Reproduction in Organisms

Reproduction: Biological process in which an organism give rise to young one (offspring) similar to itself.

The offspring, grow, mature and in turn produce new offspring.

Importance of reproduction: Ensures –

·         Survival

·         Continuity of race

·         Group immortality

Since by reproduction, the old ones are replaced by the young ones, which can feed, grow and reproduce again.

·         Increase in population

·         Vehicle of organic evolution since it transmits advantageous variations to offspring.

Lifespan: Period from birth to natural death of an organism

Every organism as a specific average life span, ranging from a day to 4000 years  (Table 1). It can be divided in to 4 stages:

Ø  Juvenility

Ø  Maturity

Ø  Aging and senescence

Ø  Death

S No.	Animal	Life Span
1	Butterfly	1-2 weeks
2	Housefly	1-4 months
3	Jelly fish	1 year
4	Rat	2-3 years
5	Squirrel	6-8 years
6	Dog	16-18 years
7	Leech	20 years
8	Horse	30-40 years
9	Ostrich	50 years
10	Man	60-80 years
11	Crocodile	60 years
12	Elephant	70-90 years
13	Tortoise	100-150 years
14	Parrot	150 years

With respect to reproduction, organisms show wide diversity. Each organism has evolved its own method of reproduction dependent on its:

Ø  Habitat

Ø  Physiology

Ø  And other factors

Types of Reproduction

Broadly of 2 types:

        I.            Asexual: offspring produced by single parent, with or without gamete formation

      II.            Sexual: two parents of opposite sex participate in reproduction. Involves fusion of male and female gamete.

        I.            ASEXUAL REPRODUCTION

A single individual is capable of producing offspring. The offspring being produced from single parent are identical and exact copies of their parents. Such genetically identical and morphologically similar individual are known as Clones.

[Apomixis: The formation of new individual through asexual reproduction without formation and fusion of gametes]

Occurrence of asexual reproduction:

§  Single celled organisms such as Protists and Monerans, parent cell divides in to two, to give rise to new individuals. Thus here, cell division is a method of reproduction

§  Plants and animals with relatively simple organization such as Algae, fungi etc by zoospores, buds (in Hydra), conidia, gemmules (in Sponges) etc.

Development of an organism from non-sexual reproductive units like buds, a fragment etc is called blastogenesis, while the asexual reproductive body is called blastos.

Characteristics of asexual reproduction:

1)      Uniparental reproduction

2)      No fusion of gametes

3)      Somatogenic reproduction

4)      Only mitotic division, no meiosis

5)      No variations produced, no role in evolution

6)      Rapid method

7)      Found in lower organisms. Absent from higher invertebrates and all vertebrates

Types of Asexual Reproduction

1.      Fission: Type of asexual reproduction in which a full grown parent organism divides in to two or more than two daughter cells. Here the reproductive unit is whole parental body.

Types of Fission: On the basis of number of daughter cells produced, fission is of two types:

·         Binary Fission: 2 equal daughter cells produced. Found in Protists (Amoeba, Euglena), bacteria and green alga; and flat worms.

·         Multiple Fission: more than 2 daughter cells produced. Found in Amoeba, Plasmodium, and some alga.

2.      Budding: type of asexual reproduction in which one or more unicellular or multicellular outgrowths, called buds are formed on the parent body. Each of these growths enlarges and develops in to a new organism. Occurs in Sponges, Coelenterates (Hydra), and among fungi in Yeast.

In Yeast, an unequal cell division occurs to produce small buds that remain attached initially to parent cell. These separate later and mature in to new yeast organism.

3.      Fragmentation: Parental body breaks in to two or more fragments accidentally and each body fragment develops in to an organism. Found in some flat worms, sea anemones and echinoderms.

4.      Zoospores: Special asexual reproductive structures, that are microscopic, flagellated and motile. Members of kingdom Fungi and simple plants such as Algae.

5.      Conidia: Non motile spores produced exogenously by constrictions at the tips of special hyphal branches known as condiophores. E.g. Penicillium

6.      Gemmules: Produced on Sponges

Vegetative reproduction: Asexual reproduction in plants is termed as vegetative reproduction. The vegetative structures in plants such as runner, rhizome, sucker, tuber, offset, bulb etc that are capable of giving rise to new offspring are known as vegetative propagules.

Plants may be propagated by vegetative propagules naturally as well as artificially.

Natural Methods:

A.      Vegetative Propagation by Roots: Roots of some plants develop adventitious buds o them e.g. Dalbergia sisoo (Sheesham), guava etc. These buds sprout under suitable conditions and form individual plants.

B.      Vegetative Propagation by Stems:

a.      Rhizome: Rhizomes are underground swollen stems that form vegetative propagules. Old and decaying branches cause isolation of newly formed branch that develop in to a new plant. E.g. ginger, Saccharum, turmeric etc.

b.      Stem Tuber: Found in potato and artichoke. Tubers are swollen apical parts of an underground stem and bear a number of nodes or ‘eyes’. New plants are produced from the buds growing on the eyes.

c.       Runner: aerial weak stems when come in contact with soil, give off adventitious roots at the nodes. When older part of plant dies, the branches separate from the parent plant and form independent plants.

