Sexual Reproduction in Organisms – An Overview, Types and its Process

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Sexual Reproduction in Plants and Animals – Characteristics, Advantages, and Disadvantages

Reproduction which takes place by the union of two different gametes, i.e., male gamete and female gamete is known as sexual reproduction. The gametes are a special type of unicellular reproductive unit produced from the gamete mother cells inside a special structure called gametangium. Gametes are haploid (n) in nature. The male body produces male gametes and the female body produces female gametes. After the union of male and female gametes one diploid (2n) cell is formed which is called a zygote. The zygote formed by the process of conjugation is called a zygospore, and which is formed by the process of fertilization is called an oospore. This zygote produces a new individual through the developmental process in the future.

Sexual Reproduction in Some Simple Organisms

Under unfavourable conditions, sexual reproduction takes place in algae, fungi, and protozoans. They usually form spores. The significance of spores is these spores have thick resistant covering to withstand natural calamities.

Characteristics of Sexual Reproduction

• It is an improved process. This reproduction process takes place through the union of gametes.
• During this reproduction two individuals are required, those are united with each other.
• Meiotic cell division takes place at any stage of this process.
• Variation always appears in the offspring.
• Due to the presence of variation which has a positive role in evolution.
• The presence of distinct alternation of generation is the life cycle.

Types of Sexual Reproduction

Sexual reproduction is of two types Syngamy, Conjugation.

1. Conjugation

The process of fusion between the two isogametes or fusion between tivo iso gametangium of the same species is known as conjugation.

Generally, the conjugation process is found in lower groups of plants and animals. Conjugation takes place between the cells of two sexually different plants and animals. In plants, the cells of two filaments that are in contact, form protrusions that lie opposite to one another. The protrusion finally produces a conjugation tube through which fusion of the connected cell takes place within one of the two conjugating cells. The fusion product of the conjugation is the zygospore. After a period of rest, the zygospore germinates into a new plant body, e.g., Spirogyra, Paramoecium (protozoa).

Conjugation of Spirogyra
In Spirogyra, sexual reproduction involves a cycle alternation between a haploid and vegetative filament (gametophyte plant) and a diploid zygospore, towards the end of the growing season the Spirogyra filament produces aplanogametes in vegetative cells called gametangia. The gametes fuse to form a zygospore (2n). At the time of germination, the zygospore nucleus undergoes meiosis to produce four haploid nuclei but only one of them survives. Thus a zygospore gives rise to a haploid new filament. Therefore, in the haplontic life cycle of Spirogyra, no true alternation of distinct plants is observed. Rather, the alternation of chromosome numbers from haploid to diploid and back to haploid conditions can be noticed.

Sexual reproduction of Spirogyra involves conjugation, which may be defined as a primitive type of isogamy called aplanogametic isogamy. In this type of isogamy, the entire protoplasmic contents of vegetative cells (viz., Gametangia) function as gametes. It always involves gametangial contact. Conjugation is of two types: Scalariform and Lateral conjugation.

(i) Scalariform Conjugation: it is the most common method in most heterothallic species of Spirogyra. It takes place at night when two filaments come close and lies opposite and parallel to each other. Now, opposite cells develop outgrowths or protuberances called conjugation processes. At the point of contact, the tips of these processes dissolve to form a conjugation tube between opposite cells. This resembles r ladder. Hence, the conjugation is called scalariform (Gr. scala – ladder; forma = shape) or H-shape conjugation.

When the conjugation tube is forming, the protoplasts of conjugation cells (gametangia) recede, round up, and function as gametes. The male gametes transfer through the conjugation tube into the opposite cell (female gametangia) and fuse with the female gamete to form a zygote (2n).

The zygote secretes a thick wall to become a zygospore. Therefore, in the late stage of conjugation, male gametangia remain empty and the female gametangia contain zygospores (2n).

(ii) Lateral Conjugation: It is of rare occurrence and involves the fusion of gametes from two adjacent cells of the same filament in monoecious or homothallic species. It is of two types: Indirect and Direct Lateral conjugation.

