A vast number of plant and animal species have existed on the earth for several thousand years. The process in living organisms that ensures this continuity is Reproduction. Reproduction is one of the most characteristic features of a living organism. Life will not exist on the earth if plants and animals do not reproduce to make offspring. By reproducing, a living organism can be sure that there is another individual of its kind to take its place when it dies. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The organism’s habitat, its internal physiology, and several other factors arc 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 a few days to as long as a number of years. Life span cannot necessarily be correlated with the size of an organism i.e., it will be a misconception to say that smaller organisms have a shorter life span and larger organisms have a 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 the average life span. It is defined as the age at which half the population still survives. Thus, maximum life span is the characteristic of species, and life expectancy is the characteristic of a population.
The maximum life span of wild animals is very difficult to estimate because signs of senility or extreme old age are seldom seen in them. Animals living under natural conditions rarely approach their maximum possible age because of very high death rates due to infant mortality, diseases, predators, bad weather, accidents, or competition for food and shelter. For this reason, the most reliable information about the duration of the life span comes from zoos, where accurate records are kept and animals live under conditions almost ideally suited to prolong life. The life span of some selected animals is shown in the table. The maximum life span of a domestic dog is about 20 years and that of a laboratory mouse is 4.5 years.
The maximum life span of humans has been estimated to be about 121 years. This rests on the fact that a man in Japan, Shirechiyo lzymi, reached the age of 120 years and 237 days in 1986. He died after developing pneumonia. The average life span and life expectancy of humans have grown dramatically. In general, the rate of mortality of humans has gone down and the life span has increased. It is 65 in India, in the United State, it is 78, whereas 81 in Japan.
Limited life span means that the death of every organism is inevitable i.e., all individuals are mortal except single-celled organisms.
Despite the limited life span and the mortality of organisms. a variety of plant and animal species are well maintained on earth through the process of reproduction. 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 is replaced through reproduction.
- It introduces variations essential for adaptability and struggle for existence.
Basic Features of Reproduction:
(i) Replication of DNA
(ii) Division of cells. It may or may not involve meiosis.
(iii) Growth due to synthesis of more protoplasm.
(iv) Formation of reproductive units.
(v) Elaboration and development of reproductive units to form new young individuals.
Semelparous organisms reproduce only once in their lifetime, such as annual plants (including all grain crops) and certain species of salmon, spiders, bamboos and century plants. Often, they die shortly after reproduction.
lteroparous organisms produce offspring in successive (e.g., annual or seasonal) cycles, such as perennial plants. Iteroparous animals survive over multiple seasons (or periodic condition changes).
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 are produced by a single parent, with or without the involvement of gamete formation. It is uniparental. On the other hand, sexual reproduction consists of the 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 the case of bisexual or hermaphrodite animals.
I. Asexual Reproduction
It is the mode of reproduction in which new individuals develop directly from specialized or unspecialized parts of a single parent without involving the 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), Sycon, Hydra, Tubularia, Planaria, Ascidia (metazoans). Asexual reproduction is also called agamogenesis or agamogeny. In this mode of reproduction, somatic cells undergo mitosis during the formation of a new individual. Therefore, it is also called somatogenic reproduction. Young ones resulting from asexual reproduction are exactly identical with the parent except in size and are called clones. Each individual of a clone is referred to as a ramet. Members of a clone can, however, differ later due to the development of different mutations.
The technical term for asexual reproduction in plants is apomixis, derived from “apo” meaning “without” and “mixis” meaning “mingling”. Apomixis thus refers to the fact that asexual reproduction lacks the mixing of genes that occurs in sexual reproduction.
In apomixis, a new individual is produced by a single parent without pollination or mixing of genetic material. A familiar example of apomixis is the production of new plants by the growth of horizontal stems (runners) in strawberries (genus Fragaria). Other familiar plants with asexual reproduction include blackberries (genus Rubus) and dandelions (genus Taraxacum), both of which produce asexually formed seeds.
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 the nucleus followed by cytokinesis. In metazoans the multicellular individual divides into two daughters by a sort of constriction or cleavage. The organisms which undergo binary fission seldom die of senescence or old age because as soon as they mature, they divide into two daughters. They are, therefore, nearly immortal. Depending on the plane of division, binary fission is of the following types:
(i) Simple Binary Fission (Irregular Binary Fission): Division can occur through any plane e.g., Amoeba
(ii) Longitudinal Binary Fission: The plane of fission passes along the longitudinal axis of the organism, e.g., Euglena, Vorticella.
