Reproduction in Organisms

 1.1 Introduction:

Panchanan Maheshwari: (1904-1966)

• He studied physiology and Morphology of Plants.

• He specialized and popularized embryological characters in taxonomy.

• He was awarded for test tube fertilization and intra ovarian pollination.

• In his leadership biology for higher secondary schools published by NCERT in 1964.

The period from birth to the natural death of an organism is called life span.

Organism	Life Span
Mayfly	1 day
Butterfly	1-2 weeks
Fruit fly	30 days
Rice plant	3-4 months
Wheat	5 months
Rose	5-7 years
Crow	15 years
Banana Tree	25 years
Cow	20-25 years
Dog	20-50 years
Crocodile	60 years
Horse	60 years
Elephant	60-90 years
Parrot	140 years
Tortoise	100-150 years
Banyan Tree	200 years
Pines	5067 years
Single celled, jelly fish and most plants	Immortal

1.1.1 Phases of Lifespan:

Lifespan of an organism usually includes four phases;

• Juvenile Phase 

• Reproductive Phase/Maturity Phase

• Aging and Senescence Phase

• Death

1.2 Reproduction: Reproduction is a biological process by which new individual organisms (offspring) are produced from their parents.

Reproduction ensures continuity of life or living organisms on an earth.

1.2.1 Types of Reproduction:

Based on whether there is fusion of gametes taking part in the process of reproduction or not, it is divided into two types as follows;

1. Asexual Reproduction

2. Sexual Reproduction

1.2.2 Asexual Reproduction:

• When offspring produced by a single parent and without involvement of sex is called asexual reproduction.

• The offspring of asexual reproduction produces an exact copy of their parent in both morphologically and genetically, this term called Clone.

1.2.2.1 Types of Asexual Reproduction:

• Budding: In this process, a new organism is developed from a small outgrowth part of the parent's body. A bud which is formed detaches to develop into a new organism.

Ex: Yeast, Hydra

• Binary Fission: This Process observes among unicellular organisms, where the parental cell divides into two daughter cells. Each daughter cell grows into a new cell.

Ex: Amoeba, Bacteria’s, Paramecium

Note: (about amoeba)

Encystation: It is a phenomenon under unfavourable conditions where the Amoeba secretes a hard protective covering or cyst around it.

Sporulation: It is a phenomenon in favourable conditions the encysted Amoeba undergoes for cell division and produces many pseudopodiospores. These spores burst from cyst and release to the surrounding and grow up into new Amoebae.

• Zoospores: It is a motile asexual spore that uses a flagellum for locomotion.

Ex: Chlamydomonas, few fungi’s, and few higher algae’s.

• Conidia: It is an asexual fungal spore produced on the hypha of conidiophores.

EX: Penicillium, Alternaria

Difference between Zoospores and Conidia:

Zoospores	Conidia
They are motile spores.	They are non-motile spores.
It formed inside the mother cell or sporangium.	It formed the tip of the conidiophores.
It is found usually in algae’s.	It is found usually in fungi.

• Gemmules: These are internal buds due to the mass of cells. Ex: Sponges.

• Fragmentation: In some organisms, if the body breaks into distinct pieces (fragments) each fragment grows into new individuals.

Ex: Hydra, Planarian, Star fish

• Fragmentation:  occurs by the breaking of the parental body into two/more parts, then each part grows into a new individual. Ex: Starfish, spirogyra and bryophytes.

• Vegetative Propagation: Vegetative Propagation is any form of asexual reproduction occurring in plants in which a new plant grows. It is also called vegetative reproduction.

• Types of Vegetative Propagation: There are two types of vegetative propagation based on with or without involvement of human,

1. Natural Vegetative Propagation and

2. Artificial Vegetative Propagation

i. Natural Vegetative Propagation: It refers to the natural development of a new plant without human intervention.

Ex: Runner, Rhizome, Sucker, Tuber, Offset, Buds and Bulb (This are vegetative propagule units)

• Runner: (Gr: Branch) stem which grows horizontally at the soil surface or just below ground. It is also called Stolons.

Ex: Strawberry, Silverweed

• Rhizome: (Gr: Mass of Roots) it is a modified subterranean stem of a plant that sends out roots and shoots from its nodes.

Ex: Ginger, Turmeric and Banana

• Sucker: A sucker is a shoot that arises from an adventitious bud on an underground root.

Ex: Banana, Blackberries

• Offset: This is found in aquatic plants and is just like the runner; only it is shorter and thicker.

Ex: Hyacinth (Eichhornia crassipes)

Note: Water Hyacinth is the most invasive weed found in standing water. It creates oxygen demand in water leads to death of fishes (Terror of Bengal)

• Tuber: it is a swollen section of stem undergoing asexual reproduction.

