Study notes on Reproduction in Organisms
A vast number of plant and animal species have existed on the earth for several thousand of years. The process in living organisms that ensures this continuity is Reproduction.
Reproduction is one of the most characteristic feature of living organisms.
Life will not exist on the earth if plants and animals do not reproduce to make offsprings.
By reproducing, a living organism can be sure that there is another individual of its kind to take its place when it dies.
In this way, a species of an organism guarantees its survival.
There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offsprings.
The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces.
- Life span can be defined as the period from birth to the natural death of an organism.
- It can vary from as short as few days to as long as a number of years.
- Life span cannot necessarily be correlated with size of an organism i.e., it will be a misconcept to say that smaller organisms have shorter life span and larger organisms have longer life span.
- Maximum Life Span : Maximum life span is the maximum number of years survived or the greatest age reached by any member of a species.
- The average life span refers to the average number of years survived or age reached by the members of a population.
- Life expectancy, the number of years an individual can expect to live for, is based on average life span.
- It is defined as the age at which half the population still survives.
- Reproduction ensures the continuity of different species.
- Reproduction is the means of self perpetuation of a race in which new, young, similar looking individuals are formed by the grown up or adult individuals.
- The adults which give rise to young ones are called parents.
Functions of Reproduction
- It replaces the individuals dying due to senescence or ageing.
- Individuals removed from population due to predation or disease are replaced through reproduction.
- It introduces variations essential for adaptability and struggle for existence.
Reproduction in Organisms
Basic Features of Reproduction :
(1)Replication of DNA.
(2)Division of cells. It may or may not involve meiosis.
(3)Growth due to synthesis of more protoplasm.
(4)Formation of reproductive units.
(5)Elaboration and development of reproductive units to form new young individuals.
Types of Reproduction
- There is no single mechanism of reproduction because of the large scale diversity in structure, physiology and habitat of organisms.
- Broadly speaking, there are two types of reproduction, asexual and sexual.
- In asexual reproduction offspring is produced by single parent, with or without involvement of gamete formation. It is uniparental.
- On the other hand, sexual reproduction consists of formation and fusion of gametes of opposite sexes. It is mostly biparental with two types of parents of different sexes but can be single/uniparental also, as in case of bisexual or hermaphrodite animals.
- It is the mode of reproduction in which new individuals develop directly from specialised or unspecialised parts of a single parent without involving fusion of gametes or sex cells.
- Asexual reproduction occurs in both single- celled and multicelled individuals.
- The parent individual splits, buds or fragments to form identical daughter cells or individuals, e.g., Amoeba, Paramoecium, Euglena (acellular protists),
Asexual reproduction occurs by fission, budding and fragmentation.
(1) Fission : It is a mode of asexual reproduction in which the body of a mature individual divides into two or more similar and equal sized daughter individuals. Fission can be binary fission or multiple fission.
(a) Binary Fission :
- It is the division of the body of an individual into two equal halves, each of which functions as an independent daughter individual.
- In unicellular organisms, binary fission is accompanied by mitotic division of nucleus followed by cytokinesis.
(b) Multiple Fission :
- The nucleus divides several times by amitosis to produce many nuclei, without involving any cytokinesis.
- Later, each nucleus gathers a small amount of cytoplasm around it and the mother individual splits into many tiny daughter cells (e.g., Amoeba, Plasmodium, Monocystis,etc.).
- In course of time, each of these daughter cells starts a free life and transforms into an adult individual. This kind of fission is called multiple fission.
Cyst formation :
- In response to unfavourable living conditions, an Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation.
- During favourable conditions, the encysted Amoeba divides by multiple fission and produces many minute amoebae or pseudopodiospores; the cyst wall bursts out and the spores are liberated in the surrounding medium to grow up into many Amoebae. This phenomenon is known as
- Acellular protists like sporozoans (e.g., Monocystis, Plasmodium, etc.)
