Wednesday, 25 September 2019

WHAT IS ASEXUAL REPRODUCTION

WHAT IS ASEXUAL REPRODUCTION

ASEXUAL REPRODUCTION

Life cannot be created de novo rather it arises from pre-existing life. Reproduction is the only method by which continuity of life is maintained. It is of two types: asexual and sexual. Asexual reproduction is the formation of a new individual without involving the fusion of gametes. It is uniparental as offspring are produced by a single parent.

CHARACTERISTICS OF ASEXUAL REPRODUCTION

  • It is more primitive than sexual reproduction as it involves only mitotic divisions.
  • New organisms are produced from the somatic part of a parental organism, so it is also called as somatogenic reproduction.
  • New individuals produced are genetically similar to the parent as well as to each other and are called clones. Hence, it plays no role in evolution.
  • Unit of reproduction may be either whole parent body, or a bud, or a body fragment, or a single somatic cell.
  • It is usually found in lower organisms like protistan protozoans (Amoeba, Paramecium), sponges (Skype), coelenterates, (Hydra, Tubularia, etc.), certain flatworms (Planaria), some worms and tunicates (Salpa, Ascidia, etc.). It is absent in higher invertebrates and all vertebrates.

TYPES OF ASEXUAL REPRODUCTION

Fission

  • It is a type of asexual reproduction in which the parent organism divides into two or more daughter cells.
  • In this type of reproduction, the whole parent body acts as the reproductive unit.
  • It is of three types:
(a) Binary fission: In this, parent organism divides into two halves, each half forming an independent daughter organism. It can be simple (occurs through any plane, e.g., Amoeba), longitudinal (plane of division is longitudinal axis of the body, e.g., Euglena), transverse (the plane of division runs along transverse axis of the body, e.g., Paramecium) and oblique (the plane of division is oblique, e.g., Ceratium).

(b) Multiple fission: In this process, the parent body divides into many similar daughter organisms. It occurs during unfavourable conditions. The nucleus of the parent divides by repeated amitosis into the nuclei which eventually form several daughter cells. E.g., Amoeba, Plasmodium (malarial parasite).

(c) Plasmotomy-Division of the multinucleate parent into many multinucleate daughter individuals without division of nuclei. Nuclear division occurs later to maintain a number of nuclei. E.g., Opalina, Pelomyxa.

fission


Fig.: Binary fission in (a) Amoeba (b) Euglena (c) Paramecium

 Multiple fission
Fig.: Multiple fission (a) Amoeba (b) Malarial parasite

Regeneration

It refers to the growth of new tissues or organs to replace lost or damaged part.
Regeneration is of two types:
morphallaxis (formation of the whole body from a fragment) and epimorphosis (replacement of lost parts). It can be reparative (regeneration of damaged tissue only) or restorative (redevelopment of a severed body part). In epimorphosis, a mass of undifferentiated cell referred to as blastema is formed after wound healing and then the blastema cells actively proliferate to restore the lost art of the amputated organ. Regeneration is found in Hydra, starfish, Planaria, etc.

Regeneration

Fig.: (a) Regeneration in (i) Planaria (ii) Hydra (iii) Starfish (b) Epimorphosis

Budding

  • Budding refers to the process formation of daughter individuals from a small projection or budding on the parent body.
  • Each bud enlarges develops parental characters and separates to lead an independent life.
  • Budding can be either exogenous (formed on the outer surface) e.g., Hydra, yeast or endogenous (formed inside parent body) e.g., Spongilla. In Spongilla, bud is called a gemmule.
budding in Hydra

Fig.: (a) Exogenous budding in Hydra (b) Exogenous budding in yeast (c) Endogenous budding (gemmule) in Spongilla

Fragmentation

  • In this type of reproduction, the parent body breaks into the two or more pieces called fragments.
  • Each fragment develops into a new organism.
  • In fragmentation, rate of reproduction is high.
  • It occurs in flatworms, sea anemones, coelenterates, echinoderms, algae-like Spirogyra, etc
Fragmentation
Fig.: Fragmentation in Spirogyra

Sporulation

Spores are minute, single-celled, thin or thick-walled propagules which are dispersive structures released from the parent body and form new individuals. Spore formation is common in members of monera, protista, algae, and fungi. Some of the commonly produced spores are:

(a) Zoospores: Motile and flagellated spores produced inside zoosporangia. Flagella help in proper dispersal in aquatic habitat. E.g., algae and lower fungi like Phycomycetes.

(b) Conidia: Non-motile spores produced singly or in chains by a constriction at the tip or lateral side of special hyphal branches called conidiophores. These are dispersed by wind and germinate to form new individuals. E.g., Penicillium.

(c) Chlamydospores: Thick-walled spores produced directly from hyphal cells. May be terminal or intercalary in position and capable of withstanding unfavorable conditions. E.g.,
Rhizopus.

(d) Oidia: Small fragments of hyphae that thin-walled and do not store reserve food material. Oidia give rise to new hyphae. These are formed under conditions of excess water,
sugar and certain salts. E.g., Agaricus.

(e) Sporangiospores: Non-motile spores produced inside sporangia. Usually, get dispersed by wind and germinate to form new mycelium. E.g., Rhizopus, Mucor.

types of spores

Fig.: Various types of spores (a) Zoospores (b) Conidia (c) Chlamydospores (d) Oidia (e) Sporangiospores

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