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

(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.

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.