Ex: Potato

• Leaf Bud: Buds arise from the leaf notches. Ex: Bryophyllum

• Bulb: It is actually an underground modified bud with a very much reduced stem (disc) and fleshy scale leaves. Bulbs produce smaller buds (lateral buds) which appear between the layers of the parent bulb. (Corm)

Ex: Agave, Onion Rheo disc plant, Garlic

• Turions: (Lt: turion means Shoot) the bulbs in aquatic plants. Ex: waterwheel.

Note:

Bulb: It is a modified underground stem or leafs. Ex: Onion, Garlic.
Bulbil: It is a modified aerial stem or leafs. Ex: Agave.

• Pseudobulbs: The pseudobulb is a storage organ derived from the part of a stem between two leaf nodes. Ex: Orchids

ii. Artificial Vegetative Propagation: It refers to the development of a new plant with human intervention.
Ex: Cutting, Grafting, Layering and Tissue Culture

Cutting: In this, a portion of a plant is cut and planted in the soil, usually a stem or a leaf.


Grafting: In this, the cutting from some other plant is added to the plant stem embedded in the soil.

Layering: In this, the stem of the plant is bent to the ground and covered with soil.

Tissue Culture: It is the growth of tissues or cells in an artificial medium separate from the parent organism. This technique is also called micropropagation.

1.2.3 Sexual Reproduction:

When offspring produced by two parents of the same species participate in the reproductive process and also involve fusion of male and female gametes is called sexual reproduction.

1.2.3.1 Different Phases in Sexual Reproduction:

1. Juvenile Phase:

It is a period of growth and maturity of an organism before it can reproduce sexually. In plants this phase is called Vegetative Phase.

2. Reproductive Phase: 

Period in which organism is capable of reproducing sexually

3. Senescent Phase:

It is the end of the reproductive phase and leads to old age, ultimately leading to death.

1.2.3.2 Features of Sexual Reproduction:

Ex: in Plants

• Easily seen in higher plants when they come to flower.

• Some plants flower seasonally & some throughout the year

• Few plants exhibit unusual flowering phenomenon

1. Bamboo species flower only once in their lifetime (50-100 years).

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

Difference between Monocarpic plants and Polycarpic Plants:

Monocarpic Plants	Polycarpic Plants
These plant flowers give rise only once in their lifetime.	These plant flowers give rise many times a year throughout its life cycle.
After flowering and fruiting they die.	Flowering and fruiting continues till completion of their life cycle.
Ex: Bamboo, Rice, Wheat, Carrot, Strobilanthes kunthiana etc.	Ex: Mango, Apple, Orange, Jackfruit etc.

Ex: In Animals

• Oestrus cycle: cyclical changes during reproduction in non-primate mammals. E.g. cows, sheep, rats, deer, dogs, tigers etc.

• Menstrual cycle: cyclical changes during reproduction in primate mammals. E.g. monkeys, apes, and humans.

• Seasonal breeders: reproductive cycle takes place in favorable seasons as in wild animals. E.g. red deer, Pigeon, Crane

• Continuous breeders: reproductively active throughout their reproductive phase. E.g. Human

• Opportunistic Breeders: It mates whenever the conditions of their environment become favorable. E.g. Mouse, Kangaroo, Parrot, Frog.

Difference between Menstrual cycle and Oestrus cycle:

Menstrual Cycle	Oestrus Cycle
This cycle occurs in primates.	This cycle occurs in non-primates.
It is a long period (27-30 days in humans).	It is short period cycle (12-24 hrs in cows)
Blood flows in the last few days of this cycle.	Blood doesn’t flow in this cycle.
Females don't permit copulation at the secretory phase.	Females permit copulation only during the Oestrus cycle.
This cycle is controlled by a hormone called Progesterone.	This cycle is controlled by a hormone called estrogen.
The broken endometrium is release out	The broken endometrium is reabsorbing.
Ex: Lemur, Mouse Lemur, Monkeys, Apes, Humans	Ex: Cow, dogs, deer, tiger etc.

Difference between asexual and sexual reproduction:

Asexual Reproduction	Sexual Reproduction
Single parent involved	Two parent involved
No fusion of gametes	Fusion of gametes
No formation of gametes	Formation of gametes
Involves mitotic division	Involves meiosis
Individual genetically identical (clone)	Individuals genetically not identical
Without involvement of sex	With involvement of sex
Ex: Hydra and Banana	Ex: Maize and Monkeys

1.3 Events in Sexual reproduction:

1. Pre-fertilization events

2. Fertilization and

3. Post-fertilization events

1.3.1 Pre-fertilization events: The events occur before zygote formation is called Pre-fertilization events.

1. Gametogenesis

2. Gamete Transfer

1.3.1.1 Gametogenesis: The process of formation of haploid male and female gametes.

1.3.1.1.1 Sexuality in organism:

1. Lower plants and fungus having both male and female thalli in the same thallus are called Homothalli. Ex: Aspergillus and Funaria.

