REPRODUCTION IN ORGANISMS
It is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.
· 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:
o Zoospores: aquatic fungi, Chlamydomonas.
o Conidia: Penicillium.
o Bud: Hydra
o Gemmules: sponges.
· Vegetative propagation units in plant: (Vegetative propagules)
o Runner, rhizome, sucker, tuber, offset, bulb.
· 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.
· 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.
· 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.
· 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.
· 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.
· 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:
o Zygote developed into embryo.
o Ovule developed into seed
o Integument of the ovule developed into seed coat.
o Ovary developed into fruit.
o Ovary wall developed into pericarp.
Stages of Embryogenesis: