Sexual Reproduction: Definition, Stages and Types

Sexual Reproduction: While you might know that birds and reptiles are born from eggs, but you might be curious about how human beings proliferate. You might wonder as to how humans reproduce and produce offspring at all? Most vertebrate animals, including humans, reproduce through sexual reproduction. In Sexual Reproduction, two haploid gametes combine to create a genetically distinct diploid offspring. Let us know about Sexual Reproduction in plants, animals and human beings, in this article.

Definition of Reproduction

Reproduction is the process in which an organism produces a new offspring or individual of its own kind.

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This is of two types:

1. Asexual reproduction
2. Sexual reproduction.

What is Sexual Reproduction?

Sexual reproduction is a biparental type of reproduction that involves the production of gametes and there is a fusion of male and female gametes to produce a diploid zygote.

CLEAR YOUR DOUBTS ON SEXUAL REPRODUCTION

Sexual Reproduction in Lower Plants

1. The plants also reproduce sexually in which fertilization of male and female gametes takes place and the zygote is formed.
2. Gametic cells like sperm and egg are produced by the meiotic division.
3. In lower plants, these gametes fuse directly through their cells and show the following types:
a. Isogamy – It is the fusion between similar gametes, where both are motile.
b. Anisogamy – It is the fusion between dissimilar gametes, where both are motile.
c. Oogamy – It is the fusion between well-defined gametes, where only the male gametes are motile.

4. In Bryophytes and Pteridophytes, these gametes are formed in well-defined structures like antheridia (for male gametes) and archegonia (for female gametes).

5. In Phanerogams (specifically Angiosperms), these gametes are located inside more pronounced structures like androecium (for male gametes) and gynoecium (for female gametes). We will discuss more about reproduction in flowering plants later in this article.

Sexual Reproduction in Animals

1. In animals, sexual reproduction occurs by the fertilization process between a haploid sperm and a haploid egg that results in the generation of diploid offspring.
2. Most individuals are dioecious where females produce large, non-motile eggs and males produce small, motile sperms.
3. In other monoecious individuals, such as earthworm and many snails, a single individual produces both sperms and egg. These individuals are called a hermaphrodite.

Types of Sexual Reproduction

Sexual reproduction is of two types, syngamy (which takes place in multicellular organisms) and conjugation (which takes place in single-celled organisms). 

It is explained through the below chart.

Other Modes of Sexual Reproduction

The other modes of Sexual Reproduction are as follows:

1. Autogamy – It is the fusion of male and female gametes produced by the same individual, e.g., Paramoecium.
2. Hologamy – It is the fusion of entire mating individuals acting as gametes, e.g., Chlamydomonas.
3. Paedogamy – It is the fusion of young individuals, e.g., Actinosphaerium.
4. Merogamy – It is the fusion of small and morphologically dissimilar gametes.
5. Macrogamy – It is the fusion of two macrogametes.
6. Microgamy – It is the fusion of two microgametes.
7. Cytogamy – It is the fusion of the cytoplasm of two individuals, but no nuclear fusion takes place. E.g., Paramoecium aurelia.
8. Plasmogamy – It is the fusion of related cytoplasm, e.g., fungi.
9. Karyogamy – It is the fusion of nuclei of two gametes, e.g., Mucor.
10. Automixis – It is the fusion of gamete nuclei of the same cell. E.g., phasmids.

Stages or Phases of Sexual Reproduction

The events/stages/phases of sexual reproduction (i.e., fertilization) can be divided into three distinct stages. These are as follows:

1. Pre-fertilization events
2. Fertilization
3. Post-fertilization events.

Pre-Fertilization Events

The events which occur before the fertilization i.e., gametic fusion are included in this. These include gametogenesis and gamete transfer.

Gametogenesis

1. The process of gamete formation is known as gametogenesis and the gametes are of two kinds, male gametes and female gametes.
2. In some lower organisms, both male and female gametes are morphologically similar and are called isogametes or homogametes. 
3. In higher organisms, both male and female gametes are morphologically distinct and are called heterogametes.
4. The gametes are usually formed by meiotic division; therefore, they are haploid in nature.

Gamete Transfer

In most organisms, the male gamete is motile and the female gamete is non-motile. The male gametes are produced in large numbers because many male gametes fail to reach female gametes. In flowering plants through the process of pollination, male gametes reach the female gametes.

Fertilisation

In this stage, the most important event is the fusion of haploid gametes and the formation of a diploid zygote. This process is called syngamy or fertilisation.

External Fertilisation

The process of fertilization that occurs outside the body of organisms is called external fertilization. (e.g., algae, amphibians, fishes, etc.)