2. Lower plants and fungus having both male and female thalli in a different thallus are called Heterothallic. Ex: Penicillium, Neurospora and Marchantia.

3. In a single Plant/part and animal having only one type of sex organ are called dioecious (2 & House) or unisexual or Dicliny ( 2 & Bed) or gonochoric. Ex: Papaya, Maize, human and Dog.

4. In a single Plant and animal having both male and female sex gamete are called monoecious or bisexual or Hermaphrodites. Ex: coconut, earthworm, pea and Leech.

5. In flowering plants a unisexual male flower is staminate (bearing stamens. Ex: Cucumber, Corn), while the female is pistillate (bearing pistils/carpel’s. Ex: Spinach, Cottonwood).

1.3.1.1.2 Types of gametes:

• Homogametes: both male and female gametes have morphologically the same. Ex: Cladophora

• Heterogametes: both male and female gametes morphologically differ. Ex: Humans, Fucus

• Male gamete is called antherozoid/sperm and the female gamete is called egg/ovum.

Difference between Haploid and diploid:

Haploid	Diploid
In the nucleus contain one set of chromosomes	In the nucleus contain two sets of chromosomes
It is denoted as (n)	It is denoted as (2n)
Zygote undergoes for meiosis	Zygote undergoes for mitosis
Ex: Monera, fungi, algae, bryophyte and gamete cells	Ex: Human skin cells, lungs and somatic cells

Meiocytes: The cell which undergoes meiosis is called meiocytes.

1.3.1.2 Gamete Transfer:

The process/mechanism to bring contacts to both male and female gametes is called gamete transfer.

• Male and female gametes must be physically brought together to facilitate fusion called fertilization.

• In most cases female gametes are non-motile, male gametes are motile.

• In the case of few fungi and algae, both male and female gametes are motile.

• In most cases the medium for gamete transfer is water.

• Number of male gametes produced is several thousand times the number of female gametes produced to compensate for the loss during transfer.

• Pollination is the method of gamete transfer in higher plants as pollen grains are carriers of male gametes.

• The unisexual animals have a special mechanism for gamete transfer called coitus/mating.

1.3.2 Fertilization:

• The process of fusion of male and female gametes is called fertilization or syngamy.

• The female gamete undergoes development to form a new organism without fertilization. This phenomenon is called parthenogenesis. Ex: Rotifers, Honeybee, turkey, pineapple and tomatoes.

Difference between external and internal fertilization:

External fertilization	Internal fertilization
Syngamy occurs outside of the body of organism	Syngamy occurs inside of the body of organism
Fusion of gametes takes place outside	Fusion of gametes takes place inside
Large number of gametes are released into surrounding medium	Large number of gametes are not released into surrounding medium
Both male and female gametes formed in large amount	Only male gamete formed in large amount
Zygote is formed usually in water	Zygote is formed usually inside the body
Ex: Bony fish, amphibians	Ex: Birds, Mammals, reptiles, plants

1.3.3. Post fertilization events: The events occur after zygote formation is called Post fertilization events.

1. Zygote

2. Embryogenesis

1.3.3.1 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.

• Development of zygote depends on the type of life cycle of the organisms and an environment it is exposed to.

• Zygote of fungi and algae develops a thick wall that is resistant to desiccation and damage.

• Organism with a haplontic life cycle, zygote undergoes meiosis to produce haploid spores.

1.3.3.2 Embryogenesis:

• The process of development of embryos from the zygote is called Embryogenesis.

• Embryogenesis involves cell division, cell enlargement/growth and cell differentiation.

1.4 In flowering Plants after fertilization:

• The zygote is formed inside the ovule in flowering plants

• After fertilization the sepals, petals and stamens of the flower fall off except pistil or carpel.

• Zygote develops into an embryo.

• Ovule develops into seed

• Integument of the ovule develops into seed coat.

• Ovary develops into fruit.

• Ovary wall developed into a pericarp.

• After dispersal, seeds germinate under favourable conditions to produce new plants.

• Locules develop into endocarp and / or mesocarp

1.5 Difference between Oviparous and viviparous:

Based on whether the development of zygote occurs inside or outside the body of the female parent, animals are categorized into 2 types;

Oviparous	Viviparous
They lay eggs	They give birth to young once
The fertilized eggs have a calcareous shell to protect them from harsh environments.	Embryo is protected inside the mother’s body.
Incubation is required	Incubation is not required
Survival of embryo is less when compared to viviparous	Survival of embryo is greater when compare to oviparous
Ex: Birds, Lizards, Reptiles, Mosquitoes	Ex: Mammals, Shark