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Special Modes of Reproduction

In angiosperms, the fruits are usually formed after the process of fertilisation. In certain plants, however, the fruits are formed without the act of fertilisation. Such fruits are called parthenocarpic fruits and the phenomenon is known as parthenocarpy.

Parthenocarpic fruits are either seedless or contain empty or non - viable seeds. In these fruits the stimulus for fruit growth is provided by the tissue of the ovary wall itself.

Natural parthenocarpy is found in oranges, cucumbers and seedless grapes. Most commonly cultivated varieties of banana and pineapple are parthenocarpic.

Even in seeded varieties of fruits, parthenocarpy can be induced by the application of low concentrations of auxin and gibberellins.

Parthenocarpy is of great commercial value. Seedless tomatoes may be produced parthenocarpically in green houses. Parthenocarpic fruits are used in the preparation of jams and fruit juices. Worlds best varieties of pine apples, banana and grapes are parthenocarpic.

 

Apomixis or Agamospermy

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It is a modified form of reproduction, where seeds are formed without the fusion of gametes. In this method, a diploid cell of the nucleus develops into an embryo. The genetic constitution of the embryo will be identical to the parent. An organism that reproduces by apomixes is called an apomict.

Parthenogenesis (Greek - Partheno - virgin genesis - descent)

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It is the development of an unfertilised ovum into a fully developed haploid organism. It is a special type of monoparental reproduction found in a number of invertebrates like aphids, rotifers and honeybees, and a few vertebrates like domestic turkey among birds and among the lizards, species of Lacerta.

Natural Parthenogenesis

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Here the parthenogenesis is regular and characteristic feature of life cycle.

Example: Honey bees

In honey bees (Apis indica) the queen produces 2 types of eggs, the fertilised and the unfertilised. The unfertilised eggs develop into drones parthenogenetically. So the drones are haploid. Drones form gametes by mitosis and not by meiosis.

Artificial Parthenogenesis

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The eggs of some normally sexually reproducing animals can be induced by artificial means to develop parthenogenetically. The stimuli inducing artificial parthenogenesis are salts, weak acids, organic solvents, urea, sucrose and physical stimuli like temperature, electric shock or mere shaking.

Parthenogenesis may be complete when there no males in the life cycle and females develop exclusively by parthenogenesis.

Examples: Aphids, Rotifers, Lacerta saxicola or Caucasian rock lizard.

It may be incomplete when males are parthenogenetically developed and females are developed from fertilised eggs.

Examples: Honey bees

Parthenogenesis is observed in some plants also. In Solanum nigram the haploid egg develops into a haploid embryo and plant. Nicotiana haploid embryos are formed from male gametes.



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