Asexual Reproduction ! Types, Definitions !! Biological Sciencess !!!

REPRODUCTION

Reproduction is the formation of new similar young living organisms by the grown up individuals of a species or race. It is meant for perpetuation of the race /species because individuals are bound to die after a life span. Reproduction provides group immortality. Four processes are
basic to reproduction – DNA replication, cell division, formation of reproductive units and development of a new individual.

Animal reproduction is of two types, asexual and sexual.

Asexual Reproduction

It is a mode of reproduction or formation of new young individuals from a specialized or unspecialized part of single parent without the formation and fusion of gametes. Besides being uniparental and absence of gametes, asexual reproduction is characterized by divisions through mitosis only, genetic similarity between parent and young ones, rapidity and absence of haploid-diploid alteration. Asexual reproductive propagule is called blastos. It contains totipotent cells like archaeocytes (sponges), interstitial cells (cnidaria), parenchyma (platyhelminthes) and neoblasts (annelids), etc. An individual produced through asexual reproduction is ramet. Clone is group of all genetically similar individuals formed through asexual reproduction.

1. Fission

It is a type of asexual reproduction in which the body of an individual undergoes division to produce two or more equal sized daughters.

(i) Binary Fission. 

The mature individual divides into two equal sized daughter individuals. Binary fission is irregular (can occur in any plane) in Amoeba, longitudinal in Euglena, oblique in dinoflagellates and transverse in Paramecium and Planaria. In unicellular forms, binary fission is accomplished through mitotic nuclear division followed by cytokinesis. In multicellular individuals like Planaria the posterior part is fixed firmly to substratum while anterior part extends forward and exerts a pull causing the middle part to break.

(ii) Multiple Fission

It is formation of a number of small daughters by division of a parent. In unicellular forms, the nucleus divides a few times followed by collection of cytoplasm around each daughter nucleus forming a number of daughter cells, e.g., Amoeba, Plasmodium, Monocystis.

(iii) Plasmotomy

 Cleavage of plasmodium or multinucleate body into multinucleate parts, e.g., Opalina, Pelomyxa.

(iv) Strobilisation

A modification of multiple fission is strobilisation in multicellular animals (body segmentation for reproduction as forming zooids in coelenterates like Aurelia and proglottides in Tapeworm).

2. Cysts and Spores

They are minute propagules, which function as dissemules as well as perennating structures. Each cyst or spore gives rise to a new individual.

3. Budding

It is a mode of asexual reproduction in which new organisms develop as
outgrowth or buds from a parent.

(i) Exogenous/ External Budding. 

The bud develops externally on the parent. In the young condition the two are internally connected to provide nourishment to the bud. Later on, the bud enlarges and becomes nutritionally independent. It then separates as a new individual, e.g., Yeast, Hydra. However, in some cases, the new individual remains joined to the parent to form colony, e.g., Scypha.

(ii) Endogenous/ Internal Budding. 

Internal buds or gemmules/ statoblasts occur in sponges/bryozoans to help in dispersal as well as perennation.

4. Fragmentation

It is breaking of an individual into two or more parts due to external (e.g., Star fish) or internal forces (e.g., Tunicates). Each fragment grows into a new individual.

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