- In budding, new individuals are formed by mitosis.
- Initially, a small outgrowth of the parent’s body develops into a miniature individual. It then separates from the mother to lead a free life (e.g., Hydra)
- This type of budding is known as exogenous budding. e.g. Yeast,Sometimes, the buds do not get separated from the mother individual and form a colony.
- For example, in Obelia, the colony consists of a number of individuals or zooids that perform different functions.
- In all fresh water sponges (e.g.Spongilla) and some marine sponges (e.g., Sycon), the parent individual
- Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
Exogenous budding in hydra
(3) Fragmentation : The body of the parent breaks into distinct pieces, each of which can produce an offspring (e.g., Hydra, some marine worms, sea-stars).
(4) Regeneration: Regeneration is a specialised form of asexual reproduction in which an organism can renew or restore lost part of body [known as epimorphosis] like in case of tail of lizard or can form whole body from small fragment like in Hydra.
- Vegetative Reproduction
- In plants, the term vegetative reproduction is frequently used.
- In plants, the units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offspring.These structures are called vegetative propagules.
- Obviously, since the formation of these structures does not involve two parents, the process involved is asexual
- The ‘terror of Bengal’ is nothing but the aquatic plant ‘water hyacinth’ which is one of the most invasive weeds found growing wherever there is standing water.
- It drains oxygen from the water, which leads to death of fishes
- This plant was introduced in India because of its beautiful flowers and shape of leaves. Since it can propagate vegetatively at a phenomenal rate and spread all over the water body in a short period of time, it is very difficult to get rid off them.
- There are invariably nodes arise in the modified stems of buds (called eyes) of the potato tuber, from the rhizomes of banana and ginger plants.
- When the nodes come in contact with damp soil or water, they produce roots and new plants.
- Similarly, adventitious buds arise from the notches present at margins of leaves of Bryophyllum.
- This ability is fully exploited by gardeners and farmers for commercial propagation of such plants. It is interesting to note that asexual reproduction is the common method of reproduction in organisms that have a relatively simple organisation, like algae and fungi and that they shift to sexual method of reproduction just before the onset of adverse conditions.
- Asexual (vegetative) as well as sexual modes of reproduction are exhibited by the higher plants. On the other hand, only sexual mode of reproduction is present in most of the animals.
- As mentioned earlier, sexual reproduction is the production of offsprings by the fusion of specialised male and female cells called gametes.
- Gametes are haploid and fuse to form the fertilized egg or zygote, which eventually develops into a new organism.
- In comparison to asexual reproduction, it is a siow process.
- Sexual reproduction has the biological advantage of promoting genetic variety among the members of a species because the offspring is the product of genes contributed by both the parents.
- By making possible the genetic recombination of inherited traits of two parents, sexual reproduction gives rise to offsprings that may be better able to survive than parents.
- Although organisms differ in their external morphology and internal structure, but all of them have nearly the same reproductive pattern.
- In the animals, sex organs are already present in young embryo.
- Most of the animals have a specific period during which they reproduce sexually. For example, birds do not lay eggs throughout the year.
- The events of reproduction take place in cycles. In animals which reproduce more than once, many cycles take place within a single life time.
- An animal reaches maturity, then goes through cycles of making and releasing gametes.
- There are species of animals which reproduce only once and then die. In such organisms, only one stage of the life cycle is reproductive phase.
- In animals, besides day length, there is also an internal system to control reproduction which involves nervous system and hormones. The hormones stimulate follicle development and ovulation (release of egg).
- Besides this, the hormones also prepare the uterus for pregnancy. In primates (Human, Ape and Old world monkey)female sexual cycle called menstrual cycle, occur all around the year with generally one ovulation in a month.
- In non-primates such as sheep, cow, rat and dog reproduce seasonally and are known as seasonal breeders. Cyclic changes in female reproductive system of non-primates is known as oestrus (estrus) cycle.
