REPRODUCTION IN ORGANISMS
Biology
Reproduction is a biological process in which an organism produces offspring of their own kind.
The period of birth to the natural death of an organisms represents its life span. The life span of an organism may vary from species to species. The life span of an organism is not necessarily correlated with their body size, it can vary from less than one day to more than 400 years.
Reproduction
Reproduction is a biological process by which an organism gives rise to young ones of their own kind.
Significance of reproduction
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Reproduction enables the continuity of the species, generation after generation.
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Sexual reproduction is responsible for variation in a population and its inheritance to future generations.
Modes of reproduction;
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Asexual reproduction
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Sexual reproduction
Asexual reproduction
A type of reproduction by which offspring is produced from s single parent with or without the involvement of gamete formation. (uniparental).
Offsprings are morphologically and genetically identical to their parents (clones).
Asexual reproduction is common among unicellular organisms, and in plants and animals with relatively simple organizations.
Types of asexual reproduction
1. Fission: a mode of asexual reproduction in which a parental cell splits up into two or more daughter cells.
a. Binary fission:
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A parental cell divides into two equal daughter cells and each rapidly grows into an adult.
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It occurs in single-celled organisms like bacteria, amoeba, and paramecium.
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It can be simple or irregular, longitudinal. Oblique or transverse, depending on the plane of division.
b. Multiple fission:
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The splitting of a parental cell into numerous daughter cells, each of which grow into an adult. E.g., Plasmodium.
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Under unfavourable conditions, organisms like Amoeba cover themselves with a three-layered hard covering or cyst. This phenomenon is called encystation. On return of favourable conditions, it divides by multiple fission within the cyst and produces many Amoebae or pseudopodiospores. The cyst bursts and spores are liberated to develop into adults. This is called sporulation.
2. Fragmentation: It is a mode of asexual reproduction in which the parental body breaks into two or more fragments and each fragment grows into a new individual, e.g., Spirogyra.
3. Budding: It is a mode of asexual reproduction in which one or more outgrowths (buds) are produced which initially remain attached to the parent cell and eventually get separated from it to grow into a new individual, e.g., yeast, Hydra.
4. Regeneration: It is a mode of asexual reproduction in which the missing parts of an organism is repaired by proliferation of cells, e.g., Planaria.
5. Spore formation:
Zoospores: These are endogenously produced unicellular, naked and motile spores with one or two flagella. Zoospores are produced in a sac-like structure called zoosporangium, e.g., Chlamydomonas.
Conidia: Asexual non-motile spores cut off externally either singly (e.g., Phytophthora) or in chains (e.g., Penicillium) from the tip of a special hyphae called conidiophore.
Gemmules: Internal asexual reproductive units or buds are called gemmules, e.g., sponges. These develop within the parental body and are released during germination.
6. Vegetative propagation: It is a mode of reproduction in which new plants are formed from vegetative parts (vegetative propagules) of the plant like root, stem, etc.
It is very common in higher angiosperms.
Following are some units of vegetative propagation:
Bulbil – Agave
Bulbs – Onion, Garlic
Eyes – Potato
Leaf buds – Bryophyllum
Offset – Hyacinth and
Rhizome – Ginger
Runner – Oxalis
Sucker – Mint
Sexual Reproduction
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Sexual reproduction involves the fusion of male and female gametes to form a zygote.
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All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. This period of growth is called Juvenile phase. Juvenile phase is also known as vegetative phase in plants.
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In non-primates (cows, sheep and dogs), the female reproductive cycle is called oestrus cycle.
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In primates (monkey, apes and humans), the female reproductive cycle is called menstrual cycle.
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The phase of life cycle where an organism possesses all the capacity and potential to reproduce sexually is called reproductive phase.
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The post-reproductive phase of life cycle where an organism slowly loses the rate of metabolism, reproductive potential and show deterioration of the physiological activity of the body is called senescent phase.
Events in Sexual Reproduction
Events in sexual reproduction may be grouped into three distinct stages as follows:
a. Pre-fertilisation
b. Fertilisation (also known as syngamy)
c. Pre-fertilisation
1. Pre-fertilisation:
This includes formation of gametes () and their transfer ().
Gametogenesis
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It involves formation of two haploid reproductive units called gametes.
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The formation of haploid male gamete (sperm) from the diploid spermatogonia is called spermatogenesis.
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The formation of haploid female gamete (ovum) from diploid oogonia is called oogenesis.
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When male and female gametes are similar in appearance and it is not possible to differentiate between them, they are called homogametes or isogametes.
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When the male and female gametes are morphologically distinct, they are called heterogametes.
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Among heterogametes, the male gamete is called antherozoid or sperm and female gamete is called egg or ovum.
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A single organism bearing both male and female sex gametes is called homothallic or monoecious, e.g., coconut, Pinus.
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Organisms in which different individuals carry male and female gametes are called heterothallic or dioecious, e.g., papaya.
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Unisexual male flowers bearing stamens are called staminate flowers.
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Unisexual female flowers bearing pistils are called pistillate flowers.
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The animals bearing both the sexes are called hermaphrodites, e.g., earthworm, sponge, tapeworm and leech.
Cell division during gamete formation
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Gametes are always haploid.
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Haploid organisms produce gametes by mitotic division. Diploid organisms undergo meiosis of specialised cells called meiocytes (gamete mother cell) to form gametes.
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At the end of both divisions, only one set of chromosomes get incorporated into each gamete.
Gamete transfer
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After gamete formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation).
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Male gametes are usually motile, whereas female gametes are static.
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In lower plants like some algae and fungi, both male and female gametes are motile.
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In simple plants like algae, bryophytes and pteridophytes, water acts as the medium for gamete transfer.
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In angiosperms, the pollen grains are transferred from anther of one flower to the stigma of another flower. This is called pollination.
2. Fertilisation or syngamy:
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It is the fusion of male and female gametes to form a diploid cell called zygote.
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The phenomenon in which the female gamete undergoes development to form new organisms without fertilisation is called parthenogenesis, e.g., rotifers, honeybees, lizards and birds.
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Fertilisation is of two types, i.e., external fertilisation and internal fertilisation.
3. Post-fertilisation events
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Events in sexual reproduction after the formation of zygote are called post-fertilisation events.
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These include development of zygote and embryo after fertilisation.
a. Development of zygote
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In external fertilisation, zygote is formed in the external medium, whereas in internal fertilisation, zygote is formed inside the body of the organism.
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Further development of zygote depends upon the life cycle and environment.
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Zygote ensures continuity of species between organisms of one generation and the next.
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In algae and fungi, the zygote develops a thick wall to resist desiccation and damage.
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In haplontic life cycle, the zygote undergoes reductional division (meiosis) to form a haploid organism.
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In diplontic life cycle, the zygote undergoes mitotic division.
b. Embryogenesis
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The processes of development of embryo from the zygote is called embryogenesis.
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During embryogenesis, zygote undergoes
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cell division (mitosis) to increase cell number, and
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cell differentiation to form specialised tissues and organs.
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Based on whether the zygote develops outside the body of female parent or inside, animals can be classified into oviparous and viviparous, respectively.
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In flowering plants, the zygote is formed inside the ovule, where the zygote develops into an embryo.
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The fertilised ovule develops into seed and ovary develops into fruit.
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The seed after dispersal in favourable condition germinates to produce new plants.
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The outermost protective covering of fruit is called pericarp or fruit wall.