SEXUAL REPRODUCTION:
Sexual reproduction is a process that involves the formation of male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote, which develops to form the new organism. It is an elaborate, complex, and slow process compared to asexual reproduction. Sexual reproduction results in offspring that are not identical to the parents or amongst themselves because of the fusion of male and female gametes.
A study of diverse organisms such as plants, animals, or fungi shows that though they differ greatly in external morphology, internal structure, and physiology, they share a similar pattern when it comes to sexual mode of reproduction. Let us first discuss what features are common to these diverse organisms.
All organisms have to reach a certain stage of growth and maturity in their life before they can reproduce sexually. That period of growth is called the juvenile phase, known as the vegetative phase in plants. This phase is of variable durations in different organisms. In some plants, where flowering occurs more than once, the inter-flowering period is considered mature.
Plants, such as annual and biennial types, show clear-cut vegetative, reproductive, and senescent phases, but in perennial species, it is difficult to define these phases clearly. A few plants exhibit unusual flowering phenomena, such as bamboo species that flower only once in their lifetime, generally after 50-100 years, produce a large number of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka, and Tamil Nadu into blue stretches and attracted a large number of tourists.
The end of the reproductive phase can be considered as one of the parameters of senescence or old age. There are concomitant changes in the body, such as slowing of metabolism, during this last phase of lifespan. Old age ultimately leads to death.
• In animals, gametogenesis is a complex process that occurs in specialized organs known as gonads. In males, the
gonads are testes which produce and release sperm, while in females, the gonads are ovaries which produce and
release eggs.
• In plants, gametogenesis occurs in specialized structures called gametangia. The male gametangia is called the
antheridium, which produces and releases sperm, while the female gametangia is called the archegonium, which
produces and releases eggs.
• Gamete Transfer
After gamete formation, the next step is gamete transfer. In animals, this can occur through a variety of methods,
such as copulation or external fertilization. In plants, gamete transfer occurs through pollination, which can be
achieved through various agents such as wind, water, or animals.
• FERTILISATION
The fusion of male and female gametes to form a zygote is called fertilisation. This event marks the beginning
of the post-fertilisation events in sexual reproduction.
• POST-FERTILISATION EVENTS
The post-fertilisation events involve the development of the zygote into a multicellular organism through a series
of cell divisions and differentiation. These events can vary greatly among different organisms, but generally
involve the formation of embryonic structures, establishment of the body plan, and eventually the formation of
reproductive organs in the adult organism.
Sexual reproduction in organisms is often characterized by the fusion of gametes from two different individuals, but this is not always the case. Some fungi and plants exhibit different sexual systems. For example, in fungi, the terms homothallic and heterothallic are used to describe the bisexual and unisexual conditions, respectively. Similarly, in plants, the terms monoecious and dioecious are used to describe the presence of both male and female flowers on the same individual or separate individuals, respectively. In flowering plants, the male flower is called staminate, bearing stamens, while the female flower is called pistillate, bearing pistils. Examples of monoecious plants include cucurbits and coconuts, while papaya and date palm are examples of dioecious plants.
• The life cycle of many organisms involves both haploid and diploid stages. In some organisms, the haploid stage is dominant while in others, the diploid stage is dominant.
• Organisms belonging to the Monera, fungi, algae, and bryophytes have a haploid plant body. In these organisms, gametes are produced by mitotic division.
• In contrast, organisms belonging to pteridophytes, gymnosperms, angiosperms, and most animals, including humans, have a diploid parental body.
• The diploid parental body undergoes meiosis to produce haploid gametes. Fertilization between haploid gametes results in the formation of a diploid zygote, which develops into a diploid individual.
• In most organisms, the male gamete is motile while the female gamete is non-motile.
• However, in angiosperms and red algae, both male and female gametes are non-motile.
• In simple plants like algae, bryophytes, and pteridophytes, water is the medium through which gamete transfer occurs. However, many male gametes fail to reach the female gametes during transport. To compensate for this loss, several thousand male gametes are produced for every female gamete.
• In bisexual, self-fertilizing plants such as peas, transfer of pollen grains to the stigma is easy as the anthers and stigma are located close to each other. Pollen grains come in contact with the stigma soon after they are shed. But in cross-pollinating plants, including dioecious plants, a specialized event called pollination facilitates 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 for fertilization.
• One of the most crucial events in sexual reproduction is the fusion of gametes, known as syngamy, which results in the formation of a diploid zygote. The terms syngamy and fertilisation are often used interchangeably.
