$\displaystyle \small \bullet$ Reproduction taking place by fusion of gametes to form zygote which develops into new organism is called sexual reproduction.
$\displaystyle \small \bullet$ Sexual reproduction is an elaborated, complex and slow process.
$\displaystyle \small \bullet$ Offspring of sexual reproduction are not identical.
$\displaystyle \small \bullet$ The organisms -plants, animals or fungus differs in their morphology, internal structure and physiology but their sexual mode of reproduction are similar.
$\displaystyle \small \bullet$ Characteristics of Sexual reproduction:
$\displaystyle \small \bullet$ It involves two parents.
$\displaystyle \small \bullet$ It requires the formation of gametes.
$\displaystyle \small \bullet$ It involves both mitosis and meiosis type of cell divisions.
$\displaystyle \small \bullet$ Fertilization takes place by fusion of gametes.
$\displaystyle \small \bullet$ Young ones genetically differ from the parents.
$\displaystyle \small \bullet$ Multiplication of individuals is not as rapid as in asexual reproduction.
Phases of Growth
Juvenile phase (Vegetative phase in plants): The stage of growth and maturity,before the organism can reproduce sexually.
Reproductive phase: Begins at the end of the Juvenile phase. In higher plants reproduction is marked by the onset of flowering. Different plants take different time for flowering. In many plants flowering occurs more than once during their life span. In such plants the period of flowering is also called as vegetative phase.
Senescence phase (old age): The end of reproductive phase. Organisms will die or spend old age.
$\displaystyle \small \bullet$ Some plants produce flower throughout the year but some are seasonal.
$\displaystyle \small \bullet$ The annual and biennial plants show clear vegetative,reproductive and senescent phases.
$\displaystyle \small \bullet$ Hormones are responsible for the transitions between the three phases.
$\displaystyle \small \bullet$ Some Plants exhibit unusual flowering phenomenon.
$\displaystyle \small \bullet$ Ex: Bamboo: flower only once in their life time generally after 50-100 years, produce a large number of fruits and die.
$\displaystyle \small \bullet$ Strobilanthus kunthiana (Neelakuranji): flowers once in 12 years.
$\displaystyle \small \bullet$ In animals, juvenile phase is followed by morphological and physiological changes prior to active reproductive behavior.
$\displaystyle \small \bullet$ Birds living in nature lay eggs only seasonally. However,birds in captivity (e.g. poultry) can be made to lay eggs throughout the year.
$\displaystyle \small \bullet$ The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts and hormones during the reproductive phase. It is called oestrus cycle in non-primates (cows, sheep, rat, deer, dog, tiger etc.) and menstrual cycle in primates (monkeys, apes & humans).
| Menstrual Cycle | Oestrous Cycle |
|---|---|
| Characteristic of Primates | Characteristic of non-primate mammals |
| Shed their uterine lining (endometrium) during menstruation | Reabsorb the lining if conception does not occur |
| Can be sexually active at any time, even when they do not ovulate | Females sexually active only during estrous, when there is ovulation |
Seasonal breeders: The mammals (living in natural conditions) exhibiting reproductive cycles only during favorable seasons.
Continuous breeders: They are reproductively active throughout their reproductive phase.
Events in Sexual Reproduction
1. Pre-fertilization
2. Fertilization
3. Post-fertilization
Pre-fertilization Events
$\displaystyle \small \bullet$ The events earlier than fusion of gametes is called pre-fertilization events, that includes gametogenesis and gamete transfer.
$\displaystyle \small \bullet$ Gametogenesis: The process of formation of gametes (male and female) is called gametogenesis.
$\displaystyle \small \bullet$ Gametes are haploid.
$\displaystyle \small \bullet$ Homogametes (isogametes): Male and female gametes are similar in appearance. Ex. Some algae.
$\displaystyle \small \bullet$ Heterogametes: Male and female gametes are morphologically distinct.
$\displaystyle \small \bullet$ Male gametes are called antherozoid or sperm. Female gamete is called the egg or ovum.
Sexuality in Organisms
Bisexual: Organisms with both male and female reproductive structures.
Unisexual: Organisms with either male or female reproductive structures.
Staminate: Unisexual male flower bearing stamen.
Pistillate: Unisexual female flower bearing pistil.
Homothallic / Monoecious: Bisexual condition in several plants and fungi. (Cucurbits, coconuts)
Heterothallic / Dioecious: Unisexual condition in several plants and fungi. (Papaya, date palm)
Hermaphrodites: Bisexual animals that possess both male and female reproductive organs. (Earthworms, sponge, tapeworm and leech).
$\displaystyle \small \bullet$ Many monerans, fungi, algae & bryophytes have haploid parental body. They produce haploid gametes by mitosis.
$\displaystyle \small \bullet$ Pteridophytes, gymnosperms, angiosperms & animals have diploid parental body. They produce haploid gametes by meiosis of meiocytes (gamete mother cell).
Gamete Transfer
$\displaystyle \small \bullet$ For the fusion of gametes, the male and female gametes must physically brought together. Commonly male gamete is motile and the female is non-motile (stationary).
$\displaystyle \small \bullet$ In few fungi and algae both the gametes are motile.
$\displaystyle \small \bullet$ For the movement of gametes it requires the medium, usually plants like algae,bryophytes and pteridophytes, gametes transfer through water.
