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Fungi – Morphology, Nutrition, Reproduction

Two Forms Of Hypha

Morphology – Cell wall is made up of chitin. Two forms of cell – 1) Single-celled form e.g. Yeast;  2) Multi-cellular and filamentous form called mycelium.
NOTE: Exception-Yeast in unicellular fungi.
Mycelium/mycelia are made up of interwoven mass of hypha. Hyphae (singular hypha) are long, slender thread -like tubular structures. Or we can say fungal cell is HYPHA.
NOTE: Mycelium is made up of network of hypha.

Hypha can be: 1) Septate hypha and 2) Coenocytic hypha.

  • In septate hypha, tubular hypha is divided into cell-like compartments by cross-walls called septa/septum. Septa have large pores enough to allow ribosomes, mitochondria, and even nuclei to flow from one cell to other cell.
  • Coenocytic hypha lacks these septa (aseptate), and has hundreds of nuclei. These types of fungi are called as coenocytic fungi. Coenocytic condition occurs due to repeated nuclear division without cytokinesis.
Two Forms Of Hypha

Two Forms Of Hypha: Septate & Coenocytic Hypha

NutritionAbsorptive i.e. secrete digestive enzymes outside its cell to break the large food particles present in environment, and then absorb the breakdown products (Extracellular digestion).
Mycelia grow in the direction of food sources and die back in areas where food is running out.

Reproduction– occurs by 3 means –
i) Vegetative Reproduction – occurs by Fragmentation, Fission and Budding.

  • Fragmentation- Simple breakage of the fragment of hypha, which grows into individual organism.
  • Fission- cell equally divides into two cells.
  • Budding- asymmetrical cell division in which a smaller daughter cell is produced called as bud.
Vegetative Reproduction

Vegetative Forms of Reproduction: Fragmentation, Budding, Fission

ii) Asexual Reproduction – During asexual reproduction, spore-forming structures are produced by a haploid mycelium, and these structures produce spores through mitosis. As a result, offspring are clones—i.e. they are genetically identical to their parent. These microscopic spores are then carried by wind to adapt a new life and to colonize new habitat.
Asexual reproduction occurs by formation of Conidia, Sporangiospores, Zoospores, Aplanospores.

  • Conidia– production of haploid non-motile spores (not enclosed in sporangia) at the tips of specialized hypha stalk called conidiophores.
  • Sporangiospores– production of (usually) haploid spores within sporangia at the tip of sporangiophore.
  • Zoospores– naked, motile, flagellated asexual spores produced within zoosporangium.
  • Aplanospores– walled, nonmotile spores.
asexual_modes

Asexual Reproduction: Conidia, Sporangiospores, Zoospores

iii) Sexual Reproduction – Fungal species have thousands of sexes/or mating types, instead of having only male and female. Sexual reproduction begins when hypha from two different compatible mating types come together and fuse to form a hybrid cell.

The sexual cycle involves the following three steps :
(i) Plasmogamy : Fusion of protoplasms between two motile or non-motile gametes or fusion of two haploid hypha of compatible mating types.
Plasmogamy does not immediately lead to nuclear fusion (in ascomycetes and basidiomycetes ). Rather, haploid nuclei (n + n) persist in the same cell or mycelium.

NOTE: When two or more genetically distinct nuclei exist within a single mycelium, this condition is called as heterokaryotic and nuclei are called heterokaryon.

Most hyphae in heterokaryotic mycelia have 2 nuclei i.e. dikaryotic stage (n + n) or dikaryon nuclei, i.e. two nuclei in one cell, one from each mating type. Also called as dikaryophase of fungi.

(ii) Karyogamy : Fusion of two nuclei (n + n form 2n). Cell becomes diploid (2n).
(iii) Meiosis in zygote: The nuclei (2n) that are produced by karyogamy then divide by meiosis (reduction division) to form haploid spores in fruiting body.

general_life_cycle_of_fungi

General Life Cycle Of Fungi

Sexual reproduction occurs by formation of Oospores, Zygospores,Ascospores and Basidiospores.

  • Oospores– thick walled spores form when male gamete enter the large spherical female cell(oogonium).
  • Zygospores– direct fusion of two hyphal protrusions from neighbouring filaments form zygosporangia, in which zyospores are formed.
  • Ascospores– spores produced within specialized cells asci /ascus (cylindrical sac like cells); spores formed by meiosis,each asci produce 8 spores in number.
  • Basidiospores– spores produced externally on basidia/ basidium (club shaped cells); each basidia produce 4 spores in number.
sexual_reproduction

Sexual Reproduction: Zygospores, Ascospores, Basidiospores

NOTE: Fruiting bodies– Multi-cellular structures on which spore producing structures are present in sexual reproduction. Also called as sporocarp.
e.g. Ascocarp (fruiting body) in Ascomycetes(class) has spore producing structure(asci) in which ascospores(spores) are formed.
Basidiocarp is fruiting body of class Basidiomycetes, which produces basidiospores on basidia.
• Like-wise is the case of asexual reproduction. Conidiophore is specialized erect hypha which produces conidia on their tip. Sporangiophore is specialized hypha that bears special cells sporangia which contain sporangiospores.

NOTE: Spore is the most fundamental reproductive cell in fungi. Spores are the dispersal stage in the fungal life cycle and are produced during both asexual and sexual reproduction.
• Fertilization in fungi occurs in two distinct steps: (1) fusion of hypha and (2) fusion of nuclei from the fused hypha.
Isogamy: Sexual fusion of gametes of similar size and form; gametes formed are isogamous gametes.
Anisogamy: Sexual reproduction involving fusion of gametes differ in both size and form ; gametes are anisogamous gametes. Also called as heterogamy. Oogamy is the part of anisogamy, having large female gamete and small male gamete.

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