Occurrence of fungi
Fungi are found in wide variety of habitats. These habitats are water, soil, dead and decaying organic matter. Fungi are heterotrophic. The./ cannot synthesize their food. Therefore, they get their food from different sources. Fungi may be:
I. 3aprophytes: These fungi get their food from dead organic matter. They secretes enzyme in their substratum. These enzymes break the organic matter into simple compounds. Then fungi absorb these substances.
- Parasites: The live on their host and get their food from them. These fungi cause some serious diseases.
- Symbionts: These fungi develop relation with other organisms.examples are mycorrhizae and lichens. General structureStructure of thallusThe body of fungi consists of hyShae. Hyphae *se minute, thread like tubular structure. The group or mass of hyphae is called mycelium. Each hypha has cell wall. In lower fungi cellulose is the chief component. But cell walls of higher fungi have chitin in their cell wall.
Fig: A, A: Septate mycelium, B,B: Septate mycelium
The protoplast of hyphae has cytoplasm and nuclei. Fungi are eukaryotes. So their nuclei have nuclear membrane and chromatin network. Cell divides by mitosis. The hyphae may be aptate or unseptate. Septa are present in septate hyphue. But it is absent in unseptate hyphae. The individual cell may be uninuc eate or multinucleate. Hyphae containing many nuclei are called coenocyte. A vacuole is present in the centre of cytoplasm in older hypnae. The reserve food material is glycogen or oil droplets.
Mycelium and tissues in fungi
Mycelium is composed of loosely organized hyphae. Mycelium has following special structures.
1. Sporophores: Sometimes, hyphae organized to form fruiting bodies. These fruiting bodies are called sporophores.
- Plectenchyma: The tissues composed of compact mass of ;typhae are called plectenchyma.
- Prosenehyma: The tissues with distinct hyphae are called xosenchyma.
- Pseudoparenchyma: The tissue in which individual hyphae oose their shape and become isodiametric is called pseudoparenchyma.
- litroinata: Some fungal tissues become very hard. Fruiting bodies are formed on such tissues. These tissues are called 3tromata.
- Sclerotium: A mass of hard fungal tissues without fructifications are called sclerotium. Sclerotiums are formed for passing over unfavourable periods. These structures germinate to
form new mycelium in favourable conditions.•
- Rhizomorphs: Several hypae unites to form rope like strand called rhizomorphs. Rhizomorphs form fruiting bodies.
Fig: C: Prosenchyma, D, Pseudoparenchyma, E: Sclerotia, F:Rhizomorph
Rep:oduction in fungi may be vegetative, asexual or sexual. Vegetative reproduction
A mall part of hyphae of fungi can produce complete mycelium. In some fungi fruiting bodies can form new mycelium. Sometimes, some parts of the fungi decompose. The remaining part develops new mycelium.
Fungi reproduce asexually by spore formation. Fungi produce a large number of spores in single cycle. Spores are produced during favourable conditions. The spores have thick wall. Unicellular fungi like yeasts reproduce by binary fission or budding. Following types of spores are produced in fungi:
- Endospore: The spores produced inside the sporangia are called
- Conidia: The spores produced at the tip of hyphae outside the worangia are called conidia. Conidia are produced or special hypha called conidiophores.
- Zoospores: The spores of aquatic fungi are motile. They contain flagella. Such spores are called zoospores. Zoospores are very are in fungi.
- Aplanospores: The non-motile aquatic fungi are called kplanospores.
- Arthrospores or oidia: In some fungi, the hyphae break up into ndividual cells. These cells behave as spores. Such spores are called arthrospores or oidia.
- •lamydospores: Sometimes, individual cells of the hypae become thick walled. These cells behave as spores. Such spores ire called chlamydospores.
Fig: Different types of spores: A-Zoospores, Conidia, Oidia.
Sexual reproduction in fungi has three phases:
(a)Plasmogamy: Two gametes fuse and bring the two haploid tuclei in the same cell.