- In mammals, the reproductive phase comes to an end as the organism grows old. This is accompanied by slowing down of metabolism and ultimately results in death.
Events in Sexual Reproduction: The entire sexual process can be divided into three phases:
- Pre-fertilization events
- Syngamy or fertilization
- Post-fertilization events
A. Pre-fertilization Events
Pre-fertilization events include two processes:
(i)Formation of gametes or gametogenesis
- The process of formation of gametes is known as gametogenesis. Gametes are of two types- male and female and are always haploid.
- In animals, the male gametes, called sperms are produced in the testes whereas the female gametes or the eggs develop in the ovaries.
- In comparison to male gametes, female gametes are always produced in much smaller number.
Sexuality in organisms
- Animals are either unisexual (ants, wasps, bees, mosquito, cockroach,frog, birds, rabbit, humans) or bisexual (or hermaphrodite, e.g., earthworm, tapeworm, leech etc.)
- Male and female cockroach are separate and exhibit sexual dimorphism.
- Male cockroach has two testes occupying dorsolateral position just beneath the 4th to 6th abdominal terga.
- Formation of three layered spermatophore occurs which is transferred to the female genital chamber by the male
- Female cockroach has a pair of ovaries, one on either side and embedded in fat bodies from 2nd to 6th abdominal segments.
- Each ovary consists of 8 ovarioles.
- Spermatheca stores the sperms received from the male during copulation.
- It is interesting that most hermaphrodites do not reproduce by self-fertilization. For example, in earthworms, two animals copulate and each inseminates the other. In some hermaphrodites, self-fertilization is prevented by development of testes and ovaries at different times.
- Earthworm has 2 pairs of testes; one pair each in segment 10 and 11.
- Two pairs of seminal vesicles(segment 11 and 12) are present for maturation of sperms
- Female reproductive system consist of oe pair of ovaries in segment 13
- In earthworm, male part matures first (protandry condition), therefore cross fertilization occurs
Cell division during gametogenesis
- Most of the animals have a diploid body, hence they form gametes by meiotic division.
- A few animal species (e.g., ants, bees, wasps) show an unusual type of sex differentiation.
- The males are haploid and make haploid sperms by mitosis.
- The females are diploid and make haploid eggs by meiosis.
- If an egg is fertilized, it develops into female while unfertilized eggs develop into males.
- Meiosis is an essential feature of sexual cycle. It results in the formation of daughter cells, each with half the number of chromosomes of the parent cell.
- During fertilization, the nuclei of two gamete cells fuse and the zygote thus formed has a fixed number of chromosomes for each species.
- In all organisms, this number of chromosomes represents the diploid condition (2n).
- If meiosis does not occur, fusion of gametes would result in doubling of the chromosomes for each successive sexually reproduced generation. This situation is prevented by reduction in the diploid number of chromosomes (2n) to haploid number (n) during gametogenesis.
- After the formation of gametes, it is essential that male and female gametes are brought together in physical contact.
- The world is full of a variety of organisms and generally their males and females go their own way.
- But to reproduce, they have to get together in right pairs,one male and one female of the same species and of the right age.
- The most primitíve mechanisms for correct gametes to come together are chemical.
- In a majority of organisms, male gamete is motile and the female gamete is stationary.
- Exceptions are a few fungi and algae in which both types of gametes are motile and they differ only in certain surface proteins. They are usually released in water and move towards each other chemotactically. They however, cannot recognize each other until they touch. In some other groups of plants (bryophytes and pteridophytes), the male gamete is motile and the female gamete is stationary. Although only one male gamete is required to fertilize the egg (female gamete) but they are produced in very large numbers to ensure fertilization.
- In seed plants, male gametes develop in the pollen and female gamete or egg lies in the embryo sac within the ovule (megasporangium). To bring male and female gametes together, it is necessary that pollens from anther are transferred to the stigma. This process is known as pollination.
- If the transfer occurs between two plants of different genetic make up, the process is known as cross-pollination.
