Types of Reproduction

Human Reproduction of Class 12

There are two types of reproduction :

  • Asexual
  • Sexual

ASEXUAL REPRODUCTION

Definition : In asexual reproduction a single parent splits, buds or fragments to give rise to two or more young ones that have hereditary traits identical with those of the parent. New individuals are produced by  any means other than fusion of gametes. Asexual reproduction is also known as agamogeny or agamogenesis. It involves only mitotic cell divisions and is also termed somatogenic reproduction.

TYPES OF ASEXUAL REPRODUCTION

  • Binary fission :The parent organism divides into two halves and each half forming an independent daughter organism. A group of genetically identical offsprings produced from single parent is called clone. Organisms that undergo binary fission are immortal.

Types of binary fission :

  • Irregular binary fission - Amoeba
  • Longitudinal binary fission - Euglena and Vorticella
  • Transverse binary fission - Paramoecium and Planaria
  • Oblique binary fission - Ceratium

Multiple fission :

The parent body divides into many daughter organisms.
e.g., Amoeba, Plasmodium, Monocystis
Budding :
It is of two types

  • Exogenous budding - takes place in Hydra
  • Endogenous budding (Gemmulation) - takes place by gemmule formation in sponges

Fragmentation :

e.g. Flatworms, Sponges, Coelenterates.
Plasmotomy :

Division of multinucleate parent into many multinucleate individuals without nuclear division. Later on nuclear division occurs to maintain chromosome number e.g. Opalina, Pelomyxa.
Differences between asexual and sexual reproduction

Types of Reproduction

Sexual Reproduction : Sexual reproduction involves union of male & female gametes with haploid nuclei.Gametes are specialised haploid cells produces in gonads. Gonads are primary sex organs which are ectodermal in origin. When the two opposite types of gametes are produced by same parent, it is regarded as bisexual, monoecious or hermaphrodite. Advantage of hermaphrodatism is that it doubles the reproductive capacity and permits self fertilization which is essential for some endoparasites such as tapeworm, that leads a solitary life . When the two opposite types of gametes are produced by two different parents (of opposite sex) then parents are regarded as unisexual or dioecious. Higher animals show sexual dimorphism i.e. male and female look different with different features / structure. On the basis of morphology  gametes are generally of two types isogametes and anisogametes. Anisogamets are dissimilar in size & shape. If the gamete is smaller in size,  motile and deficient in stored nutrients. It is called microgamete or sperm or spermatozoa. If female gamete is much larger in size, non-motile  and laden with stored nutrients. It is called macrogamete or ovum.

Isogametes : Male and female gametes are morphologically similar as in monocytis. Sexual reproduction contributes to evolution of species by introducing variations in a population much more rapidly than asexual  reproduction.
Polyandry : When one female mates with more than one male.
Polygyny : When one male mates with more than one female.

Events in Sexual reproduction

The events in sexual reproduction can be categorized as following.

  • Pre-fertilization events
  • Fertilization events
  • Post-fertilization events

Pre-fertilization events

Gametogenesis : Formation of gametes inside the gonads is called gametogenesis. Formation of sperm is called spermatogenesis & formation of ovum is called oogenesis.
Gamete transfer : In case of animals, male gametes are transferred to female gamete by three distinct patterns depending on whether fertilization and embryonic development occur within or outside the maternal body.
External fertilization and external development : This pattern is found in many aquatic animals such as Obelia Nereis, Labeo, frog. For this pattern to succed male and female must shed their gametes at same time and place and in large numbers. Parents may or may not make a physical contact for releasing gametes.
Internal fertilization and external development : Sperm are passed from male into female with an intromittent organ like penis in shark & lizards or by cloacal apposition in birds, with modifed arm in cuttle fish (Sepia), Octopus.Fertilized egg forms zygote which passes down the female reproductive tract to exterior and acquires secretions, membranes, or shell for protection of developing embryo.
Internal fertilization and internal development : Mother’s body provides exactly the right chemical conditions and in mammals, warmth and nourishment also. Developing embryo is not vulnerable to predators who attack externally developing eggs.

Fertilization and syngamy

Syngamy is fusion of two entire gametes (one male and other female) to form zygote. It involves close association of gametes and all their acts resulting in formation of a zygote including the fusion of nuclei.On the basis of source of fusing gametes, syngamy may be divided into two types i.e. exdogamy & exogamy.

Endogamy : It involves self fertilization, i.e. fusion of two gametes of same parent. It is uniparental e.g Taenia solium (Tape worm)
Exogamy : It involves cross fertilization ie fusion of two gametes formed by different parents. It is biparental, Eg. frog, rabbit, humans On the basis of structure of fusing gametes, syngamy may be divided into two types i.e. Isogamy & Anisogamy.
Isogamy : Fusing gametes are similar morphologically e.g. monocystis. isogametes of same parent do not fuse with each other.
Anisogamy or hetrogamy : Fusing gametes are different in form and structure.

Post fertilization events

All the events that occur during sexual reproduction after the fusion of gametes are called post fertilization events.

 Zygote : It is a single celled diploid structure formed as a result of fusion of two haploid cells (gametes) zygote is the first cell of diploid organisms including human. Whether the zygote divides by meiosis to form haploid spores (e.g. chlamydomonas) or divides by mitosis to form diploid body depends upon the type of life cycle occuring in the organism.