(iii) Oblique Binary Fission: The plane of binary fission lies at an angle to the transverse axis e.g., Ceratium, Gonyaulax.
(iv) Transverse Binary Fission: The plane of binary fission runs along the transverse axis of the individual, e.g., Paramoecium, diatoms, bacteria, Planaria.
In Paramecium, transverse binary fission is preceded by amitotic division of meganucieus and mitotic division of micronucleus. In it, binary fission produces two dissimilar daughters, one proter (anterior) and the other opisthe (posterior).
Both develop deficient components and become similar.
(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 unfavorable living conditions. An Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed encystations. 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 sporulation. Acellular protists like sporozoans (e.g., Monocystis, Plasmodium, etc.) typically exhibit sporulation in their life cycles.
(2) Budding: 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 freshwater sponges (e.g., Spongilla} and some marine sponges (e.g., Sycon), the parent individual releases a specialised mass of cells enclosed in a common opaque envelope, called the gemmule. On germination, each gemmule gives rise to an offspring and the archeocytes present in it give rise to various cells of the body of the sponge as they are totipotent. Gemmules are thought to be internal buds formed by endogenous budding during unfavorable conditions.
(3) Fragmentation: The body of the parent breaks into distinct pieces, each of which can produce offspring (e.g., Hydra, some marine worms, sea-stars).
(4) Regeneration: Regeneration is a specialized form of asexual reproduction in which an organism can renew or restore lost part of the body (known as epimorphosis) like in the case of the tail of a lizard or can form a whole body from a small fragment (known as Morphallaxis) like in Hydra.
Advantages of Asexual Reproduction:
(i) It is uniparental.
(ii) It is a rapid mode of reproduction.
(iii) The young ones are exact replicas of their parent.
(IV) Asexual reproduction is simpler than sexual reproduction.
Disadvantages of Asexual Reproduction:
(i) As there is rapid multiplication, a large number of young ones are formed which causes overcrowding.
(ii) There is no mixing of genetic material, so no new combination or variation takes place.
(iii) There is no crossing over, hence new linkages are not formed.
(IV) It has no role in evolution.
(v) Adaptability to changes in the environment is low due to the absence of new variations.
II. Sexual Reproduction
As mentioned earlier, sexual reproduction is the production of offspring by the fusion of specialized 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 slow process.
Sexual reproduction has the biological advantage of promoting genetic variation 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 offspring that may be better able to survive than parents.
Although organisms differ in their external morphology and internal structure, all of them have nearly the same reproductive pattern.
In animals, sex organs are already present in young embryos.
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 that reproduce more than once, many cycles take place within a single lifetime. An animal reaches maturity and then goes through cycles of making and releasing gametes. There are species of animals that reproduce only once and then die. In such organisms, only one stage of the life cycle is the reproductive phase.
In animals, besides day length, there is also an internal system to control reproduction which involves the nervous system and hormones.
The hormones stimulate follicle development and ovulation (release of an egg). Besides this, the hormones also prepare the uterus for pregnancy. In primates (Human, Ape, and Old world monkey) female sexual cycle called the menstrual cycle, occurs all around the year with generally one ovulation in a month. In non-primates such as sheep, cows, rats, and dogs reproduce seasonally and are known as seasonal breeders. Cyclic changes in the female reproductive system of non-primates are known as oestrus (estrus) cycle.
In mammals, the reproductive phase comes to an end as the organism grows old. This is accompanied by a slowing down of metabolism and ultimately results in death.
Events In Sexual Reproduction:
The entire sexual process can be divided into three phases:
A. Pre-fertilization events
B. Syngamy or fertilization
C. Post-fertilization events
A. Pre-fertilization Events
Pre-fertilization events include two processes:
- Formation of gametes or gametogenesis
- Gamete transfer
(i) 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 numbers.
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.)
As already studied in cockroaches in the XI portion, male and female cockroaches are separate and exhibit sexual dimorphism. The male cockroach has two testes occupying a dorsolateral position just beneath the 4th to 6th abdominal terga. The formation of a three-layered spermatophore occurs which is transferred to the female genital chamber by the male phallomeres. The female cockroach has a pair of ovaries, one on either side and embedded in fat bodies from the 2nd to 6th abdominal segments. Each ovary consists of 8 ovarioles. Spermatheca stores the sperm 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 the development of testes and ovaries at different times.