(a) Indirect Lateral Conjugation: Two outgrowths emerge on both sides of a transverse septum of two adjunct cells and after some growth, the two protuberances unite to form a conjugation tube. Of the two adjacent cells, one behaves as a male gametangium whiles the other as a female gametangium. The content (now behaving as gamete) from the male gametangium passes through the conjugation tube and enters into the female gametangium. By the fusion of male and female gametes a diploid zygospore is formed. Thus, in each second cell of a filament, a zygospore is formed, e.g., S. tenuissima, S. afflnis.

(b) Direct Lateral Conjugation: In this type of conjugation, the male gametangium after passing through an aperture in the transverse septum of adjoining gametangium enters the female gametangium and fuses with the female gamete and a diploid zygospore is formed, e.g., S. jogensis.

Germination of Zygospore
The final product of conjugation is a zygospore which is a diploid thick wall structure. Zygospore is the only diploid phase in the sexual life cycle. The decay of female gametangia causes the liberation of zygospore which remains dormant in the pond bottom till favorable conditions return. The Zygospore wall is thick and differentiated into 3-layers. Outer exosporium (cellulose), middle mesosporium (cellulose and chitin), and inner endosporium (cellulose). The fusion of male and female cells is the zygospore.

In return for favourable conditions, the diploid nucleus of the zygospore undergoes meiosis to produce 4 haploid nuclei out of which 3 degenerate. The zygospore with one haploid nucleus gradually enlarges and burst open to release a germ tube. The repeated transverse divisions of the germ tube form a new filament.

The schematic representation of the sexual life cycle of Spirogyra is as follows:

Conjugation of Paramoecium
Paramoecium is a ciliate protist organism. They live in saline water. Generally, Paramoecium reproduces asexually, by binary fission. Due to asexual reproduction, Paramoecium becomes physiologically and metabolically weak. Then the species of Paramoecium involves in the special method of sexual reproduction. It takes place by conjugation method. During the process following events occurs:

• Two Paramoecia come close and get attached together from the side of the oral groove by some sticky substances. The Paramoecia that take part in conjugation are called conjugants.
• At the point of attachment, the pellicle degenerates to form a cytoplasmic bridge.
• After the formation of the cytoplasmic bridge, the macronucleus of each conjugant disappeared.
• The micronucleus undergoes meiosis division in each conjugant to give four nuclei.
• In each conjugant out of four nuclei, three nuclei degenerate and only one remains functional.
• The remaining nucleus of each conjugant undergoes mitosis division to produce two nuclei. Out of two nuclei, one is larger and the other is smaller.
• A small nucleus of each conjugant migrates crosswise between two Paramoecia through the cytoplasmic bridge. That nucleus is called the migratory nucleus or male nucleus.
• The larger nucleus remains stationary and is called the stationary nucleus or female nucleus.
• The migrated nucleus fuses with a stationary nucleus in each conjugant to form a zygote nucleus.
• Now two paramoecia separate together and then they are called exconjugant.
• In each exconjugant, the zygote undergoes mitosis division 3 times to produce 8 nuclei.
• Out of eight nuclei, four become larger and 4 become smaller in each exconjugant. The larger nuclei are termed macronucleus and the smaller is micronucleus.
• Out of four micronuclei, 3 degenerate and one remains functional.
• The functional micronucleus in each exconjugant divides into two and the conjugant divide by binary fission into two daughter Paramoecia from each exconjugant. The macronuclei are shared equally.
• Again the micronuclei of two daughter Paramoecia divide into two and macronuclei are again shared and later the Paramoecia divide again to produce 8 Paramoecia.

Conjugation can be induced by a shortage of food, at certain points in the Paramoeciurn life cycle.

2. Syngamy

The process of fusion between two different male and female gametes where one is smaller and flagellated and the other is larger and non-flagellated is called syngamy.