$\displaystyle \small \bullet$ During transfer of gametes, large numbers of male gametes fail to reach the female gametes. Hence male gametes are produced in more number.
$\displaystyle \small \bullet$ In angiosperms (flowering plants), the male gamete is carried by pollen grains which are produced in anthers. The female gamete (egg) is present in the ovules of ovary.
$\displaystyle \small \bullet$ Pollination: the process of transfer of pollen to the stigma is called pollination.
Fertilization
$\displaystyle \small \bullet$ The fusion of male and female gamete is called fertilization or syngamy. It results in the formation of diploid zygote.
$\displaystyle \small \bullet$ The process of development of new organisms without fertilization of female gametes is called parthenogenesis. ex. honey bee, rotifers, and lizards
Types of Fertilization
External fertilization: Syngamy occurs in the external medium (water), i.e. zygote is formed outside the body.
$\displaystyle \small \bullet$ E.g. most aquatic organisms (many algae, bony fishes etc.) and amphibians.
$\displaystyle \small \bullet$ Such organisms show synchrony between the sexes and release large number of gametes into the surrounding medium to ensure syngamy.
$\displaystyle \small \bullet$ Disadvantage: The offspring are extremely vulnerable to predators threatening their survival up to adulthood.
Internal fertilization: Syngamy occurs inside the body of the organism. E.g. terrestrial organisms, belonging to fungi, animals (reptiles, birds, mammals) & plants (bryophytes, pteridophytes, gymnosperms & angiosperms).
$\displaystyle \small \bullet$ In this, non-motile egg is formed inside the female body to where motile male gamete reaches and fuses.
$\displaystyle \small \bullet$ In seed plants, the non-motile male gametes are carried to female gamete by pollen tubes.
$\displaystyle \small \bullet$ There is large number of sperms produced but the number of eggs is very low.
Post Fertilization Events
$\displaystyle \small \bullet$ Events in the sexual reproduction after formation of zygote.
$\displaystyle \small \bullet$ Zygote is the vital link that ensures continuity of species between organisms of one generation and the next.
$\displaystyle \small \bullet$ Every sexually reproducing organism, including human beings, begin life as a single cell–the zygote.
$\displaystyle \small \bullet$ In the organisms, having external fertilization, zygote is formed in external medium (water) and those having internal fertilization zygote is formed inside the body of female.
$\displaystyle \small \bullet$ In algae and fungi, zygote develops a thick wall resistant to desiccation and damage. This germinates after a period of rest.
$\displaystyle \small \bullet$ In the organisms having haplontic life cycle, zygote divides to form haploid spores that germinate to form haploid individual.
Embryogenesis
$\displaystyle \small \bullet$ It is the development of embryo from the zygote.
$\displaystyle \small \bullet$ During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.
$\displaystyle \small \bullet$ Cell divisions increase the number of cells in the embryo.
$\displaystyle \small \bullet$ Cell differentiation causes the modifications of groups of cells into various tissues and organs to form an organism.
$\displaystyle \small \bullet$ Types of animals based on place of zygote development:
Oviparous: Animals lay fertilized/unfertilized eggs. E.g. Reptiles & birds lay fertilized eggs covered by hard calcareous shell in a safe place. After incubation, young ones hatch out.
Viviparous: The zygote develops into a young one inside the female body. Later, the young ones are delivered out of the body. E.g. most of mammals. Because of proper care and protection, the chances of survival of young ones are greater in viviparous animals.
Ovoviviparous: Animals those give birth to young ones after internal hatching. Ex.Electric ray (Narcine), Scorpion fish (Pterois), Aquarium fishes(Poecilia, Xiphophorus), Common skink (Mabuya)
$\displaystyle \small \bullet$ In flowering plants, zygote is formed inside the ovule. After fertilization, sepals, petals and stamens of flower fall off.
$\displaystyle \small \bullet$ The zygote develops into embryo and ovules into seeds. The ovary develops into fruits which develop a thick wall called pericarp, protective in function.
$\displaystyle \small \bullet$ After dispersal, seeds germinate under favorable condition to produce new plants.
Advantages of Sexual Reproduction
1. There is diversity in the genetic makeup of the individuals produced by sexual reproduction.
2. Since both the parents are involved, the newly formed individuals have the attributes of both.
3. Variations are more successful in sexual mode than in asexual one.
4. The species produced by sexual reproduction survive more than those produced by asexual reproduction. This is because genetic variations help them to adapt to different environments.
| Asexual Reproduction | Sexual Reproduction | |
|---|---|---|
| 1. Parents | Single parent is involved. No Sexual Dimorphism | Two (male and female) parents are involved. Sexual Dimorphism exists |
| 2. Gametes | No gametes formed | Gametes are produced |
| 3. Divisions | Mitosis is the major division | Meiosis for gametes, while for growth, repair regeneration Mitosis occurs |
| 4. Fertilization | No fertilization | Fusion of gametes(fertilization) occurs |
| 5. Multiplication | Rapid and faster multiplication | Considerably slow and gradual |
| 6.New individuals | Offsprings are exactly similar to parents | Offsprings are different to parents |
| 7. Variations | Variations are very less | Variations are more |
| 8. Evolution | Evolutionarily less significant | Evolutionarily more significant |
| 9. Occurrence | Occurs in lower forms of microbes, plants and Animals | Occurs in Higher forms of Plants and Animals |
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