• Syngamy can occur either inside or outside the body of an organism. In most aquatic organisms, such as algae, fishes, and amphibians, syngamy occurs in the external medium, i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation.
• In many terrestrial organisms, such as fungi, reptiles, birds, mammals, and a majority of plants (including bryophytes, pteridophytes, gymnosperms, and angiosperms), syngamy occurs inside the body of the organism. Hence, the process is called internal fertilisation. In these organisms, the egg is formed inside the female body where it fuses with the male gamete.
• In organisms exhibiting internal fertilisation, such as bryophytes and pteridophytes, the male gamete is motile and has to reach the egg to fuse with it. Although the number of sperms produced is very large, there is a significant reduction in the number of eggs produced.
• In seed plants, the non-motile male gametes are carried to the non-motile female gamete by pollen tubes.
• In some organisms, such as certain species of lizards and birds, the female gamete can develop into a new organism without fertilisation. This phenomenon is called parthenogenesis.
Events in sexual reproduction after the formation of zygote are caused post fertilization events.
The Zygote :-
• Formation of the diploid zygote is universal in all sexually reproducing organisms. In organisms with
external fertilisation, zygote is formed in the external medium (usually water), whereas in those exhibiting internal
fertilisation, zygote is formed inside the body of the organism.
• In organisms belonging to fungi and algae, zygote develops a thick wall that is resistant to
dessication and damage. It undergoes a period of rest before germination.
• Zygote is the vital link that ensures continuity of species between organisms of one generation and next.
It undergoes a series of developmental events to eventually form a mature organism.
Embryogenesis:
• In organisms exhibiting internal fertilisation, zygote undergoes a series of mitotic divisions to form a multicellular
embryo.
• In plants, this embryo is called a sporophyte, which is diploid and will give rise to the adult plant.
• In animals, the developing embryo may undergo various stages of morphological and physiological changes, including
the formation of tissues and organs, ultimately resulting in the formation of a fully functional organism.
Seed formation:
• In seed plants, after fertilization, the zygote develops into an embryo within the ovule. The ovule develops
into a seed after fertilization, which has an embryo and a food source (endosperm) enclosed by a protective seed coat.
• This process of seed formation is unique to seed plants and ensures that the embryo is protected and has access to
nutrients even after it is dispersed away from the parent plant.
Placenta formation:
• In mammals, the zygote implants in the uterus and develops into an embryo that is nourished by the placenta.
• The placenta is a specialized organ that forms from the embryonic and maternal tissues and facilitates nutrient
and waste exchange between the mother and the developing embryo.
Germ cell formation:
• In some organisms, such as humans, germ cells are set aside early in embryonic development to eventually form
the gametes for the next generation.
• This process is essential for the continuity of species and ensures that genetic information is passed on from one
generation to the next.
In viviparous animals (including most mammals, such as human beings), the zygote develops into a young one inside the body of the female organism. The young ones are nourished and protected by the female’s body until they are ready to be delivered. Because of the proper embryonic care and protection, the chances of survival of the young ones are greater in viviparous organisms.
Natural Parthenogenesis: • In natural parthenogenesis, the unfertilised egg develops into a complete organism without any external intervention. This process is commonly observed in many invertebrate species such as aphids, rotifers, and some species of bees and crustaceans. • In some cases, this process can occur in certain vertebrates such as reptiles, amphibians, and fishes. However, the frequency of natural parthenogenesis in vertebrates is very low.
Artificial Parthenogenesis: • Artificial parthenogenesis is the process of inducing the development of an unfertilized egg into a complete organism through external manipulation such as heat shock, chemicals, or electrical stimulation. • This technique has been used in various fields of research such as cloning, stem cell research, and in vitro fertilization. It has also been used in the production of some commercially important species like fish and bees.
Advantages and disadvantages of parthenogenesis:
Advantages:
- Parthenogenesis can result in high rates of reproduction and rapid population growth, as the adult organism can devote all its energy to reproduction rather than searching for a mate.
- Parthenogenesis avoids the wastage of germplasm as no sperm is needed for fertilization.
- Offspring produced by parthenogenesis are genetically identical to the parent, which can be useful for maintaining desirable traits in agriculture or animal breeding.
- Haploid parthenogenesis can provide direct evidence for the chromosomal theory of sex determination.
Disadvantages:
- Parthenogenesis eliminates the possibility of new gene combinations that can result from sexual reproduction, which may limit the ability of the population to adapt to changing environmental conditions.
- In some cases, parthenogenesis can result in reduced genetic diversity and an increased risk of extinction.