The earthworm has 2 pairs of testes; one pair each in segments 10 and 11. Two pairs of seminal vesicles (segments 11 and 12) are present for the maturation of sperms. The female reproductive system consists of one pair of ovaries in segment 13. In earthworm, the male part matures first (protandry condition), therefore, cross-fertilization occurs.
An entirely different sexual behaviour is found in the American oyster, Crassostrea virginica. It is a hermaphrodite organism and after attaining maturity it first produces sperms. Next year it produces eggs. The process continues in a regular annual alternation of sexes.
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 females while unfertilized eggs develop into males.
Meiosis is an essential feature of the 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, a fusion of gametes would result in the doubling of the chromosomes for each successive sexually reproduced generation. This situation is prevented by the reduction in the diploid number of chromosomes (2n) to haploid number (n) during gametogenesis.
(ii) Gamete transfer:
After the formation of gametes, it is essential that male and female gametes are brought together in physical contact. But to reproduce, they have to get together in the right pairs, one male and one female of the same species and of the right age. The most primitive mechanisms for correct gametes to come together are chemicals. In a majority of organisms, the 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) 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 makeup, the process is known as cross-pollination. If the transfer takes place between flowers of the identical genetic constitution, the process is known as self-pollination. It is common to think of self-pollination occurring within a single flower on a plant such as a garden pea where petals enclose the stamens in such a way that the pollen has little chance of escaping. 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 fanned in different individuals. For fertilization to occur, sperm and egg must get together. Animals have evolved different strategies for this. For example, many animals secrete sex pheromones to attract their partners. Male and female insects like the silkworm moth (Bombyx mon) produce tiny amounts of very volatile pheromones that diffuse very long distances. Only male moths of this species are attracted towards female moths by distinctive alcohol (bombykol) produced by the female. Several marine invertebrates release their gametes into the water, thus there is no need for the parents to make direct contact. In others, mating is important to bring male and female gametes in close contact.
B. 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, 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.
Now, what would happen if syngamy does not occur?
There are some organisms that 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 parthenogenesis.
Types of parthenogenesis:
Natural parthenogenesis is classified as below:
(i) Arrhenotoky: In this type, only males are produced by parthenogenesis. It occurs in honey bees, wasps, and Turkey (bird).
(Ii) Thelytoky: In this type, only females are produced by parthenogenesis. It occurs in Lacerta saxicola armeniaca, Typhlina brahmina and Rotifers, Whiptail.
(iii) Amphitoky: In this type, the parthenogenetic egg may develop into the individual of any sex (i.e., male or female). It occurs in Aphis (aphid).
Where does the syngamy occur?
External and internal fertilization: Fertilization between male and female gamete is species-specific due to the presence of protein fertilizin on the surface of an ovum (female gamete) and antifertilizin present on the surface of sperm (male gamete). This is the reason that one species of gamete fertilizes with the same species of gametes. 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. Frogs do pseudocopulation as they do not have a copulatory organ. Even fishes, which cannot easily hold on to one another, may have courtship leading to the 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 the coordination 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. Snails, insects, and mammals have developed a special copulatory organ for the delivery of sperms.
C. Post-fertilization Events
As mentioned above, in all sexually reproducing organisms, a fusion of male and female gametes results in the formation of a zygote. In organisms with external fertilization, the zygote is formed outside the body, usually in water as in frogs, bony fish, etc. In organisms exhibiting internal fertilization, a zygote develops inside the body of the organism. The post-fertilization events include
The development of an embryo from the diploid zygote is known as embryogenesis. Embryonic development is a complex process that 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 synthesize 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 the 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, frogs, 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 a 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, the development of a fertilized egg into an embryo takes place within the uterus of the female parent, and the offspring are born as a juvenile. The embryo in viviparous organisms receives nourishment from the mother’s blood through the placenta.
Many species of sharks are ovoviviparous. In such organisms, the eggs are incubated within a modified portion of the oviduct called the uterus and the young ones are born alive after hatching. During their development, they depend on stored yolk for their nourishment.
1. Life span extends from birth to the natural death of an organism. It varies from organism to organism and is irrespective of size.
2. Due to limited lifespan, it is reproduction, which guarantees the continuity of a species.
3. Reproduction can be asexual or sexual.
4. Sexual reproduction Involves the formation and fusion of male and female gametes – gametogenes1s, fertilization, and embryogenesis.
5. If the female gamete develops into a complete individual without fertilization, it is called parthenogenesis.
6. Fertihzallon can be internal or external.
7. Zygote develops outside the body of female parent in case of oviparous animals.
8. Zygote develops inside the body of female parents in the case of viviparous animals.