REPRODUCTION IN ORGANISMS
- Reproduction is the biological process in which an individual gives rise to an offspring similar to itself.
Types of Reproduction:
Based on whether there is one or two organisms taking part in the process of reproduction
- ASEXUAL REPRODUCTON
- SEXUAL REPRODUCTION
Some of the major differences between asexual and sexual reproduction are as follows:
- It occurs in lower invertebrates and lower chordates and plants with simple organisations.
- It is always uni-parental.
- Gametes are not formed.
- No fertilization.
- It involves only mitosis.
- Daughter organisms are genetically identical to the parent.
- Multiplication occurs rapidly.
- Since there is no variation, so it does not contribute to evolution of the species.
MODES OF ASEXUAL REPRODUCTION
Parent cell divides into 2 daughter cells.
E.g.. Amoeba, Paramecium
Parent cell divides to form large cell and small cell, called bud, attached to the large cell; the bud gets separated and grows into an adult. E.g., Yeast
- In fungi and algae specialized asexual reproductive units are formed
- Zoospores in many algae and certain aquatic fungi, Conidia (Penicillium), Buds (Hydra), Gemmules (Sponge).
- In higher plants, vegetative propagation is followed using units such as rhizome, sucker, offset, bulb, tuber etc. – known as vegetative propagules.
Examples of Vegetative Propagation:
WATER HYACINTH (Terror of Bengal)
- One of the most invasive weeds
- Grows wherever there is standing water
- Drains oxygen from water- leads to death of fishes.
- Introduced in India because of its pretty flowers & shape of leaves
- Vegetative propagation occurs at a phenomenal rate
- It occurs almost in all types of animals and mostly in higher plants.
- It is usually bi-parental.
- Gametes are always formed.
- Fertilization takes place.
- It involves both meiosis and mitosis.
- Daughter organisms genetically differ from the parents.
- Multiplication is not as rapid as in asexual reproduction.
- Since there are variations, so it contributes to evolution of the species.
- Involves formation of male and female gamete by two individuals of the opposite sex.
- Offspring produced by fusion of male and female gametes not identical to each other or to the parents.
- All sexually reproducing organisms share a similar pattern of reproduction.
DIFFERENT PHASES IN SEXUAL REPRODUCTION
1. JUVENILE PHASE
It is a period of growth and maturity of an organism before it can reproduce sexually.
2. REPRODUCTIVE PHASE
Period in which organism is capable of reproducing sexually
- Easily seen in higher plants when they come to flower.
- Some plants flower seasonally & some through out the year
- Few plants exhibit unusual flowering phenomenon
For e.g.. 1. Bamboo species flower only once in their lifetime. 2. Srobilanthus kunthiana (neelakurinji) found in hilly areas of Kerala, Karnataka & Tamil Nadu flower once in 12 years the latest being in September 2006.
- In animals sexual reproduction is usually seasonal
- In placental mammals there is occurrence of cyclical changes in the activities of ovaries & accessory ducts as well as hormones.
- In non primate mammals like dogs, rats etc these cyclical changes during reproduction is called oestrus cycle – no bleeding occurs.
- In primate mammals like apes & humans, these cycles are called menstrual cycles.
3. SENESCENT PHASE
- It is the end of reproductive phase.
- Old age ultimately leads to death
EVENTS IN SEXUAL REPRODUCTION
- The gametes are usually haploid
- Gametes called homogametes / isogametes when both have same appearance.
- When gametes are different, they are called heterogametes; male gamete known as antherozoid /sperm, female is called ovum / egg.
- Bisexual organisms called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.
- In most of the organisms, male gametes are motile & female gametes are stationary.
- In algae, bryophytes & pteridophytes water is the medium for gamete transfer.
- Pollination is the method of gamete transfer in higher plants as pollen grains are carriers of male gametes.
2. SYNGAMY / FERTILIZATION:
- Syngamy results in formation of a diploid zygote.
- In certain animals, female gamete develops into the adult organism without fertilization; such a process is called parthenogenesis – seen in rotifers, lizards turkeys etc.
- Fertilisation may be external or internal
The fertilization process occurs within the body of a female is called internal fertilization. It is a specialization for the protection of egg, but it depends on the birth method . Reptiles and birds have thick shell, covering the egg to protect it from dehydration and destruction. However, fertilization occurs inside the body otherwise, sperm has to enter through a thick wall.