C.      Vegetative Propagation by Leaves: Leaves of some plants bear adventitious buds e.g. Bryophyllum, Begonia etc. These buds are borne in the notches of the margins of succulent leaves. When these nodes come in contact with damp soil or water, they form roots and new plants.

This ability is exploited by farmers and gardeners to produce new plants.

Artificial Methods of Vegetative Reproduction

a.      Cutting

b.      Grafting

c.       Budding

d.      Layering

e.      Bulbil

Artificial methods of vegetative reproduction are used by farmers and gardeners for:

·         Quick production of new plants

·         Combining good qualities of two different varieties

Water Hyacinth (Eicchornia): Terror of Bengal

Aquatic plant which is the most invasive weeds found growing wherever there is standing water. It drains oxygen from water which leads to death of fishes.

The plant was introduced in India because of its beautiful flowers and shapes of leaves. Since it can propagate vegetatively at a very high rate and spread all over the water body in a short period of time, it is very difficult to get rid of them.

Significance of vegetative reproduction:

·         Characters of parent plants preserved

·         Quicker

·         Easier and cheaper

·         It is the only method of perpetuation and reproduction in plants that does not produces viable seeds.

·         Easier to get rid of pathogen from any part of plant by vegetative reproduction

·         Method of multiplying plants which have poor seed viability

·         Some methods like grafting allow physical as well as physiological union of two different individuals to the best of economic advantage.

In organisms with simple organisation such as algae, fungi etc; asexual methods of reproduction are the preferred methods. Sexual methods are restricted to adverse conditions. Higher plants undergo both asexual and sexual reproduction while animals show only sexual reproduction.

Sexual Reproduction

Involves the formation of male and female gametes; either by the same individual or by different individuals of the opposite sex. The gametes so formed fuse to form the zygote which develops in to a new organism.

Because of fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parent and among themselves.

Despite the wide diversity of living organisms, it has been observed that all living organisms share a similar pattern in terms of sexual reproduction. The major characters of sexual reproduction are:

·         Involves 2 parents; biparental

·         Involves formation and fusion of gametes

·         Involves meiosis during gametogenesis and mitosis during development of zygote

·         Also known as germinal reproduction as organisms are produced from germ cells

·         Offspring genetically different from parent as variations appear due to new combinations of genes during crossing over. Plays important role in evolution

·         Slow

·         Units of reproduction of haploid gamete

·         Found in higher plant and organism

·         Also found in some protists e.g. Plasmodium, paramecium etc.

DIFFERENT PHASES IN SEXUAL REPRODUCTION

Sexual reproduction is an elaborate and slow process compared to asexual reproduction. Since the organism must reach a certain stage of growth and maturity in their life cycle, before they can produce sexually. That period of growth is known as juvenile phase and is also known as vegetative phase in plants. The end of juvenile or vegetative phase is known as reproductive phase. It can be identified in plants by the appearance of flowers.

1. JUVENILE PHASE
   It is a period of growth and maturity of an organism before it can reproduce sexually. It is of varying duration in different plants. For e.g. it is very small for annual plants, longer for biennial and longest for perennial plants. Perennial plants have many years to complete their life cycle; while annuals have a limited period of 1 year to do so.

Also, the phases are clearly defined in annuals and biennials; but in perennial plants it is difficult to define these phases. In plants where flowering occurs more than once, i.e. seasonal flowering plants; the interflowering period is known as mature phase.

 2. REPRODUCTIVE PHASE
   Period in which organism is capable of reproducing sexually

• Easily seen in higher plants when they come to flower.
• Some plants flower seasonally & some throughout the year
• Few plants exhibit unusual flowering phenomenon

 For e.g.;

·         Bamboo species flower only once in their lifetime, produce large number of fruits and die.

·         Srobilanthus kunthiana (neelakurinji) found in hilly areas of Kerala, Karnataka & Tamil Nadu flower once in 12 years. The latest being in September 2006.

• In animals juvenile phase is followed by morphological and physiological changes prior to active reproductive behavior.
• Reproductive phase is also of variable duration
• Sexual reproduction is usually seasonal for e.g. birds in nature lay eggs only seasonally. However, birds living in captivity (as in poultry farms) can be made to lay eggs throughout the year. This is not related to reproduction but for commercial purposes.
• In placental mammals there is occurrence of cyclical changes in the activities of ovaries & accessory ducts as well as hormones.
• In non primate mammals like dogs, rats etc these cyclical changes during reproduction is called oestrus cycle – no bleeding occurs.
• In primate mammals like apes & humans, these cycles are called menstrual cycles.
• On the basis of reproductive activity these mammals are classified in to 2 groups:

ü  Seasonal Breeders: mammals that breed only during favorable seasons during their reproductive cycle

ü  Continuous breeders: mammals that are reproductively active throughout their reproductive cycles

3. SENESCENT PHASE

• Marked by the end of reproductive phase
• Many other changes also occur in the body like slowing of metabolism.
• This leads to last phase of life span, death

In both plants and animals, transition from one phase to next occurs due to hormonal changes. Interactions between hormones and certain environmental factors regulate the reproductive process and the associated behaioural expressions of the organisms.

