Sexual Reproduction, sexual reproduction in flowering plants, events of sexual reproduction, gamete transfer, fertilization,

   Sexual Reproduction   

In sexual reproduction, two parents of opposite sex participate along with fusion of male and female gametes. All organisms have to reach a certain stage of growth and maturity in their life cycle before they reproduce sexually. Life cycle of an organism is divided into the following three phases: - 

(i) Juvenile or Vegetative phase - pre-reproductive period 

(ii) Reproductive phase - Reproductive period 

(iii) Senescent phase - End of reproductive period leading to ageing.

Sexuality in Organisms: -

On the basis of gametes, sexuality in organisms can be classified as follows Sexuality in Plants: -

(i) Homothallic or Monoecious plants Both male and female flowers are present on same plant body (bisexual condition) and the gametes belong to the same parent, e.g. in several fungi like Mucor and plants like maize, cucurbits and coconuts. 

(ii) Heterothallic or Dioecious plants Male and female flowers are present on separate plant body- (unisexual condition) and the gametes belong to different parents, e.g. papaya and date palm. In flowering plants, the unisexual male flower is staminate, while female is pistillate.

 Sexuality in Animals: -

(i) Bisexual animals have both male and female organs in single individual, e.g. earthworm, sponge, leech, tapeworm, etc. These are also called as hermaphrodites. 

(ii) Unisexual animals have male and female organs in different individuals, e.g. cockroach human, dog, etc.

Features of Sexual Reproduction: -

(i) Reproduction is a seasonal phenomenon in some organisms. Based on the season of reproduction in majority of plants, following categories can be made 

(a) Monocarpic plants Flowering occurs once in their life cycle after which they bear fruits and die, e.g. annual plants like rice, wheat, biennial plants like radish, henbane, etc. Bamboo species flower only once in their lifetime generally after 100 years. These produce large number of fruits and ultimately die. Similarly, Strobilanthes kunthiana (neelakurinji) flowers (blue in colour) bloom once in every 12 years.

(b) Polycarpic plants Flowering occurs every year in a particular season after reaching maturity, e.g. perennial plants like mango, apple, orange, etc. Few perennial plants flower throughout the year, e.g. China rose. 

(ii) Similarly, in animals, on the basis of time of breeding, the categories are: -

(a) Seasonal breeders breed at a particular time of the year, e.g. frogs, lizards, birds, etc. 

(b) Continuous breeders reproduce throughout their lifespan of sexual maturity, e.g. cattle, poultry, rabbit, etc. 

(iii) During reproductive phase in placental mammals, many cyclic changes occur in the reproductive system of females, i.e. 

(a) In non-primate mammals such as cow, sheep, rat, deer, dog, tiger, etc., the cyclic changes occurring during reproduction are termed oestrus cycle

(b) In primate mammals, such as monkeys, apes and humans, the changes cyclic occurring during reproduction are termed as menstrual cycle.

Events in Sexual Reproduction 

The events of sexual reproduction occur in the following three stages: - 

1) Pre-fertilisation the two main pre-fertilisation events are as follows Gametogenesis It is the formation of male and female gametes. Types of gametes are: - 

(i) Homogametes or Isogametes Gametes, which are not differentiated as male and female gametes, e.g. algae such as Ulothrix. The fusion of these gametes is called isogamy.

(ii) Heterogametes, which are morphologically distinct, e.g. gametes in human, i.e. sperm (male) and ovum (female). The fusion of these gametes is called anisogamy.

Cell Division during Gamete Formation: -

(i) Gametes are always haploid, but they divide either by mitotic or meiotic division. The organisms belonging to Monera, Fungi, algae and Bryophyta. have haploid plant body. These organisms produce gametes by mitotic division.

(ii) In pteridophytes, gymnosperms, angiosperms and most of the animals including humans, the parental body is diploid. In these organisms, specialised cells called meiocytes or gamete mother cells undergo meiosis that results in the formation of haploid gametes.

Gamete Transfer: -

It occurs in various ways in different organisms.: - 

(i) In most organisms, male gamete is motile and female gamete is non-motile. 

(ii) Both gametes are motile in few fungi and algae.

(iii) In algae, bryophytes and pteridophytes, gamete transfer takes place through water by producing many male gametes to ensure fertilisation because during gamete transfer, large number of male gametes are lost. In bryophytes and pteridophytes, male gametes are known as antherozoid. 

(iv) In self-fertilising or bisexual plants, e.g peas, gamete transfer is easy as the anthers and stigma are closely located. In Cross-fertilising plants, gamete transfer occurs by pollination. It is the process of transfer of pollen grains from anther to stigma, e.g. dioecious plants. 

2) Fertilization (Syngamy): -

It is the process of fusion of male and female gametes, which leads to the formation of a diploid cell, known as zygote. Depending upon, where does the syngamy occur, fertilization may be of two types 

(i) External fertilisation It is the fusion of gametes that takes place outside the body of an organism in external medium such as water, e.g. in bony fishes, frogs, etc. A large number of gametes are released in the surrounding medium by such animals. 

(ii) Internal fertilisation It is the fusion of gametes that takes place inside the body e.g. in fungi, higher animals Such as birds, mammals and majority of plants. The number of ova produced is less, but a large number of male gametes are formed, as many of them fail to reach the ova. In some lizards, birds, honeybees and rotifers, female gamete undergoes development to form offspring without fertilisation. This is called parthenogenesis. In honeybee queen, 32 chromosomes are present, During the development of eggs within a queen, a diploid cell with 32 chromosomes divides to generate haploid cells called gamete with 16 chromosomes. Now this haploid egg with 16 chromosomes forms new offspring (known as drone) without fertilisation.

3). Post-fertilisation Events: -

After the fusion of male and female gametes, following processes occur: - 

(i) Formation of zygote is the beginning of new life. It is always diploid and ensures the continuity of race from generation to generation. Organism such as fungi, develops a thick wall around it that is resistant to external damaging factors. Zygote divides by meiosis to form haploid Spores that grow into haploid individuals in case of organisms which lead haplontic life cycle, e.g. Volvox, Spirogyra, etc. In organisms with diplontic life cycle, zygote undergoes mitotic division and develops into an embryo, e.g. all flowering plants. 

(ii) Embryogenesis is the process of development of embryo from zygote. The events which occur in animals during embryogenesis are 

(a) Mitosis (Cell division or Cleavage) occurs leading to the growth of embryo. 

(b) Cells of embryo undergo differentiation and attain specific shape, size and function.

(c) Cell differentiation leads to the production of different tissues, organs and organ systems. 

(d) Depending on the development of embryo the animals can be, 

✅Oviparous animals, e.g. reptiles and birds. Here, embryo develops outside the body of female (eggs). In these animals, the fertilised eggs are covered by hard calcareous shell. 

✅Viviparous animals, e.g. majority of mammals including humans. Here, embryo develops inside the body of female. 

The post-fertilisation events that occur in flowering plants are: -

✅ Sepals, petals and stamens wither and shed off. 

✅Pistil remains attached to the plant. 

✅Zygote develops into embryo and the ovules develop into seed. 

✅Ovary develops into the fruit. 

✅Pericarp is produced as the wall of ovary. Seeds disperse by different agents and germinate into new plants after getting suitable conditions.

Organisms such as aphids, slime Moulds, sea-anemones and many plants switch from asexual to sexual mode of reproduction, when environmental conditions are unfavorable. Sexual reproduction leads to variation in offspring, thus providing a mechanism for selective adaptations to occur.