Sexual Reproduction in Animals - Pre-fertilisation Events in Organisms

Pre-fertilisation Events in Organisms:

• These include all the events of sexual reproduction prior to the fusion of gametes.
• The two main pre-fertilisation events are gametogenesis and gamete transfer.

A) Gametogenesis:

• Gametogenesis refers to the process of formation of the two types of gametes – male and female.
• Gametes are haploid cells. Gametes divide into two types - 
  a) Homogametes (isogametes): The two gametes are similar so it is not possible to classify them as male or as female gametes. Therefore, they are called homogametes (isogametes). For example - some algae like Cladophora.
  

  Isogametes of Cladophora (an alga)

  b) Heterogametes: As in the majority of sexually reproducing organisms, the gametes produced are of two morphologically distinct types and are known as heterogametes. The male gamete is called an antherozoid (sperm or microgametes) and the female gamete is called an egg (ovum or macrogametes). For example Fucus (an alga) and Homo sapiens (Human beings).
  

  Heterogametes of Fucus (an alga)

  Heterogametes of Homo sapiens (Human beings)

Sexuality in organisms:

• Sexual reproduction in organisms, in general, involves the fusion of gametes from two different individuals. But this is not always true.
• Bisexual: Both male and female reproductive structures are present in the same individual.
  (i) Bisexual Plants and Fungi: Both male and female reproductive structures are present in the same plant. In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition. In some flowering plants, both male and female flowers may be present on the same individual, it is called monoecious. Some examples of monoecious plants are cucurbits and coconuts.
  

  Monoecious plant (Chara)

  Bisexual flower (sweet potato)

  (ii) Bisexual animals: Both male and female reproductive organs, are hermaphrodites. Earthworms, sponges, tapeworms and leech, are typical examples of bisexual animals.
  

  Bisexual animal (Earthworm)

• Unisexual: Male and female reproductive structures are present in different individuals.
  (i) Unisexual Plants and Fungi: Both male and female reproductive structures are present in the different plants, it is called unisexual plants. In several fungi and plants, Heterothallic and dioecious are the terms used to describe unisexual conditions. In flowering plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils. In some flowering plants, both male and female flowers may be present on separate individuals, it is called dioecious. Some examples of dioecious plants are papaya and date palm.
  

  Dioecious plant (Marchantia)

  (ii) Unisexual Animals: The cockroach, ant, wasps, bees and humans, are examples of a unisexual species.
  

  Unisexual animal (Cockroach)

Cell division during gamete formation:

• A haploid parent produces gametes by mitotic division. 
• Several organisms belonging to monera, fungi, algae and bryophytes have haploid plant body, but for organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid. It is obvious that meiosis, the reduction division, has to occur if a diploid body has to produce haploid gametes.
• In diploid organisms, specialized cells called meiocytes (gamete mother cells) undergo meiosis. At the end of meiosis, only one set of chromosomes gets incorporated into each gamete.

(B) Gamete Transfer:

• Male gametes need a medium to move towards female gametes for fertilisation. In a majority of organisms, the male gamete is motile and the female gamete is stationary.
• Exceptions are a few fungi and algae in which both types of gametes are motile.

  Homogametic contact in alga

• In several simple plants like algae, bryophytes and pteridophytes, water is the medium through which this gamete transfer takes place. A large number of the male gametes, however, fail to reach the female gametes. To compensate for this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.
• In seed plants, pollen grains are the carriers of male gametes and the ovule has the egg. Pollen grains produced in anthers have to be transferred to the stigma before it can lead to fertilisation. 
  

  Germinating pollen grains on the stigma of a flower

• In bisexual, self-fertilizing plants, e.g., peas, transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other. Pollen grains soon after they are shed, come in contact with the stigma. But in cross-pollinating plants (including dioecious plants), pollination facilitates the transfer of pollen grains to the stigma. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg. 
• In dioecious animals, since male and female gametes are formed in different individuals, the organism must evolve a special mechanism for gamete transfer. Successful transfer and coming together of gametes is essential for the most critical event in sexual reproduction, the fertilisation.