This type of sexual reproduction is found in higher groups of plants and animals. Sexual reproductive units, i.e., gametes, are produced within the gametangium. According to the structure of fusing gametes syngamy is of four types:

• Isogamy: In syngamous sexual reproduction, where two morphologically identical gametes of equal size and form, i.e., isogametes fuse in pair forming a zygote is called isogamy, e.g., Mucor, Monocystis.
• Anisogamy: In this type, the morphologically non-identical gametes, i.e., gametes of unequal size fuse to form a zygote, known as anisogamy. Of the two non-identical gametes, the smaller male gametes are called microgametes and the larger female gametes are called macrogametes. These two gametes after their union produce zygotes outside the body, e.g., Chlamydomonas.
• Oogamy: Oogamy is a complex type of sexual reproduction. In this type, fusion takes place between two heterogametes, one is smaller, active, and flagellate known as the male gamete (antherozoids, spermatozoids, sperm) and the other one is larger, passive, and non-flagellate known as as the female gamete (oospore, ovum, egg). As a result of such union, i.e., fertilization, an oospore (zygote) is formed. The zygote gives rise to a new plant, e.g., humans and other mammals.
• Hologamy: Hologamy means when two organisms act as gametes and fuse with each other, e.g., Yeast (Saccharomyces cerevisiae).

According to the source of fusing gametes syngamy is of two types:

• Endogamy: In this type of syngamy, fusion occurs between male and female gamete of the same parent i.e., uniparental, e.g., Tapeworm (Taenia).
• Exogamy: In this type of syngamy, fusion occurs between two gametes, produced by different parents i.e., biparental, e.g., Rabbit.

Differences between Asexual and Sexual Reproduction:

Asexual Reproduction	Sexual Reproduction
1. Asexual reproduction is a simple process.	1. Sexual reproduction is a complex process.
2. It is seen in the lower groups of organisms.	2. It is seen in the higher groups of plants and animals.
3. It involves only one individual.	3. It involves both male and female individuals.
4. Spore formation takes place.	4. Gametes are formed.
5. Offsprings are always identical to the parents.	5. Offsprings may be identical or may be associated with new characters.
6. Offsprings are produced larger in number.	6. Offsprings are produced smaller in number.
7. It takes a short time for its completion.	7. It takes a long time for its completion.
8. There is no chance for evolution.	8. There is a chance for evolution.
9. Variation does not take place in this process. So they cannot adapt themselves to the changed environment and have a chance of extinction.	9. Due to the mixing of new characters in the off-springs, they can easily adapt themselves to the changed environment.

Differences between Senescence, Ageing, and Death:

Senescence	Ageing	Death
1. It is a progressive, collective, and deteriorative process, leads to a complete loss of organization and function.	1. Structural and chemical changes occur during the life span of a plant.	1. It is the termination of the functional life of a plant.
2. Only degenerative and deteriorative changes occur in plants.	2. Ageing refers to the sum of total changes of metabolic activities.	2. It is a regular feature of the annual cycle of plants.
3. It is the result of ageing that leads to death.	3. It is the durable feature of all living organisms.	3. It is a permanent feature of all living.

Differences between Isogamy and Anisogamy:

Isogamy	Anisogamy
1. In this case, fusing gametes are morphologically similar but physiologically different.	1. In this case, fusing gametes are morphologically different.
2. Smaller microgametes and larger macrogametes are not formed.	2. Micro and Macrogametes are formed.
3. It is found in Mucor, Monocystis, etc.	3. It is found in Chlamydomonas.

Importance or Advantages of Sexual Reproduction

• Sexual reproduction is an improved method of reproduction.
• The offspring, which is produced by the process of fertilization is to be adapted in the new environment.
• It creates opportunities for mixing two contrasting parental characters.
• Parental characters are transmitted to the offspring from generation to generation.
• It expresses new characters resulting from the union of two parents.
• It helps to increase the vital force of an organism by changing the hereditary materials.
• Sexual reproduction helps in the appearance of variation at the time of gamete formation by crossing over.
• Variation is the raw material of evolution.
• Sexual reproduction helps to attain diploid conditions in the life cycle of an organism, by which alternation of generations of haploid (n) and diploid (2n) phases is maintained.
• The offspring which is produced by sexual reproduction becomes strong, stout, healthy, and disease resistant.

Disadvantages of Sexual Reproduction

• Sexual reproduction is usually a slow and complex process.
• Sometimes there are some obstacles to the union of male and female gametes in sexual reproduction.
• Both male and female individuals are necessary for sexual reproduction.
• If sexual reproduction does not take place properly in time, wastage of gametes occurs.
• It takes much time to complete the process.