- If the transfer takes place between flowers of identical genetic constitution, the process is known as self-pollination. However, self-pollination also occurs if pollen is transferred between different flowers on the same plant. It is helped by insects moving from flower to flower collecting nectar,
- Pollens are also transferred between flowers on two different plants which are genetically identical, This is called cross-pollination.
- In dioecious animals, male and female gametes are formed in different individuals. For fertilization to occur, sperm and egg must get together.
- In others, mating is important to bring male and female gametes in close contact.
- Syngamy and Fertilization
- The most important step in the process of sexual reproduction is the fusion of male and female gametes.
- Although the terms syngamy and fertilization are used synonymously, but the actual act of fusion of gametes is syngamy whereas fertilization includes all the events that ultimately lead to syngamy.
- The result of syngamy and fertilization is the formation of a diploid zygote which is the vital link that ensures continuity of species between organisms of one generation to the next generation.
- There are some organisms which show a special type of reproduction in which the female gamete develops into a new individual without being fertilized by a male gamete. This process is known as parthenogenesis and occurs naturally in some animals.
- The embryos developing from unfertilized haploid eggs are naturally haploid and this is known as haploid or generative
- External and internal fertilization : Fertilization may occur inside or outside the female body. Animals with external fertilization may or may not undergo mating but it is essential when the fertilization is internal because the male gametes need to be placed close to the egg inside the body.
- Many marine invertebrates release their gametes in water. The sperms swim to reach the eggs.
- Frogs have external fertilization, yet a form of mating takes place. A male clings to a female’s back for hours, until she releases eggs.
- Frog do pseudocopulation as they do not have copulatory organ.
- Even fishes, which cannot easily hold on to one another, may have courtship leading to pairing of male and female, before eggs are laid and fertilized. Courtship is an important prelude to mating.It gives females an opportunity to assess the quality of a male, Also it allows time for co-ordination of the male and female reproductive organs.
- Internal fertilization requires direct contact between the two sexes, In most birds, the openings of the reproductive systems are simply brought together through which sperms are transferred. This is a brief encounter, typically a very few seconds.
- Post-fertilization Events
- As mentioned above, in all sexually reproducing organisms, fusion of male and female gametes results in the formation of a zygote.
- In organisms with external fertilization, zygote is formed outside the body, usually in water as in frogs, bony fish etc.
- In organisms exhibiting internal fertilization, zygote develops inside the body of the organism. The post fertilization events include :
- Development of embryo from the diploid zygote is known as embryogenesis. Embryonic development is a complex process which involves cell division and cell differentiation.
- These events proceed according to the genetic information contained in the zygote and ultimately lead to the formation of mature animals. How amazing it is that from one fertilized zygote, cells as different as liver, muscle,nerve and skin are produced.
- These cells differ from one another in that they synthesise different enzymes and structural proteins.
- The developing embryo grows in size at the expense of food derived from outside.
- The mammalian embryo is nourished by the placenta.
- Depending upon the development of zygote inside or outside the body of the female parent, organisms have been classified into oviparous or viviparous.
- Oviparous organisms lay eggs (e.g., some species of sharks, skates, bony fishes, frog, lizards, birds), the yolk in the egg supplies food to the embryo.
- In some oviparous organisms (e.g., reptiles and birds), the fertilized eggs are covered by hard calcareous shell or in some cases with a leathery coat. Such eggs are laid in a safe environment where they are incubated for a certain period and then young ones hatch out.
- In viviparous organisms, on the other hand, development of fertilized egg into embryo takes place within the uterus of the female parent and the offspring is born as a juveniles.
- The embryo in viviparous organisms receives nourishment from the mother’s blood through placenta.
- Many species of sharks are ovoviviparous. In such organisms, the eggs are incubated within a modified portion of the oviduct called uterus and the young ones are born alive after hatching
- During the development, they depend on stored yolk for their nourishment.