Sexual Reproduction-Reproduction in Fungi ”Biological Classification” Botany Notes Chapter 2, for Class 12-CBSE

Sexual Reproduction : It takes place by the union of two cells(more often similar in size). The copulating pair of cells may be vegetative cells or ascospores. Yeasts may be homothallic or heterthallic. The stages of fusion are extremely variable and are discussed separately.

1. a) Haplontic life cycle : This is exhibited by Schizosaccharomyces octosporus (fission yeast) which is homothallic. In this type of life cycle. Haploid stage (haplophase) is very elaborated. Whereas the diploid stage (diplophase ) is very short (being confined to the zygote cell only).

During sexual reproduction two cells come in contact. A beak-like protuberance develops from each conjugating cell at the point of contact. A continuous passage is developed by the dissoluitonof intervening walls at the point of contact where the (+) nuclei migrate. Karyogamy takes place and forms the zygote.

The zygote ultimately develops into an ascus. The diploid zygotic nucleus undergoes divisions of which the first one is meiotic division followed by mitotic division producing eight haploid nuclei. Each nucleus  with cytoplasm develops into an ascospore, hence the ascus contains eight ascospores. The ascospores liberated by breaking down of the ascus wall, now behave as somatic cells.

1. b) Diplontic life cycle : This is exhibited by Saccharomycoldes ludwigii(Helobial yeast). Here the diploid somatic stage is long and the haploid stage is very short. The diploid somatic cells produce buds which eventually enlarge to function as asci. The diploid nucleus divides meiotically forming four haploid nuclei that develops into four ascospores. The ascospores remain confined in the ascus and copulate there, forming two diploid cells. Each diploid cell germinates by a germ tube which pushes out through the ascus wall and ultimately forms a tubular structure. This tubular structure behaves as a sprout mycelium from which diploid cells are produced by budding. Thus, the haploid stage is represented by the ascospores only.
2. c) Diplohaplontic life cycle : This is exhibited by Saccharomyces cerevisiae (Budding yeast ). In this life cycle, both haploid and diploid phases are equally well represented constituting alternation of generations. Two haploid cells copulate to form a diploid cell. The diploid cell multiplies by budding producing large number of diploid cells. Eventually, each diploid cell behaves as an ascus bearing four ascospores (meiosis takes place during the development of ascospores). The ascospores on being liberated from the ascus multiply by budding and produce haploid cells. It is evident, that in this life cycle there are two distinct stages, the diploid stage and the haploid stage which alternate in cyclic order. The vegetative cells may be both haploid and diploid reproducing asexually by budding.

Did You Know?

In addition to the above given methods of sexual reproduction of yeast, the following methods are also found :

a)Adelphogamy : Copulation between two adjoining sister cells. This is isogamous and the cells which fuse, do no separate after fission but remain united to form short chains.

1. b) Pedogamy : It is copulation between mother and the daughter cell formed by budding. The daughter remains attached to the mother and the nucleus of the bud migrates into the mother.

EXERCISE

1. Mark the odd on(w.r.t fungi)
   1. Unicisternal golgi bodies
   2. Show a great diversity in morphology and habitat
   3. Most of the members are aquatic
   4. Reverse food material is stored in the form of oil and glycogen.
2. Fungi with cellulosic wall belong to the class
   1. Oomycetes
   2. Zygomycetes
   3. Ascomycetes
   4. Basidiomycetes
3. Select incorrectly matched pair
   1. Rhizopus – Sporangiospore
   2. Penicillium – Ascocarp
   3. Mucor – Dikaryophase
   4. Aspergillus – Conidia
4. Which one of the following organism performs plasmogamy by gametangial contact?
   1. Puccinia
   2. Albugo
   3. Rhizopus
   4. Agaricus
5. Coenocytic dimorphic vegetative mycelium is found in
   1. Neurospora
   2. Rhizopus
   3. Penicillium
   4. Ustilago

6. Choose incorrect match w.r.t. different classes of fungi
   1. Oomycetes – zoospore – gametic copulation
   2. Zygomycetes – sporangiospore – zygophore
   3. Ascomycetes – conidia – monokaryotic aseptate mycelium
   4. Phycomycetes – algal and conjugation fungi – coenoctic mycelium
7. Fungi often employed in experimental genetics is the member of
   1. Egg fungi
   2. Conjugation fungi
   3. Sac fungi
   4. Club fungi
8. Select correct match

Column –I                                       Column – II

1. Soft rot or apple i) Absidia
2. Bronchomycosis ii) Rhizopus
3. White rust disease iii) Pythium
4. Damping off disease iv) Albugo

• a(ii), b(i), c(iv), d(iii)
• a(i), b(ii), c(iv), d(iii)
• a(iv), b(iii), c(ii), d(i)
• a(ii), b(i), c(iii), d(iv)

9. in which of the following yeast ascus contains eight ascospores?
   1. Saccharomyces
   2. Saccharomycoides
   3. Schizosaccharomyces
   4. More than one option is correct
10. Asci are not organized into ascocarps in
    1. True yeast
    2. Drosophila of plant kingdom
    3. Pigmented mould
    4. Morels