Sexual Reproduction

Sexual reproduction includes the genesis of the male and female gametes, either by the same individual or by distinct individuals of the opposite sex. A zygote is formed by the fusion of these two gametes which further develop into a new organism. It is a slow, elaborate and complex process and more difficult to understand than asexual reproduction. The fusion of different male and female gametes results in the formation of a new organism that does not completely look like its parents. Thus, the organisms that reproduce through sexual reproduction have the same features but still look very dissimilar amongst themselves.

When we study diverse creatures found on the earth – animals, plants or fungi – we find that even if these organisms differ extremely in external structure or morphology, internal morphology or anatomy, and even physiology, they share similar mode of sexual reproduction.

Further, in this article, we will see how these organisms are so different and yet have common or similar patterns of sexual reproduction.

Organism go through three phases in their lifespan

Juvenile Phase

All living beings have to reach a certain stage of maturity and growth to be able to reproduce sexually. This period of growth is known as Juvenile Phase. It is termed as Vegetative Phase in reference to plants. The juvenile phase can be of different periods in different living beings.

Reproductive Phase

The end of the vegetative or juvenile phase marks the starting of the reproductive phase.

In plants

It can be observed more clearly in the higher plants when they mature or flower. In plants, you must have seen that the time taken to reach on flowering stage is different for different plant species. While Mango tree may take some years, Marigold blooms in a few months in a certain season. Even, some plants bloom more than once. You must have also noticed that the flowering happens during the same month every year. Also, the availability of the fruits is seasonal. In fact, some plants flower throughout the year. The vegetative or reproductive phase is easily identifiable in the plants that reproduce annually or biannually, but these phases are blurred in the perennial species.

Interesting Fact – Bamboo trees (Bambusoideae)  flower only once in their lifetime, normally after fifty to hundred years, producing a large number of offsprings (fruits) and then die.

Another plant is known as Strobilanthus kunthiana (Neelakuranji), flower only once in twelve years.

In Animals

Morphological and physiological changes occur before the initiation of the reproductive phase in animals. In animals, too, these phases are of different duration in different species or organisms. You must have noticed the changes in human beings when they grow and reach the reproductive stage.

Among other animals, for example, birds, lay eggs during a certain time of the year. Similarly, frogs and lizards also tend to follow the same pattern. However, we have seen that poultry birds in the captivity are made to lay eggs throughout the year. There is no season in which you won’t find eggs. But, this is not a reproduction; it is exploitation of nature by a human for his selfish needs.

The cyclical changes in the functioning of the ovaries and accessory ducts integrated with hormonal changes during the reproductive phase are exhibited by the placental mammals (female).

In non-primate mammals like rats, sheep, cow, deers, tiger, dogs, etc., these cyclical changes are termed as oestrus cycle.

Similarly, in primates such as apes, monkeys, and humans, these cyclical changes are termed as the menstrual cycle. Some mammals that live in wild and natural conditions show these cycles only in favourable conditions (seasons). These animals are called seasonal breeders.

However, the animals that reproduce throughout the year are known as continuous breeders.

Senescence

We all grow old one day (if not dead in young age). Growing old in science is just the end of the reproductive phase, or we can say that the end of this reproductive stage is one of the criterions of old age. Growing old is known as Senescence. At this stage, we see consequential changes in the body (slow metabolism) which finally leads to death.

Note – Hormones are one of the key factors that are responsible for the changeover between these three phases of life. The hormones interact with the environmental factors and regulate the functioning of the reproductive system and also bring out the related behavioural expressions in the living organisms.

Events in sexual reproduction

The sexual reproduction is a complex phenomenon but for the sake of convenience, it is grouped into three unique stages – Pre-fertilization, Fertilization and the Post-fertilization.

Types of gametes on the basis of morphological structure

Isogametes

In some organisms, such as Algae, the gametes are so identical that it is very difficult to identify them as male and female. Such gametes are known as homogametes or isogametes.

Heterogametes

However, most of the organisms that reproduce sexually produce gametes that are morphologically different. These gametes that are different from each other are called heterogametes. The human female egg or ovum and the sperm or antherozoid is a good example.

On the basis of sexuality, the organisms can be categorised as bisexual or unisexual

Bisexual Organisms

Those organisms which have both male and female sex organs. For example- In some plants, both stamen and pistil are found that are responsible for self-fertilization. Terms like Monoecious or Heterothallic are used for plants or fungi that are bisexual. Example of monoecious plants is coconut and cucurbits. In animals, Earthworm is an example of a Bisexual organism.

Unisexual Organisms

Those organisms which only possess one reproductive organ – either male or female. Example – Human. Many species of plants are also unisexual and have either male stamen or female pistil. These plants reproduce through cross-pollination. Terms like Dioecious or Heterothallic are used to describe these unisexual organisms. In animals, the cockroach is also an example of a unisexual organism.

