores provided with one or more flagella. They
are produced in zoosporangia and require free water for movement. After
swimming for a period, a zoospore loses its flagellum, develops a wall and
germinates. Zoospores are characteristic of many chytrids; for example, Allomyces produces flagellated
zoospores. Similar motile spores also occur in fungus-like oomycetes such as Phytophthora.
3. Aplanospores
Aplanospores are
non-motile spores formed within a sporangium. Unlike zoospores, they lack
flagella and are dispersed mainly by air, water splash or mechanical
disturbance. The sporangiospores of Rhizopus
and Mucor are typical examples of
non-motile aplanospores.
4. Conidia
Conidia are
non-motile asexual spores produced externally, usually at the tips or sides of
specialized hyphae called conidiophores. They are not enclosed within a
sporangium. Conidia may occur singly, in chains or in clusters and may be
one-celled or multicellular. Aspergillus
produces chains of conidia from a swollen conidiophore tip, while Penicillium produces chains of conidia
on characteristically branched, brush-like conidiophores.
5. Arthroconidia (Arthrospores)
Arthroconidia
are formed by fragmentation of an existing hypha into short, thick-walled
cells. Each separated segment behaves as a spore and may germinate into a new
mycelium. This method is therefore both a form of vegetative fragmentation and
asexual reproduction. Arthroconidia are well developed in fungi such as Geotrichum candidum and Coccidioides species.
6. Blastoconidia (Blastospores)
Blastoconidia
are produced by budding from a parent cell. A small outgrowth develops on the
mother cell, receives a daughter nucleus and enlarges before separating or
remaining attached. This type of spore formation is typical of yeasts. Saccharomyces cerevisiae commonly
reproduces by budding and forms blastoconidia, while Candida species may form both blastoconidia and pseudohyphae.
7. Chlamydospores (Chlamydoconidia)
Chlamydospores
are thick-walled resting asexual spores formed from ordinary hyphal cells or
from swollen terminal or intercalary cells. Their thick walls and accumulated
food reserves help the fungus survive drought, nutrient shortage and other
unfavourable conditions. When conditions improve, they germinate and produce
new hyphae. Fusarium species may form
chlamydospores, and Candida albicans
characteristically produces terminal chlamydospores under suitable laboratory
conditions.
8. Macroconidia and Microconidia
In some fungi,
conidia occur in two distinct size classes. Macroconidia are relatively large,
often multicellular and septate, whereas microconidia are smaller and usually
one-celled. This distinction is especially useful in identifying many
filamentous fungi. Fusarium species
typically produce sickle-shaped multicellular macroconidia as well as smaller
microconidia. Dermatophytes such as Microsporum
and Trichophyton may also produce
characteristic macroconidia and microconidia.
Sexual Spores
Sexual spores
are formed as a result of sexual reproduction. During this process, compatible
cells or hyphae undergo plasmogamy, in which cytoplasm fuses, followed sooner
or later by karyogamy, in which nuclei fuse. Meiosis then restores the haploid
condition and produces genetically recombined spores. The major sexual spores
of fungi are zygospores, ascospores and basidiospores. Oospores are also
commonly discussed in introductory mycology in relation to oomycetes, although
modern classification places oomycetes outside the true fungi.
9. Zygospores
A zygospore is a
thick-walled resting sexual spore formed after fusion of two compatible
gametangia or hyphal branches. It is resistant to adverse environmental
conditions and generally undergoes a period of dormancy. On germination,
meiosis occurs and new haploid spores are produced. The classical example is Rhizopus, in which compatible mating
strains form suspensors and gametangia that fuse to produce a dark,
thick-walled zygospore. Zygospores are traditionally associated with the former
group Zygomycota.
10. Ascospores
Ascospores are
sexual spores produced inside a microscopic sac called an ascus. Karyogamy
occurs in the young ascus, producing a diploid nucleus, and this is followed by
meiosis. Frequently, one additional mitotic division occurs so that a typical
ascus contains eight ascospores, although the number can vary. Ascospores are
characteristic of the phylum Ascomycota.
Examples include Saccharomyces, Neurospora crassa, Peziza and Morchella. In
many ascomycetes, numerous asci are enclosed within a fruiting body called an
ascocarp.
11. Basidiospores
Basidiospores
are sexual spores produced externally on a club-shaped cell called a basidium.
In the basidium, two compatible nuclei fuse and the resulting diploid nucleus
undergoes meiosis, usually producing four haploid nuclei. Each nucleus enters a
developing basidiospore borne on a small projection called a sterigma.
Basidiospores are characteristic of Basidiomycota.
They occur in mushrooms such as Agaricus,
bracket fungi such as Polyporus, and
in rust and smut fungi such as Puccinia
and Ustilago.
12. Oospores
An oospore is a
thick-walled sexual resting spore formed after fertilization of a female
gametangium, the oogonium, by a male gametangium, the antheridium. The
resulting diploid oospore can survive unfavourable conditions and later
germinate. Oospores are found in oomycetes such as Pythium and Phytophthora.
These organisms were historically treated as fungi because of their filamentous
growth and absorptive nutrition, but they are now classified among the
stramenopiles rather than the true fungi.
13. Specialized Spores in Rust Fungi
Rust fungi have
exceptionally complex life cycles and may produce several morphologically and
functionally different spores. In a typical macrocyclic rust such as Puccinia graminis, pycniospores
(spermatia) are involved in mating, aeciospores infect the alternate host,
urediniospores repeatedly infect the primary host during the growing season,
teliospores function as thick-walled resting structures in which karyogamy
occurs, and basidiospores are formed after meiosis and infect the alternate
host. This diversity of spores allows rust fungi to complete complex seasonal
and host-dependent life cycles.
.jpg)
Comments
Post a Comment