(b)Karyogamy: The two haploid nuclei fuse to form a single diploid or zygotic nucleus
(c)Meiosis: Reduction division occurs in zygotic nucleus. It Detail of sexual reproduction is different in different fungi:
- lsogamy and anisogamy: lsogamy and anisogamy occurs in lower aquatic fungi and some terrestrial higher fungi. These fungi pmcuce motile gametes in gametangia. These gametes fuse to form zygote. The gametes may be isogametes or anisogametes. In anisogametes, smaller gamete is called microgamete and the larger gamete is called macrogamete. The process of union of isogametes is known as isogamy. The union of anisogametes is called anisogamy.
Some–terrestrial fungi also reproduce sexually by direct conjugation of similar gametangia. The product of plasmogamy is zygote. Zygote secrAes thick wall and is changed into a zygospore. Karyogamy occurs in zygospore. Its nucleus undergoes reduction division (meiosis).
- Oogamy: In this case, two gametangia are different in size and shape. The smaller is the male gametangium. It is called antheridium. The larger is the female gametangium. It is c died the oogunium. The fungi may be homothallic or heterothallic. In case of hete-othallic the hypae may be + or – strains. The two gainetangia come near each other. The contents of the antheridium are trarderred to the oogonium. These contents are transferred by a pore or by fertilization tube. The contents of the antheridium may be uninucleate or multinucleate. In multinucleate forms only one nucleus left. All other nuclei disintegrate before plasmogarny. The connnts of the oogonium are called egg or oosphere. This type of plasmogamy is called oogamy or heterogamy.
- .3permatization: In some fungi antheridia are not formed. The fungi develop conidia like spermatia. Spermatia unite with the sex organ or vegetative hyphae of female. Plasmogamy occurs. Thus dikaryotic hyphae are produced.
- Direct union of vegetative hyphae and parthenogenesis: In some forms plasmogamy takes place by the direct union of monokaryotic vegetative hyphae. Parthenogenesis is also common in fungi.
Plasmogamy and karyogamy
(a) Lower fungi: In the lower fungi karyogamy takes place Immediately after fertilization. The oosphere is transformed into an oospore. Meiosis takes place at the time of oospore germination. In some forms the contents of the antheridium are
transformed into motile male gametes or antherozoies. They move towards the oogonium, enter into it and fertilize the egg
Higher fungi: In the higher fungi karyogamy does not take place immediately after fertilization. The oogonium forms special hyphae. Each cell of this hypha contains two nuclei. Both nuclei divide at the samz time. This type of division is known as conjugate division. Such hyphae are called dikaryotic hyphae. The stimulus of plasinogamy produces fruiting bodies or sporophores. Kaiyogamy takes place in special cells. Meiosis follows karyogamy. It produces special spores.
These are peculiar structure formed in almost all basidiomycota. The, are formed on the dilcaryotic mycelium usually im terminal hyphae cells during conjugate division of the diWyous. It ensures that the sister nuclei separated into two daughter ea,
- In the beginning a lateral outgrowth arises from a cell of tlikaryotic hyphae.
- The nuclei of the terminal cell of dikaryotic hypha divide. The •ower daughter nucleus of the upper cell passes .nto the outgrowth.
- Now septum forms in between the outgrowth and upper cell. The outgrowth joins with the adjacent cell.
- Partition in between outgrowth and second cell dissolves. The lateral pocket thus formed by the outgrowth. It becomes a passage for the transference of nuclei and is called as clamp reconnection.
Fig: Stages of clamp connection formation
Basidia in Fungi
The members of Basidiomycota are characterized by the presence of basidia. The basidia vary in form in different groups of the basidiomycota. Following are main types of basidia hund in basiliomycota
I. Stichobasidia: In this type the basidium divides by tr tnsverse .3epta into a four celled structure. The terminal cell produces a sterigma at its apex. The other three cells push On lateral sterigmata one each more than half way up the cell. It is quite common in Smuts and Rusts
- Chlastobasidia: This type of basidia is globular. It divides vertically and forms four cells. Each cell forms sterignea at the iip of which basidiospores are formed.
- Tuning fork type: In this case the basidium is slender hypha ike structure. lit is deeply divided and resemble a tuning form.