Sexual reproduction can be divided into three main phases

1. Pre-Fertilization Phase
These include all the activities of sexual reproduction before the fusion of gametes. The two important events of this stage are gametogenesis and gamete transfer.

a. Gametogenesis

  • It is simply the process of creation of the two types of gametes (male and female). These are haploid cells.

b. Cell division during gamete formation

  • There are two types of heterogametes – Male and Female. Both of these are haploid even though the parents from which they are produced are either haploid or diploid. A haploid organism gives rise to haploid gamete through mitotic division. Most of the organisms that belong to Monera, Fungi, Bryophytes, and Algae are haploids. On the other hand, the Pteridophytes, Angiosperms and Gymnosperms and even the majority of the animals including human beings possess a diploid body. When organisms possess diploid body, they produce gametes that are haploid.
  • Note- Haploid – When a cell or nucleus has only a single set of unpaired chromosomes, it is called haploid.
  • Diploid – When a cell or nucleus contains two complete sets of chromosomes, one from each parent it is called Diploid.
  • In a diploid organism, some kind of specialised cells is present. These are known as meiocytes or gamete mother cell that undergo meiosis. Only one set of chromosomes get integrated into each gamete which makes them haploid in nature.

c. Gamete Transfer

  • Two gametes can only lead to fusion when these brought close physically. In most of the organisms, male gamete is mobile and female tends to be static. Though some exception are also there like some fungi and algae have both gametes motile. The gametes need some medium for their movement. In many of the simple plants, like bryophytes, algae, and pteridophytes, water is the best medium for this transfer. These organisms produce a lot of gametes because there are chances that most of these gametes do not even get the chance to meet each other and hence no fertilization takes place. This is the reason; male gametes are produced a thousand times more than the female gametes.
  • In seed plants, the carriers of the gametes of male and female are pollen grains and ovule respectively. The production of pollen grains happen in the anthers; therefore, these are transferred to stigma for fertilization. In bisexual plants, self-fertilization occurs. Example – peas. In these plants, the transfer process is easy because both stigma and anthers are closely located. After shedding, pollen grains contact stigma. Though, this process is not so simple in cross-fertilizing plants (dioecious). A specialized activity called pollination induces transfer of pollen grains to the stigma. The germination of pollen occurs on stigma, and the male gametes are carried through pollen tubes. The gametes reach the ovule through these tubes where it is finally discharged near the ovum or egg. In dioecious animal, the presence of the male and female gametes in different individuals demands for the situation and a special mechanism through which both the gametes can come in contact. It is an essential event in sexual reproduction as, without successful gamete transfer, fertilization cannot occur.

2. Fertilization Phase

The most important event in the process of sexual reproduction is fertilization. Without any fusion of gametes, further process is not possible. Fertilization is also called syngamy as this results in the creation of diploid zygote.

Interesting Fact – Some organisms like honeybees, lizards, rotifers, and birds (like Turkey) do not need a fusion of gametes to form a new organism. The new individual is formed without the fertilization. Such a phenomenon is known as Parthenogenesis. In most aquatic organisms, like algae, fishes, and amphibians, external medium like water is required for syngamy. It means the fusion happens outside the body of the organism. This type of fusion of gametes is called external fertilization. Organisms that reproduce through external fertilization show great coordination between the different sexes and deliver a large number of gametes into the medium (water) so that they can increase the chances of fusion. This occurs in frogs and bony fishes where they produce a large number of offsprings. A major drawback is that the offsprings are highly vulnerable and their survival is often threatened by the big predators. Only a few of these offsprings get chance to reach adulthood.

In many terrestrial living beings, like fungi, higher animals like birds, reptiles, mammals and in many plants like gymnosperms, pteridophytes, and angiosperms, fertilization occurs inside the body of the organism. Therefore, the process is known as internal fertilization. In all of these beings, the formation of the egg occurs inside the body of the female where it fuses with the male gamete. In most of these organisms, the male gametes are motile and try to reach the egg to fuse with it.
The number of eggs is usually way smaller than the number of sperms that are produced. However, in seed plants, the male gamete is non-motile and reach female gamete through pollen tubes.

3. Post-fertilization Phase
The activities that occur after the zygote is formed are called post-fertilization activities.

a. Zygote

  • The process of formation of a diploid zygote is the same in all the sexually reproducing beings. In the case of external fertilization, the zygote is formed outside in an external medium (normally water). On the other hand, in case of internal fertilization, the formation of zygote occurs inside the body of the organism.
    The development of this zygote depends on the environment and the kind of life cycle it goes through. For example, in fungi and algae, a thick wall is developed around the zygote resistant to the damage and desiccation. It goes through a period of rest (dormancy) before it finds a suitable place and environment to germinate.

b. Embryogenesis

  • It is the process of formation of an embryo from a zygote. The zygote undergoes mitosis (cell division) during embryogenesis. The cells divide rapidly continuously increasing their number. Further, the cells are also differentiated to give them modification so that they can take the form of specialised tissues that has a specific function. These tissues form the organs and develop to form a full-fledged organism.

Based on whether the zygote is developed outside or inside of the body of the parent, i.e., whether they lay unfertilised or fertilized eggs or produce young ones, animals are divided into two categories.

1. Oviparous – These animals lay eggs. In reptiles and birds, a hard calcareous shell surrounds the fertilised eggs providing them with a safe environment.

2. Viviparous – These animals give rise to young ones. Most of the mammals, even human beings, the give rise to young ones because the zygote is developed inside the body which attains further advanced features inside the body of the organism. Young ones are delivered only after attaining a certain stage in growth. This method provides proper protection and care to the new individual and also increases the chances of survival.

In flowering plants, the formation of zygote occurs inside the ovule. The sepals, stamens, and petals of the flower desiccate and fall off leaving the fruit behind. However, in some plants speaks remains attached like that of tomato and brinjal. The zygote grows into the embryo, and the ovule develops into the seed. The ovary takes the form of fruit with a thick wall known as pericarp which is responsible for protecting the seed. Seeds are dispersed into the environment when conditions are suitable for the formation of new plants.