Reproduction:
defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.
Asexual reproduction:
- Offsprings produced by single parents.
- Without involvement of gamete formation
- Offsprings are genetically identical to their parents.
Methods of asexual reproduction:
- Cell division as a method of asexual reproduction as in Protista and monera.
- Binary fission e.g. Amoeba, Paramecium.
- Budding: e.g. yeast.
- Asexual reproductive structures:
- Zoospores: aquatic fungi, Chlamydomonas.
- Conidia: Penicillium.
- Bud: Hydra
- Gemmules: sponges.
- Vegetative propagation units in plant: (Vegetative propagules)
- Runner, rhizome, sucker, tuber, offset, bulb.
Sexual reproduction:
- Involvement of single or two individual.
- Production of male and female gametes ( haploid)
- Gametes fused to form a diploid zygote.
- Zygotes developed into new organism.
- The offsprings are not genetically identical with their parents.
Features of sexual reproduction:
- Period between birth and sexual maturity is called juvenile phase. It is known as vegetative phase in plant.
- Bamboo species flower only once in their life time generally after 50-100 yr.
- Strobilanthus kunthiana (neelakranji) flowers once in 12 years.
- Oestrus cycle: cyclical changes during reproduction in non-primate mammal like cows, sheep, rats, deers, dogs, tiger etc.
- Menstrual cycle: cyclical changes during reproduction in primate mammals like monkeys, ape, and humans.
- Seasonal breeders: reproductive cycle takes place in favourable seasons as in wild animals.
- Continuous breeders: reproductively active throughout their reproductive phase.
Pre-fertilization events:
- Process of gamete formation is gametogenesis.
- Two gametes are similar in appearance are called homogametes (isogametes).
- Gametes produced are of two morphologically distinct types called heterogametes.
- Male gamete is called antherozoids or sperm and the female gamete is called ovum or egg.
Sexuality in organism:
- Plant having both male and female sex organ called homothallic or monoecious.
- Plants having only one sex organ is called heterothallic or dioecious.
- In flowering plants, the unisexual male flower is staminate, i.e. bearing staments, while the female is pistillate or bearing pistils.
- Animal having one type of reproductive system, called unisexual.
- Animal having both male and female reproductive system, called hermaphrodite or bisexual.
Cell division during gamete formation:
- Gametes in all heterogametic species two types namely male and female.
- Gametes are always haploid irrespective of parent’s ploidy.
- A haploid parent produces gametes by mitotic division.
- Diploid parent produces gametes by meiotic division.
- In diploid organisms specialized cells called meiocytes (gamete mother cell) undergo meiosis to produce haploid gametes.
Gamete transfer:
- Male and female gamete must be physically brought together to facilitate fusion called fertilization.
- In most cases male gametes are motile, female gametes are non-motile.
- In case of few fungi and algae, both male and female gametes are motile.
- In most cases water is the medium for gamete transfer.
- Male gametes are produced in several thousand times the number of female gametes produced to compensate the loss during transfer.
Fertilization:
- Fusion of male and female gamete is called fertilization or syngamy.
- The female gamete undergoes development to form new organism without fertilization. This phenomenon is called parthenogenesis.
- Gametic fusion takes place outside the body i.e. water is called external fertilization.
- Their must be synchrony of gamete release, large number of gametes released to enhance the chance of fertilization.
- Enable the individual to produce large number of offsprings.
- A major disadvantage is that the offsprings are extremely vulnerable to predators.
- Fertilization takes place inside the body is called internal fertilization.
Zygote:
- Formation of zygote after fertilization is universal in all sexually reproducing organisms.
- Zygote is formed usually in water in case of external fertilization.
- Zygote is formed inside the body of the organism in internal fertilization.
- Zygote of fungi and algae develops a thick wall that is resistant to dessication and damage.
- Organism with haplontic life cycle, zygote undergoes meiosis to produce haploid spores.
Embryogenesis:
- Development of zygote into an embryo is called embryogenesis.
- Zygote undergoes cell division (mitosis) and cell differentiation.
- Oviparous animal which lays eggs and development takes place inside egg.
- Viviparous animal gives birth to the young. The development takes place inside the body of the female.
- In plants:
- Zygote developed into embryo.
- Ovule developed into seed
- Integument of the ovule developed into seed coat.
- Ovary developed into fruit.
- Ovary wall developed into pericarp.
