REPRODUCTION IN ORGANISMS: AN OVERVIEW

Reproduction is defined as a biological process in which an organism gives rise to young ones, similar to itself. The offspring grow, mature and in turn produce new offspring. Thus there is a cycle of birth, growth and death.

Broadly classifying, the reproduction is of two types

1.     ASEXUAL REPRODUCTION: when the reproduction involves only one parent and the reproduction takes place without formation of gametes. Then the reproduction is known as asexual reproduction.

·        As a result the offspring are not only identical to one another but are also exact copies of their parent.

·        Asexual reproduction is common among different group of single celled organisms, and in plants and animals with relatively simple organisation.

·        In single celled organisms, cell division is itself a mode of reproduction.

DIFFERENT MODES OF SEXUAL REPRODUCTION

·        BINARY FISSION: it is a type of sexual reproduction in which the parent cell divides into two daughter cells. Ex. Amoeba, Paramecium

·        BUDDING: A small bud is formed in the parent cell, which primarily remains attached to the parent and then after attaining ability to remain independently, the bud detaches form the parent cell. Ex Yeast.

·        In fungi and algae, specialized asexual reproductive structures are formed.

Zoospore in algae

Conidia in Penicillium

Buds in Hydra

Etc

·        VEGETATIVE REPRODUCTION/PROPAGATION

In plants the asexual reproduction is rather called as vegetative propagation. In plants vegetative propagation such as runner, rhizome, sucker, tuber etc

2.     SEXUAL REPRODUCTION: it involves two parents and formation of male and female gametes either by same individual or by different individuals of opposite sex. The gametes fuse to form zygote which develops to form new organisms. Because of fusion of male and female gametes which results into the fusion of two different genetic combination, the offspring produced are not identical to their parents.

The sexual reproduction has different phases

·        Juvenile phase: it is a period of growth and maturity of an organism before it can reproduce sexually.

·        Reproductive phase: it is a period in which organism can reproduce sexually.

Easily seen in higher plants, when they come to flower

Some plants flower seasonally and some throughout the year

Few plants exhibit unusual flowering phenomenon.

For eg. Bamboo species flower only once in their life time, Neelakurinji (Tamil Nadu) flower only once in 12 years, (the latest being September, 2006)

In animals, sexual reproduction is usually seasonal.

In placental mammals there is occurrence of cyclical changes in the activities of ovaries and accessory organs.

·        SENESCENT PHASE: it is a phase in which reproductive phase ends,  and old age ultimately leads to death of the organism.

EVENTS IN SEXUAL REPRODUCTION:

1.       PRE-FERTILIZATION EVENTS:
a)      GAMETOGENESIS

• The gametes are usually haploid
• Gametes called homogametes / isogametes when both have same appearance. 
• When gametes are different, they are called heterogametes; male gamete known as antherozoid /sperm, female is called ovum / egg.
• Bisexual organisms called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.

b)GAMETE TRANSFER

• In most of the organisms, male gametes are motile & female gametes are stationary.
• In algae, bryophytes & pteridophytes water is the medium for gamete transfer.
• Pollination is the method of gamete transfer in higher plants as pollen grains are carriers of male gametes.

2.       SYNGAMY / FERTILIZATION:

• Syngamy results in formation of a diploid zygote.
• In certain animals, female gamete develops into the adult organism without fertilization; such a process is called parthenogenesis - seen in rotifers, lizards turkeys etc.
• Fertilisation may be external or internal

EXTERNAL FERTILIZATION Syngamy occurs outside the body of the organism Large numbers of gametes are released in the surrounding medium. Ex. Bony fishes and Amphibians.

INTERNAL FERTILIZATION Syngamy occurs inside the body of the organism Numbers of ova produced are less, but large numbers of male gametes are released and they travel towards the ovum. Ex. Birds and Mammals.

3.       POST FERTILIZATION:

• Events in sexual reproduction after the formation of zygote
• Zygote – vital link that ensures continuity of species between organisms of one generation and the next.
• Development of zygote depends on 

             - the type of life cycle of the organisms.
             - the environment it is exposed to.

• Zygote develops into embryo.

Embryogenesis involves:
             - cell division to increase the number of cells
             - cell differentiation for the formation of different kinds of tissues

• Based on whether the development of zygote occurs inside or outside the body of the female parent, animals are categorized into oviparous and viviparous.

OVIPAROUS These animals lay eggs. The fertilized eggs have a calcareous shell to protect them from harsh environment.

VIVIPAROUS These animals give birth to young ones Embryo protected inside the mother’s body.

• In flowering plants the zygote formed inside the ovule; zygote develops into embryo after which the ovule becomes the seed and ovary becomes the fruit.