Differences between Vegetative, Asexual, and Sexual Reproduction:

Vegetative Reproduction	Asexual Reproduction	Sexual Reproduction
1. It is a comparatively simple and primitive process.	1. It is slightly improved than vegetative.	1. It is an improved process.
2. It takes place simply by cell division or by separation from the mother’s body.	2. It takes place by the formation of various types of spores.	2. It takes place through the union of gametes.
3. Only one individual is required.	3. Only one individual is required.	3. Two individuals are required.
4. Mitotic cell division takes place in this process.	4. Mitotic cell division takes place in this process.	4. Meiotic cell division takes place at any one stage of this process.
5. Offspring bears similar parental qualities.	5. Offspring bears similar parental qualities.	5. Variation always appears in the offspring.
6. Variations never appear in this process.	6. Variations never appear in this process.	6. Variations always appear.
7. No role in further evolution.	7. No role in further evolution.	7. Presence of a positive role in evolution.
8. Absence of alternation of generation in the life-cycle.	8. Absence of alternation of generation in the life-cycle.	8. Presence of distinct alternation of generation in the life-cycle.
9. Generally, it is found in lower groups of plants and animals.	9. Generally, it is found in plants and lower groups of animals.	9. Generally, it is found in higher groups of plants and animals.

Sexual Diversity Among Various Organisms

There is a large diversity among the sexually reproducing organisms. In the case of plants, where the male and female reproductive organs are present in the same mother plant i.e., bisexual, it is termed as homothallic and monoecious; Similarly, when reproductive organs are present in different plants i.e., unisexual, it is termed as heterothallic and dioecious. In the case of animals, sex organs develop and demarcate male and female organisms. As an example: an earthworm is a bisexual organism because it possesses both male and female organs in the same body whereas a cockroach is a unisexual insect because it does not contain both sex organs in its body.

Some organisms, belonging to monera, fungi, algae, and Bryophyta, have haploid parental bodies whereas those belonging to Pteridophyta, gymnosperms, angiosperms, and animals, have diploid parental bodies. The following tabular representation shows the different chromosome numbers in meiocytes and gametes in some organisms:

Plants
Name of an Organism	Chromosome number in Meiocyte (2n)	Chromosome number in Gamete (n)
1. Rice	24	12
2. Maize	20	10
3. Potato	48	24
4. Onion	16	8
5. Apple	34	17

Chromosome number in Meiocytes and Gametes of some Organisms

Animals
Name of an Organism	Chromosome number in Meiocyte (2n)	Chromosome number in Gamete (n)
1. Butterfly	380	190
2. Fruit fly	8	4
3. Rat	42	21
4. Cat	38	19
5. Dog	78	39
6. Human	46	23

Chromosome number in Meiocytes and Gametes of some organisms

Different Events of Sexual Reproduction

During sexual reproduction different eventual stages are namely-

Pre-fertilization Events

These events are prior to the fusion of male and female gametes. This stage is divided into gametogenesis and gamete transfer.
(i) Gametogenesis: It is characterized as the process of the formation of two different types of gametes i.e., male and female gametes. These are haploid cells. In some algae, gametes are similar in appearance, so they cannot be categorized into male and female gametes. Such gametes are called homogametes or isogametes. In the case of Spirogyra, Ulothrix, this type of gametes is found. In another case, when two gametes are structurally and functionally different, then these gametes are called heterogametes. Fucus, Oedogonium, Chara and in the case of humans, this type of gametes are found.

In most organisms, the male gamete is recognized as sperm, and female gamete is known as egg or ovum.

(ii) Gamete transfer: After the formation of male and female gametes, they are brought together for fertilization. The male gamete is motile whereas the female gamete is non-motile. Due to the fusion of gametes, the zygote is formed which gives rise to a new individual.

In seed plants, the male gamete carries pollen grains and the female gamete carries an egg. Pollen grains produced in another must be transferred to the stigma of the carpel. This process is called pollination. These pollen grains germinate at the site of the stigma of the carpel through pollination. Pollen grains germinate on stigma. The pollen tube carries the male gametes. Male gametes reach the ovule and at last discharge the male gametes near the egg.

