Reproduction in Organisms

Life Span: Period from birth to natural death of an organism.

• It doesn’t depend on size. eg. sizes of crow and parrot are similar but life span show wide difference, also mango  tree has much shorter life span than Peepal tree.
• No individual is immortal except single celled.

Reproduction: Biological process in which organism gives rise to young ones similar to itself.

• It enables continuity of species, generation after generation.
• Its of 2 types - asexual and sexual reproduction.

Asexual Reproduction:

• Offsprings are produced by single parent with or without involvement of gamete formation.
• Offsprings are identical and exact copies of their parent.
• Clone: Morphologically and genetically similar individuals.
• Common among single celled, plants and animals with simple organisation.

Cell division: Parent cell divides into 2 and give rise to new individuals.

eg. Protists and Monerans

2. Binary fission: Cell divides into 2 halves and grow into an adult.

eg. Amoeba, Paramecium.

3. Budding: Unequal division, bud remains attached to parent cell initially and then gets separated and mature to new organism. eg. Yeast and Hydra.

4. Multiple fission:

• Encystation: Under unfavourable condition Amoeba withdraws its pseudopodia and secretes 3 layered hard covering (cyst) around itself.
• Sporulation: Under unfavourable condition, it divides by multiple fission and produce minute spores (Pseudopodiospores), cyst wall bursts out and spores are liberated.

5.         Special reproductive structures: in algae and members of kingdom fungi

(a)       Zoospores: eg. Chlamydomonas

(b)       Conidia: eg. Penicillium

(c)       Buds: eg. Hydra

(d)       Gemmules: eg. Sponges

6.         Vegetative Propagation: in plants.

• Vegetative Propagules: runner, sucker, rhizome, tuber, offset, bulb give rise to new offspring.
• eg. rhizome of ginger, offset of hyacinth, bulbil of Agave.

7.         Fragmentation: body breaks into fragment which give rise to new offspring. eg. Hydra.

8.         From nodes: nodes come in contact with damp soil and produce roots and new plants, for commercial purpose also. eg. rhizome in banana, ginger and buds (eyes) in potato.

9.         From leaf: adventitious buds arise from notches at leaf margin. eg. Bryophyllum.

• Simple organisms like algae and fungi shift to sexual reproduction before onset of adverse condition.
• Higher plants exhibit both sexual and asexual (vegetative) mode of reproduction while most of the animals exhibit sexual reproduction.

Terror of Bengal: Scourge of water bodies.

• Water hyacinth: Most invasive weed growing in standing water.
• It drains oxygen from water and lead to death of fishes.
• It spreads over the water body in short period of time so its difficult to get rid of them.
• It was introduced to India because of beautiful flowers and leaf shape.

Sexual Reproduction:

Involve fusion of male and female gamete.

• 2 parents involved.
• offspring aren’t indentical to parents.
• fusion of male and female gametes result in zygote.

1.         Juvenile phase: Period of growth, also called vegetative phase (in plants)

2.         Reproductive phase: at end of juvenile phase, followed by morphological and physiological changes.

3.         Senescence phase: end of reproductive phase, slowing of metabolism, lead to death.

• Annual and biennial plants show clear cut vegetative, reproductive and senescent phases but perennial species don’t due to inter-flowering period.
• Bamboo: flower once in 50-100 years, produce fruits and die. It is monocarpic.
• Strobilanthus kunthiana (neelakuranji): Flower once in 12 years, last in Sep-Oct 2018, found in hilly areas of Kerala, Karnataka and Tamil Nadu forming blue stretches.
• Birds living in nature lay eggs seasonally whereas in captivity lay eggs throughout the year (commercial exploitation).
• Oestrous cycle: in non-primates like cow, sheep, rat, deer, dog, tiger.
• Menstrual cycle: in primates like monkeys, apes, human.
• Seasonal breeders: Reproductively active during favourable season in reproductive phase.
• Continuous breeders: Reproductively active throughout reproductive phase.
• Interaction between hormones and environment regulate reproductive process and expression.