Internal Fertilization

If the syngamy occurs inside the body of organisms, it is called internal fertilization. (e.g., fungi, reptiles, birds, higher animals, and plants)

Parthenogenesis

In organisms like rotifers, honey bees, lizards, and some birds, the female gametes form new organisms without fertilization. This phenomenon is called parthenogenesis.

Post-Fertilisation Events

These are the events that take place after fertilization and are majorly described under zygote and embryogenesis.

Zygote Formation

The zygote is formed in all sexually reproducing organisms. Further, the development of the zygote depends upon the type of life cycle and the environment of an organism.

Embryogenesis

Organisms that lay eggs from which their young ones are hatched out are called oviparous animals.

Examples: reptiles, birds, amphibians, etc.

Organisms that give birth to young ones are termed viviparous animals.

Examples: primates, non-primates, etc.

Sexual Reproduction in Flowering Plants

Many plants can reproduce sexually. The reproductive part of higher plants is the flower.

The flower is the structure in all flowering plants that is responsible to produce seeds that will grow into a new plant. The colours and distinctive fragrances of flowers attract birds and insects that help in pollination. Many flowers make both male and female sexual units called gametes and are known as a hermaphrodite.

Sexual reproduction is carried out in two steps: Pollination and Fertilization.

Pollination

Pollination is the transfer of pollen from the anther of a plant to the stigma of the same plant or a different plant of the same species. Pollination is a part of sexual reproduction in plants. It is usually followed by fertilization and later by the formation of a seed. The two types of pollination:

Self-Pollination: It is the transfer of pollen from the anther of a flower to the stigma of the same flower.

Cross-Pollination: It is the transfer of pollen from the anther of one flower to the stigma of another flower of the same species.

Study About Sexual Reproduction in Flowering Plants

Fertilization in Plants

In plants, pollination is followed by fertilization. The fertilization process is shown in the following flow chart.

Double Fertilization Process

Double fertilization is a process in which two male gametes are utilized. i.e., one male gamete fuses with an egg to form a diploid zygote and another male gamete fuses with the diploid secondary nucleus to form a triploid primary endosperm nucleus and is called double fertilization. This is unique in Angiosperms.

Sexual Reproduction in Humans

Reproduction in human beings is naturally by sexual means only. The process entails courtship and mate selection, copulation, pregnancy, childbirth, and prenatal care. The partner chooses a potential mate essentially based on the qualities that ensure raising an offspring. During sexual acts, the sperm are deposited in the female genitalia where the sperm meets the ovum for internal fertilization.

This takes place in the female’s fallopian tube. Later, it develops to form an embryo by moving towards the uterus and gets attached or implants to the placental wall. Later, the foetal development takes place and finally, a baby is born.

Summary

Well, we have studied that sexual reproduction is the major process that takes place in an organism leading to the variation of offspring and it helps in the evolution of species. Human beings, animals like dogs, cats, etc., reproduce through sexual reproduction. Additionally, the individuals produced during asexual reproduction are genetically identical to the parent. We also understood that sexual reproduction is better than the asexual type of reproduction, as we read in detail about the stages in sexual reproduction, including fertilization, pre-fertilization, and post-fertilization process.

Frequently Asked Questions (FAQs) on Sexual Reproduction

Let us look at some of the commonly asked questions about Sexual Reproduction:

Q.1. What are examples of sexual reproduction?
Ans: Human beings, animals like dogs, cats, etc., reproduce through sexual reproduction.

Q.2. Which process leads to the formation of gametes?
Ans: When two distinct haploid cells, male and female, are formed, it is known as gametogenesis. The entire process takes place in the pre-fertilization stage of sexual reproduction.

Q.3. What is required for sexual reproduction to occur?
Ans: In sexual reproduction, two parents are involved where a sperm and an ovum is required.

Q.4. Why is sexual reproduction important?
Ans: Sexual reproduction involves the fusion of genetic material and this leads to the creation of varieties in offspring, thus, it ensures the survival of species in a population.

Q.5. What happens during sexual reproduction?
Ans: In the sexual reproduction process, a haploid sperm from the male parent fertilizes the haploid egg from the female leading to the formation of a diploid zygote.

Q.6. Why is sexual reproduction better than asexual?
Ans: Sexual mode of reproduction is better than the asexual mode of reproduction because, in asexual reproduction, a single parent gives rise to another individual which is exactly similar to the parent (called a clone) as it passes its genetic material to the clone but sexual reproduction, promotes diversity of characters in an offspring due to combinations of genes which can lead to a variation of offspring.