Embryogenes is The process of development of embryo from the zygote is called embryogenesis. Development of offsprings from reproductive units such as  buds or fragments in asexual reproduction is called blastogenesis. Both embryogenesis and blastogenesis have same target to develop a new individual by process of cell division and differentiation. Whether zygote development occurs inside the female body or outside the female body, the animals are grouped into following three categories.
Oviparous animals : These animals lay fertilized or unfertilized eggs. Many invertebrates, fishes and amphibians lay unfertilized eggs. Reptiles and birds lay fertilized eggs covered by hard calcareous shell (cleidoic eggs) in a safe place. After passing a variable incubation period, the young ones hatch out from the fertilized eggs. Prototherians or monotremes are egg laying mammals.
Viviparous animals : These animals give birth to young ones. Zygote develops into embryo inside uterus of mother and derives nutrition from maternal body through placenta. Eg. Eutherian mammals. (Humans, Monkeys, Elephants, Dogs etc.)
Ovoviviparous animals : Fertilized eggs hatch out in the reproductive tract of mother. Embryo is nourished by egg yolk and not from maternal body. The placenta is not formed. However mother’s body does provide gas exchange. Eg. sharks (Dog fish scoliodon) some amphibians. Parthenogenesis (Virgin Birth or Virginal Reproduction)
Definition : Development of an egg (ovum) into a complete individual without fertilization is known as parthenogenesis. It is a modified form of sexual reproduction.

Types : Parthenogenesis is of two main types|: natural and artificial.

 Natural Parthenogenesis.

It occurs regularly in the life cycle of certain animals. It may be complete, incomplete or paedogenetic.
(a) Complete (Obligatory) Parthenogenesis : It occurs in those animals which breed exclusively by parthenogenesis. There are no males and, therefore, individuals are represented by females only.

Examples :
(i) Lacerta saxicola (Caucasian Rock Lizard)
(ii) Typhlina brahmina perhaps the smallest snake of India (15.2 cm long).

(b) Incomplete (Cyclic) Parthenogenesis : It is found in those animals in which both
reproduction and parthenogenesis occur.

Examples :
(i) In honey bees, fertilized eggs (zygotes) give rise to queens and workers (both females) and unfertilized eggs (ova) develop into drones (males).
(ii) In spring, eggs (ova) of aphids develop into females which produce many generations of females by parthenogenesis through the summer months. At the end of summer some females produce males and females by parthenogenesis. Both of these males and females mate to produce fertilized eggs (zygotes) that hatch in the spring as parthenogenetic females to continue parthenogenesis. Thus cyclic parthenogenesis is found in aphids. It means several generations of parthenogenetic reproduction alternate with biparental reproduction in which the eggs are fertilized.
(iii) Some species of wasps produce alternately a parthenogenetic generation and another one which develops from fertilized eggs.
(iv) About 40% male turkeys are produced by parthenogenesis and all females are produced by sexual reproduction.

(c) Paedogenetic Parthenogenesis (Paedogenesis) :

When larva produces a new generation of larvae by parthenogenesis, it is called paedogenetic parthenogenesis or paedogenesis. It occurs in sporocysts (larvae) and rediae (larvae) of liver fluke. It is also found in the larvae of some insects. (e.g., gall fly) Based on the sex of offspring, there are following three types of natural parthenogenesis.

  • Arrhenotoky : In this type of parthenogenesis, only males are produced by parthenogenesis. It occurs in rotifers, bees (honey bees), wasps, ticks, mites and certain spiders.
  • Thelytoky : In this type of parthenogenesis; only females are produced by parthenogenesis. It occurs in Solenobia of Lepidoptera, Lacerta sexicola armaniaca, Typhlina brahmina, etc.
  • Amphitoky : In this type of parthenogenesis, parthenogenetic egg may develop into individual of any sex (i.e. male or female). It occurs in Aphis (aphid).

Artificial Parthenogenesis :

In this type of parthenogenesis, the egg (ovum) is induced to develop into a complete individual by artificial stimuli. Artificial parthenogenesis may be induced by physical as well as chemical stimuli.

  • Physical stimuli : These include changes in temperature and pH, electric shock, ultra violet light, and mechanical stimulus (e.g., prick by a needle).
  • (Chemical Stimuli : These include changes in the salt concentration of the surrounding water, application of chloroform, ether, alcohol, urea, fatty acids, etc.

Examples :
Eggs (ova) of annelids, molluscs, echinoderms (sea urchin, star fish), frogs, salamanders, birds (turkey, hen) and even mammals (rabbit) may be induced by physical or chemical stimuli to develop parthenogentically into complete individuals.

Significance of Parthenogenesis

Advantages :

  • It is a simpler and easier means of reproduction.
  • It represents a method of rapid multiplication.
  • Parthenogenesis permits establishment of triploid and aneuploid chromosomal combinations.
  • Parthenogenesis is a means of sex determination in some animals such as in honeybees. Thus it supports the chromosomal theory of sex determination.

Disadvantages :

Parthenogenesis eliminates variation in a popultaion so it does not play any role in organic evolution

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