Two types of pollination are there. These are

1. Self-pollination: It occurs within the same plants.
2. Cross-pollination: It occurs between two flowers of different plants of the same species. The method of cross-pollination is carried out by-
   • Insect pollination: Insects like honey bees play a significant role in pollination when they visit a flower for nectar. At that time, some pollen grains get deposited on the body of insects.
   • When the insects visit another flower, some pollen grains automatically stick to the stigma of another flower. This results in pollination.
   • Wind pollination: Wind pollination occurs in the case of small, light-weighted flowers. They don’t produce any nectar. Here, a large quantity of pollen grains are produced.
   • Water pollination: In this case, the flowers are dormant, small. The amount of production of pollen grains occurs in large quantities. Male and female flowers are borne on separate plants, i.e., dioecious plants.

Fertilization Events

The union of male and female gametes is known as fertilization. Gamete is the unit of sexual reproduction. The fusion product of two gametes, in general, is called a zygote. Fertilization involves the physical entry of the sperm and also physiological processes in both the egg and sperm. During fertilization, the female gamete remains confined within the oogonium. The antherozoids when matured, are liberated from the antheridium, which swims around in the water and enters the oogonium. But only one sperm can unite with the ovum.

Types of Fertilization
Fertilization is of different types. They are as follows:

• Internal Fertilization: It is the most common method of fertilization where the union of gametes takes place inside the body of the female. The male gamete is transferred to the female body from the male by sex organs or other means, e.g., Birds and Mammals.
• External Fertilization: In this type of fertilization, the eggs and sperm are simultaneously shed into the water and the sperm swim through the water to fertilize the egg in a process known as broadcast fertilization. It is common in many aquatic animals e.g., Hydra, coral, and in most of the fishes. While, in frogs, females deposit unfertilized eggs, and the males swim by and fertilize them.
• Self-Fertilization: When fertilization union of male and female gametes takes place within the same individual, i.e., is known as self-fertilization, e.g., Bisexual flowers, Tapeworm, etc.
• Cross Fertilization: When fertilization takes place between two different individuals, i.e., the union of male and female gametes of different individuals of the same species, is known as cross-fertilization, e.g., unisexual plants like palm, coconut, papaya, etc., and unisexual animals like toad, reptiles, fish, mammals, etc.

In flowering plants, fertilization occurs in the following ways:

• A pollen grain lands on the stigma and it germinates.
• On germination, a pollen tube emerges from the pollen grain and grows rapidly down the style to the ovary.
• During the growth of the pollen tube, the large tube nucleus gradually disappears and the small generative nucleus divides into two sperm nuclei that represent male gametes.
• The pollen tube enters the embryo sac.
• The tip of the pollen tube bursts and the two male gametes are released in the embryo sac.
• One of the two male gametes fuses with the egg and forms a diploid zygote. The second one fuses with the definitive nucleus (2n) and forms a triploid (3n) endosperm nucleus. Such fertilization is called double fertilization.

Differences between Self-fertilization and Cross-fertilization:

Self-Fertilization	Cross Fertilization
1. In this type of fertilization, a fusion of male and female gametes of the same individual takes place.	1. In this type of fertilization fusion of two gametes takes place, which are produced by different individuals.
2. It is uniparental, e.g., Taenia (Tapeworm).	2. It is bi-parental, e.g., Mammals.
3. Though being a method of sexual reproduction, variations do not occur.	3. Variation always occurs in the offspring.

Differences between External and Internal Fertilization:

External Fertilization	Internal Fertilization
1. External fertilization takes place outside the body.	1. Internal fertilization takes place inside the body.
2. Medium is necessary for external fertilization, e.g., water.	2. Medium is not necessary for internal fertilization.
3. Development of the embryo takes place in the medium (water) outside the body.	3. Embryo is developed inside the organism.
4. A large number of gametes are released in the surrounding medium where fertilization takes place.	4. The number of gametes produced is less. The male gametes are deposited in the body of the female with the help of a copulatory organ.
5. External fertilization found in fish, toads, etc.	5. Internal fertilization is found in reptiles, birds, and mammals.

Post-fertilization Changes

During sexual reproduction, changes that occur after the formation of the zygote (2n) are recognized as post-fertilization events. These events occur through zygote formation and embryogenesis.
(i) Zygote formation: In the case of all sexually reproducing organisms, a diploid zygote is formed. In external fertilization, the zygote is formed in the external medium, whereas in internal fertilization, the zygote is formed inside the body of the organism.