Animals such as mammals also have internal fertilization, where embryo develops inside mother, which enhances the protection to the embryo1.
Internal fertilization facilitates the survival of the embryo, selective fertilization, and longer protection and minimizes the wastage of gametes.
In external fertilization fusion of sperm and egg occurs externally of the female body. External fertilization needs water to facilitate their fertilization, so it occurs in wet environments. Female and male gametes are released to the water, and male gamete is mostly mobile. This type of fertilization can be seen in lower plants. The advantage of external fertilization is that it produces a large number of offspring due to the external hazards. So survival of the embryo is comparatively lower. Amphibians and fish are examples for these types of animals.
What is the difference between Internal fertilization and External fertilization?
• The internal fertilization process occurs within the body of female whereas, in external fertilization, fusion of sperm and egg occurs externally of the female body.
• After the internal fertilization, egg will come out of the body having a thick shell whereas, in external fertilization, eggs are produced with thin tertiary membrane or without membrane .
• External fertilization needs water, whereas internal fertilization does not need water to fertilize.
• Organisms involved in external fertilization have mobile male gametes with flagella, whereas organisms involved in internal fertilization has immobile male gametes.
• In internal fertilization, wastage of gametes is lower, whereas wastage of gametes is higher in external fertilization.
• Organisms that involved in internal fertilization produce lower number of gametes, whereas organisms involved in external fertilization produce a large number of gametes.
• Survival of organisms that involved in internal fertilization is higher than the survival of organisms involved in external fertilization.
3. POST FERTILIZATION:
- Events in sexual reproduction after the formation of zygote
- Zygote – vital link that ensures continuity of species between organisms of one generation and the next.
- Development of zygote depends on
- the type of life cycle of the organisms.
- the environment it is exposed to.
- Zygote develops into embryo.
– cell division to increase the number of cells
– cell differentiation for the formation of different kinds of tissues
- Based on whether the development of zygote occurs inside or outside the body of the female parent, animals are categorized into oviparous and viviparous.
What is the Difference between Oviparous and Viviparous?
The adjective “Ovi” means an egg while “Parous” stands for birth. Therefore, an offspring coming out of an egg are characteristics of Oviparous animals. There are many species that are called oviparous in our animal kingdom. Over the whole, to protect the embryo that is been developed inside the egg from the external trauma, has a hard covering on it. When the maternal gene enters the ovum or egg it start forming a hard shell around it. The reproduction process and forming of an egg occurs after the mating of an adult male and female animal. In other words it is a sexual reproduction. As the egg develops outside the body of the mother, it is a result of external fertilization. External fertilization is when the ovum is thrown out of the body by the mother and then the sperm travels to it, unites with an egg in open rather then is specialized inside environment. The external fertilization where sperm and ova are ejaculated out mostly works for aquatic species in of the sea environment. Some mimic internal fertilization. Therefore those who o for external fertilization lay multiple eggs while those with “mimic” internal fertilization, especially non aquatic species, lay one egg at a time. All invertebrates are oviparous in nature. They lay egg and let the embryonic development happen within the egg.
Viviparous means the baby born as live birth, no egg covering or shelter around it. In other words, it can be said that the offspring stays inside the mother womb till it is mature enough for its own survival. The developing embryonic phase is carried out inside the mother womb. The young one is kept inside the body from where directly it gets all the food, nourishment, shelter and protection. The waste produced by the young one inside a mother’s womb is removed naturally through excretion system of the mother. Internal fertilization takes place in viviparous animals. It means that the ovum is produced by the mother inside the setup and then sperm travels to it. The fusion occurs inside the body of the female and the embryo starts developing.
- An oviparous is egg laying animals while viviparous gives birth to live offspring.
- The ovum is thrown outside the body in the oviparous while the ovum stays inside the viviparous animals.
- Fertilized egg in oviparous is covered by shell while in viviparous there is no cover, just mother’s womb.
- Oviparous give birth to multiple eggs at a time while viviparous gives birth to one or two.
- Oviparous internal (mimic) plus external fertilization takes place while in viviparous only internal fertilization occurs.
- In oviparous eggs survival is less while in viviparous survival is great.
- Fish, birds, amphibian or reptiles are some examples of oviparous while whales, dolphins or mammals are example of viviparous.