EVENTS IN SEXUAL REPRODUCTION:

After reaching maturity, all sexually reproducing organisms show similar events and processes that are involved in sexual reproduction. Though the structures involved in these processes are different. The events even though complex and elaborate, follow the same sequence.

1.       PRE-FERTILIZATION EVENTS: Events occurring before gamete fusion. Involve 2 events: gametogenesis & gamete transfer
a)      GAMETOGENESIS

• Process of formation of gametes: male & female
• The gametes are usually haploid
• Gametes called homogametes / isogametes when both have same appearance e.g. algae
• When gametes are morphologically different, they are called heterogametes; male gamete known as antherozoid /sperm, female is called ovum / egg.

Sexuality in organisms

The male and female gametes formed in sexually reproducing organisms may be produced in same or 2 different individuals.

·         Plants may have both male and female reproductive structures in same plants (Bisexual), or on different plants (unisexual).

·         Bisexual organisms are called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.

·         In flowering plants the unisexual male flower is known as staminate, i.e. bearing stamens.

·         The female flower bearing pistils is known as pistillate.

·         If both male and female flowers on same individual: monoecious

·         If male and female flowers on different individuals: dioecious

o   Among animals, individuals having both male and female reproductive organs (bisexual) are earthworms, sponges, tapeworm leeches. These are known as hermaphrodites.

o   Mostly animals are unisexual

Cell Division during gamete formation: Gametes are usually haploid, produced on parent body that may be haploid or diploid.

ü  On a haploid parent body as in monera, alga, fungi and bryophytes; the gametes are produced by mitosis. Such organisms undergo gametic fusion leading to formation of diploid zygote. The zygote undergoes meiosis or reduction division and then form haploid plant body.

ü  Organisms having diploid parent body such as Pteridophytes, gymnosperms, angiosperms and most animals including humans; the formation of haploid gamete involves meiosis. In these organisms specialized cell called meiocytes (gamete mother cells), undergo meiosis. At the end of meiosis, one set of chromosome is present in each of the gametes: n.

Gamete Transfer:

Bringing together male and female gametes to facilitate fusion/fertilisation

·         In most of the organisms, male gametes are motile & female gametes are stationary.

·         In algae, bryophytes & pteridophytes water is the medium for gamete transfer.

·         A large number of male gametes fail to reach the female gametes. In order to compensate for this loss and to ensure fertilization; the number of male gametes produced is several thousand times the number of female gametes.

·         In higher plants, pollination is the method of gamete transfer as pollen grains are carriers of male gametes; and ovule has the egg.

·         Pollen grains are produced in anthers and need to be transferred to stigma for fertilization.

·         In bisexual plants, male and female reproductive parts on same individual, hence gamete transfer is easier.

·         In unisexual or dioecious plants; where male and female reproductive structures are located on different individuals; a specific event called pollination is required. Pollination involves the transfer of pollen grains to stigma.

2.       SYNGAMY / FERTILIZATION:

• Most important event of reproduction
• Syngamy/fertilisation 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 High level of synchrony between male and female sexes Large numbers of gametes are released in the surrounding medium. Both male and female gametes are motile Number of offspring produced is large Ex. Bony fishes and Amphibians.

INTERNAL FERTILIZATION Syngamy occurs inside the body of the organism High level of synchrony not required Numbers of ova produced are less, but large numbers of male gametes are released and they travel towards the ovum. Female gametes are non motile, male may or may not be motile. Number of offspring produced is small Ex. Birds and Mammals.

A major disadvantage of external fertilization is that the offspring are highly vulnerable to predators.

3.       POST FERTILIZATION EVENTS:

• Formation of diploid zygote is a universal events in all sexually reproducing organisms
• Zygote – vital link that ensures continuity of species between organisms of one generation and the next. Every living organism, including humans begin its life cycle as a single cell zygote
• Development of zygote depends on 

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

• In organisms undergoing external fertilization, zygote is formed in external medium; while in those with internal fertilization; the zygote is formed inside the body of organism.
• Further development of zygote varies among species:

ü  In fungi and algae; zygote develops a thick wall resistant to desiccation and damage. It undergoes a period of rest before germination

ü  In organism with haplontic cycle; the zygote undergoes meiotic division; forms haploid spores; that develop in to haploid individuals.

• Zygote develops into embryo. The process of development of embryo from zygote is known as embryogenesis.
  Embryogenesis involves:
               - cell division (mitosis) to increase the number of cells
               - cell differentiation for the formation of different kinds of tissues

The different types of cells form undergo certain modifications to form specialized tissues and organs that together form the complete organism.

• 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. The fruit develops a thick pericarp that is protective in function. The sepals, petals and stamens of the flower wither and fall off. In some plants e.g. Brinjal, tomato etc, the sepals remain attached and are known as persistent. After dispersal, seeds germinate under favourable conditions.