- Normal type e Basidia: These are spherical or clavate in shape. They are one celled or unseptate. These bear four basidiospores born on short sterigrpata, e.g. Agaricus.
Fig: Different types Basidia: A.Stichobasidia, B.Chlastobasidia,
C.Tunning forked basidia, D. Normal type of basidic
Fruiting bodies or Ascocarp in Ascomycota
Different type of fruiting bodies is formed in Ascomycota. Ascocrarps may be formed singly or in groups. The shapes of these ascocarps vary in different groups. Some common forms as follows:
1. Cleistothecium: Its shape is spherical. It completely closed ascocarps. In this case, ascospores are liberated by the rupturing of its wall.
- Perithecium: It is more or less closed. But at maturity, a pore ippears in it. Ascospores are librated through these pores
- Apothecium: In this case, asci are formed in open ascocarp.
- Ascostoroma: In this case, asci are produced as a cavity within the stroma. The stroma itself form the wall of the ascocarp.
Fig: Different type of Ascocarp
Fungi have following life cycles:
- Haploid phase: In this case, predominant phase is haploid. Llaploid hyphae fuse to form diploid nuclei. These zygote nuclei undergo meiosis and haploid phase is restored.
- Diploid phase: In this case, the predominant phase of life is diploid. The mature hyphae produce gametes by meiosis. These gametes fuse to form diploid zygote. Zygote develops by mitosis to form new diploid hyphae. Thus only gametes are hpploid in these fungi.
- Haploid and diploid phases: In this case, both hap’oid and diploid phases alternate with each other. This is very rare in fungi.4. Dikaryotic phase: In some fungi like ascomycota, plasmogamy takes place without karyogamy. It produces dikaryotic hyphae. Thus most of the reproductive hyphae of these fungi are d ikaryotic.Classification of fungi
Fungi are divided into following groups:
- Chytridiomycota: They do not have true mycelium. These are unicellular. Their body may be naked or enclosed by chitin. Some parasitic species have rhizoids for anchorage. Asexual repriduction takes place by zoospore formation. Sexual reproduction is very rare. It is isogamous. Example: Synchytrium
- Monohlepharidiomycota: Their mycelium consists of branched coenocytic hyphae. They develop rhizoidal hyphae. It passes to host tissues. Asexual reproduction occurs by uniflagellate Z005 pores. Sexual reproduction is oogamous. Example: Monoblepharis.
- Plasmodiophoromycota: Their thallus is naked. It is composed of snores at maturity. The spores germinate to form swarm cells. The swarm cells bear two unequal flagella. Swarms cells may acts as zoospores. Example: Plasmodiophora
- Oomycota: Mycelium consists of branched coenocytic hyphae. Septa appear in the older hyphae. They absorb food from their host by haustoria. Asexual reproduction occurs by motile or non-motile spores. Non-motile spores may be conidia. Motile spores are biflagellated zoospores. Sexual reproduction is oogamous type. Examples: Phythium, Albugo
- Zygomycota: Mycelium is composed of coenocytic hyphae. Ase:aml reproduction occurs by spores (aplanospore) fumation. These spores are produced in sporangia. Sexual reproduction takes plac: by conjugation between two hyphae. Zygote is changed into thick walled zygospore. Example: Rhizopus
- Ascomycota: This group shows diversity of organisms. Some are unicellular like yeasts. Some are large cup fungi like morels. The hyphae may be monokaryotic or dikaryotic. Asexual reproduction takes place by conidia formation. Sexual reproduction occurs by ascospore formation. Examples: Yeasts, morels, truffles
- Basidiomycota: Their hypae is mononucleate during first phase but become dikaryotic during second phase. They produce structures basidium. Basidiospores are produced on basidium. They form characteristic fruiting bodies. Examples: Agaricus, smut, rust etc.8. Deutromycota (fungi imperfecti): Deutromycota includes all such fungi in which sexual reproduction is absent. It is a hete-ogenous group. Most of these fungi are related to the sexually reproducing Ascomycetes. Some are related to other two groups, Zygomycota and Basidiomycota. Parasexuality occurs in them. Example: Aspergillus