An organism’s life cycle is an important factor in the development of a zygote. In the case of the haplontic life cycle, the zygote forms haploid spores by meiosis and ultimately forms haploid individuals. Actually, the zygote is the important link between the organisms of one generation and the next.

Importance of Zygote :

• It plays a key role between two successive generations.
• It is always hybrid in nature because it carries a genome from two different organisms.
• It is the first cell (single) of a multicellular organism.
• It is diploid, i.e., with 2n number of chromosomes, (e.g., 2n = 46 chromosomes in humans).

(ii) Embryogenesis: In this process the development of the embryo takes place from the zygote which undergoes cell division that increases the number of cells in the developing embryo. Cell differentiation helps the groups of cells to undergo several modifications to form specialized tissues and organs to form an organism.

Differences between Gametogenesis and Embryogenesis:

Gametogenesis	Embryogenesis
1. Due to this haploid gametes are formed.	1. Due to this process development of a multicellular embryo from a unicellular diploid zygote occurs.
2. In this process meiosis occurs.	2. In this process mitosis occurs.
3. This process leads to fertilization.	3. This process leads to organ formation.

According to embryogenesis animals are divided into three categories, depending on the development of the zygote which either takes place outside the body of the female parent or inside the body.

Maintenance of Chromosome Number:

• The numbers of chromosomes are half in the case of haploid gametes (n).
• During sexual reproduction, haploid male and female gametes fuse to form a diploid zygote through the process of fertilization.
• This fusion forms a diploid number (2n) of chromosomes.
• During meiosis, random segregations of chromosomes and exchanges of genetic material between homologous chromosomes (crossing over) result in new combinations of genes.
• Genetic diversity is increased by this reshuffling of genes.
• In a zygote, two sets of chromosomes from two different parents come together, leading to variations and evolution.

Development of Embryo in Vertebrate Animals

1. Oviparous: The process of development of the embryo in the fertilized or unfertilized egg outside the body of some animals, is called oviparous animals, e.g., Fish, Amphibia, Reptiles, Birds, etc.

2. Ovoviviparous: The process by which the embryo is developed inside the animal body but does not depend on the mother’s body for nutrition, is called ovoviviparous animals, e.g., sharks.

3. Viviparous: The process where the embryo is developed and fully grown inside the mother’s body by taking shelter and nutrition, is called viviparous animal, e.g., Guineapig, Cow, Man, etc.

Differences between Oviparous and Viviparous Animals:

Oviparous Animals	Viviparous Animals
1. The zygote is developed outside the female’s body.	1. The zygote is developed within the female’s body.
2. Here, the female lays fertilized or unfertilized eggs. e.g., reptiles, all birds.	2. Here, the female gives birth to young ones. e.g., mammals.

Differences between Conjugation and Fertilization:

Conjugation	Fertilization
1. Conjugation is a simple process.	1. Fertilization is a complex process.
2. It takes place in lower groups of plants and animals.	2. It is mainly found in higher groups of plants and animals.
3. The participating gametes are morphologically different.	3. The participating gametes are both morphologically and physiologically different.
4. The union of gametes takes place through the conjugation tube.	4. No such conjugation tube is formed in the fertilization process.
5. The zygote which is formed by the process of conjugation is called a zygospore.	5. The zygote which is formed by the process of fertilization is called an oospore.

Differences between the Menstrual and Oestrous Cycles:

Menstrual Cycle	Oestrous Cycle
1. It can be seen in female primates i.e., monkeys, apes, and human beings.	1. It can be found in non-primate females i.e., cows, dogs, etc.
2. There are four different phases: menstrual phase, follicular phase, ovulatory phase, and luteal phase.	2. It contains a short heat period (12-24 hrs. in cow) followed by an anoestrous or passive period.
3. In the menstrual phase, blood loss occurs.	3. No blood loss in this phase.
4. No irresistible sex urge.	4. Female shows strong sexual urges.
5. During menstruation, the broken endometrium is outstripped.	5. Broken endometrium is reabsorbed.
6. Females does not permit usually sex during the menstrual phase.	6. Female allows for sex during oestrous cycle.