Reproduction in Fungi- Part-3: Sexual Reproduction

Fungi reproduce by vegetative, asexual and sexual methods. This post describes the Process of Sexual Reproduction in Fungi. Sexual reproduction takes place in all groups of fungi except Deuteromycetes. Fungi may be monoecious (bisexual) or dioecious (unisexual). Moneoecious species produce two sex organs (male and female) in the same thallus, thus they are called homothallic. Dioecious forms produces sex organs in separate thalli, hence called heterothallic.

Sexual Reproduction in Fungi

Ø  Based on the complexity of sexual reproduction, fungi can be grouped into two categories:

(1). Eukarpic fungi
(2). Holocarpic fungi

Ø  Eukarpic fungi: In most fungi only a part of the vegetative mycelium forms the reproductive unit and the rest remain vegetative, such a fungi is called eukarpic fungi (advanced type)

Ø  Holocarpic fungi: in some unicellular forms the whole vegetative cell is transformed to a reproductive unit at the time of maturation (primitive type)

Ø  The process of sexual reproduction in fungi is completed in three distinct phases.

(A). Plasmogamy: fusion of cytoplasm

(B). Karyogamy: fusion of nuclei

(C). Meiosis: reduction division

(A). Plasmogamy

Ø  Plasmogamy is the first phase of sexual reproduction in fungi

Ø  It is the fusion of protoplasts of two compatible gametes or sex cells or hyphae

Ø  As a result of plasmogamy, two compatible nuclei are come close to each other

(B). Karyogamy

Ø  Fusion of two nuclei to form a diploid nucleus

Ø  In some fungi (Phycomycetes) karyogamy occurs immediately after plasmogamy

Ø  In some other fungi (Ascomycetes and Basidiomycetes) karyogamy is much delayed

Ø  In the latter case (delayed karyogamy) two opposite strains of nucleoli get themselves arranged in pairs (dikaryon) in a single cell

Ø  This dikaryon divide mitotically as usual along with the simultaneous division of cytoplasm

Ø  This results in the establishment of a separate karyotic phase in the life cycle of fungi

Ø  This phase of life cycle is called dikaryotic phase

Ø  The process by which the dikaryotic phase is accomplished is called dikaryotization

Ø  After the establishment of dikaryotic phase, the two nuclei of a dikaryon fuse to form the diploid zygote

Ø  Zygote is the only diploid phase in the life cycle of fungi

(C) Meiosis

Ø  Zygote immediately undergo reduction division (meiosis) to produce meiospores

Ø  After karyogamy reduction division takes places in the diploid nucleus and thus haploid stage is established

Ø  In sexual reproduction of fungi, the two compatible nuclei (male and female) are brought together by the following processes.

(1). Planogametic copulation

(2). Gametangial contact

(3). Gametangial copulatin

(4). Spermatization

(5). Somatogamy

(1). Planogametic copulation

Ø  Planogametic copulation involves the fusion of two naked motile gametes (planogametes)

Ø  Based on the structure and size of fusing gametes, three types of planogamy is can occur in fungi

(a). Isogamy

(b). Anisogamy

(c). Oogamy

(a). Isogamy

Ø  Fusing gametes are morphologically similar but physiologically they are two strains (+ and -)

Ø  The gametes are formed on different thalli

Ø  Primitive type of reproduction

(b). Anisogamy

Ø  Fusing gametes are both morphologically and physiologically different

Ø  Male gamete is smaller and active than female gametes

Ø  Female gametes are larger and less active than male gametes

(c). Oogamy

Ø  Female gamete is large and non-motile

Ø  Male game is smaller and motile (flagellated)

Ø  Male gametes are called antherozoids

Ø  They are formed on specialized reproductive structures called antheridium

Ø  Female gamete is called oogonium

Ø  Oogamy is an advanced type of sexual reproduction

(2). Gametangial contact

Ø  Here gametes are not released from gametangia

Ø  Instead male and female gametangia come in close contact with help of fertilization tube

Ø  Then one or more male nuclei migrate in to the female gametangium

Ø  The gametangia never fuse or lose their identity during the sexual act

Ø  Male and female gametangia are called antheridia and oogonia (Ascogonium in Ascomycetes) respectively

Ø  Example: Albugo, Aspergillus, Pythium

(3). Gametangial copulation

Ø  Here the entire content of two compatible gametangia fuse each other

Ø  The gametangia come in close contact, wall at the point of contact dissolves and their contents mix each other

Ø  Then karyogamy is established

(4). Spermatization

Ø  In some advanced fungi sex organs are completely absent

Ø  Here sexual process is accomplished by minute spore like spermatia and specialized (receptive) hyphae acting as male and female structures respectively

Ø  Spermatia is carried by air, water or insect to the receptive hyphae

Ø  Spermatia attached to the trichogyne of receptive hyphae and migrate in to the cytoplasm

(5). Somatogamy

Ø  Here sex organs are not formed

Ø  Occurs in advanced fungal groups such as Basidiomycetes

Ø  Two vegetative cells or vegetative hyphae take over the sexual function and fuse together

Ø  Eg. Morchella, Peziza, Agaricus

Sexual spores in Fungi

Ø  Important sexual spores in fungi are

(1). Zygospores

(2). Ascospores

(3). Basidiospores

(1). Zygospore

Ø  Zygospores are formed by the fusion of male and female structures during sexual reproduction

Ø  They are diploid spores

Ø  They are thick walled resting spores of some fungi

Ø  Usually produced by lower groups of fungi

Ø  Fungal species which produce zygospores are grouped in the class Zygomycete (Rhizopus)

Ascus with Ascospores (source wikipedia)

(2). Ascospores

Ø  Sexual spores produced in the class Ascomycetes

Ø  Ascospores are endospores

Ø  Ascospores are produced in specialized sac like structure called ascus

Ø  Ascospores are haploid spores

Ø  Typically a single ascus contain eight ascospores

Ø  The eight ascospores are produced after a meiosis, immediately followed by a mitosis

Basidiospores (source wikipedia)

(3). Basidiospores

Ø  Sexual spores produced in the class Basidiomycetes

Ø  Basidiospores are exospores

Ø  Basidiospores are produced on specialized structure called basidium

Ø  Basidiospores are also haploid spores

Ø  Typically single basidium produce four basidiospores

Ø  The four basidiospores are produced after the meiosis of a diploid zygote

Learn more: Vegetative reproduction in fungi

Learn more: Asexual reproduction in fungi

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