Events in Sexual Reproduction:

1.         Pre-fertilisation: Gametogenesis + gamete transfer.

(a)       Gametogenesis: formation of male and female gametes.

• Gametes are haploid cells.
• Homogametes: 2 gametes are similar in appearance. eg. Algae also called isogametes.
• Heterogametes: 2 gametes are morphologically distinct, in majority of organism, male gamete- antherozoidlsperm and female gamete- egg/ovum.

• Bisexual: Both male and female flowers in same plant, also called monoecious/homothallic. eg. cucurbits, coconut, sponges, earthworm, tapeworm, leech, chara, sweet potato.
• Unisexual: male and female flowers on different plants, also called dioecious/heterothallic. eg. papaya, datepalm, cockroach.
• Staminate: Unisexual male flower. i.e. bearing stamens.
• Pistillate: Unisexual female flower. i.e. bearing pistils.
• Haploid plant produces gamete by mitotic division.
• eg. Monera, Fungi, Algae, Bryophyte
• Diploid body produce haploid gamete by meiosis.
• eg. Pteridophyte, Gymnosperms, Angiosperms

They have specialized cells-meiocytes (gamete mother cell) and only one set of chromosomes get incorporated into each gamete.

They have specialized cells-meiocytes (gamete mother cell) and only one set of chromosomes get incorporated into each gamete.

• In bisexual plants, its easier as anther and stigma are located close to each other e.g. peas.
• In cross pollinating or unisexual plants, pollination facilitates transfer of pollen grains, which germinate on stigma and pollen tubes carry male gamete to ovule.

2. Fertilisation: Fusion of gametes (syngamy) results in diploid zygote.

• Parthenogenesis: Female gamete undergoes development to form new organisms without fertilisation.

eg. rotifers, honeybee, some lizards, turkey.

(a). External fertilisation: Syngamy occurs in external medium. i.e. outside the body of organism.

• Release large no. of gametes in surrounding medium to enhance chances of fertilisation.
• These offspring are vulnerable to predator.
• eg. most aquatic organism, majority of algae and fish and amphibians, bony fish, frogs.

(b). Internal fertilisation: Syngamy occurs inside body of organism.

• egg is formed inside female where they fuse with male gamete (motile)
• no. of sperms are large but eggs produced are less.
• In seed plants, non-motile male gamete are carried by pollen tubes.
• eg. terrestrial organisms-fungi, higher animals like reptiles, birds, mammals, majority of plants-bryophytes, gymnosperms and angiosperms.

3.Post-Fertilisation:

(a)       Zygote: Occurs in all sexually reproducing organisms, single-celled.

• Its formed in external medium in external fertilisation whereas inside the body in internal medium.
• Its a vital link that ensures continuity of species between organisms of one generation and next.
• In haplontic life cycle, zygote divides by meiosis to form haploid spores.
• In fungi and algae it develops a thick wall resistant to dessication and damage. It undergoes period of rest before germination.

(b)       Embryogenesis: Development of embryo from zygote.

• Zygote undergoes cell division (increase in no. of cells in embryo) and cell differentiation (cells undergo modification to form specialised tissues and organs).
• Oviparous: fertilised eggs are covered by hard calcareous shell and laid in safe place, after incubation young ones hatch out. eg. reptiles and birds.

• Viviparous: Zygote develops inside female body and after certain time young ones are delivered out female body. eg. majority of mammals, human being.

• Because of proper embryonic care and protection, chances of survival are more in viviparous.
• In flowering plants, zygote is formed inside ovule. After fertilisation, sepals, petals and stamen of wither and fall off. Pistil remain attached to plant.
• Zygote develops to embryo and ovules into seed .
• Ovary develops to fruit which develops thick wall pericarp (protective). After dispersal, seed germinate under favourable condition to produce new plants.