Types of reproduction: There are two principal types of reproduction, asexual and sexual. The organisms produced through asexual reproduction are genetically identical among themselves and also to their parents. These organisms fail to cope with the changing environment and if continue to reproduce this way, ultimately become extinct/ asexual reproduction is advantageous for the organisms as this method is efficient, a shorter time is taken for the species to inhabit or colonize the habitats. In the long term the organisms must reproduce by sexual reproduction to become successful over longer periods of time.
“A population of genetically identical individuals produced from a single parent is called a clone”
In asexual reproduction no gametes are involved, all the offspring’s are produced from single parent by mitotic cell division.
The most important difference between asexual and sexual reproduction is that in the former, there is no mixing of hereditary material prior to the formation of new organism, but in the latter there is always mixing of hereditary material, either between two cells of the same organism or between two cells of different organisms of the same species.
The none mixing of hereditary material in asexual reproduction does not improve the genetic make up and does not produce variety among the descendents. So if environmental conditions, to which these organisms are well adapted, are changed, this would affect all the members of that species. Suppose if asexually produced plant has no resistance to a particular disease the whole population could be wiped off. In contrast the genetic variability is produced by sexual reproduction, which results in most cases by meiosis and mixing of hereditary material. This genetic variability helps the organisms to live successfully in changing environment.
So some of the offspring would survive, even if most of them are wiped off by a particular disease, because the surviving organisms have the resistance to that disease. Some of the common methods of reproduction in plants and animals are binary fission, multiple fission, budding, spore formation, vegetative propagation, regeneration, parthenogenesis and sexual reproduction.
Asexual means without sex and this method does not involve the fusion of gametes in most cases. There are many different forms of asexual reproduction employed by different kinds of organisms including plants. Some of these methods are:
Binary fission in bacteria
It is a type of asexual reproduction in which one unicellular organism divides into two by simple division (fig 16.1). Bacteria belong to kingdom monera and exhibit binary fission. Under favorable conditions one bacterium would divide into two bacteria after about every 20 minutes. They gave a single circular chromosome made of DNA. Steps involved in binary fission are:
- when the DNA molecule replicates, it results in the formation of two chromosomes.
- These two chromosomes move towards opposite sides.
- The middle portion of the cell membrane invigilates inwards from the two sides and when meet in the centre it separates the two halves of the bacterial cell.
- New cell wall is deposited between two cross cell membranes.
- The daughter bacteria grow to their normal size, and then separate from one another.
Budding in yeasts
Yeast, a fungus, consists of a single microscopic spherical cell. The cells of yeast reproduce by budding. It is a special type of asexual reproduction, in which a small bud or out-growth is formed on one side of cell. The nucleus divides by mitosis into two nuclei, and one of the two nuclei is passed on to the bud. Bud enlarges and is finally cut off as an independent cell. When the process of budding is rapid, the individual cells so formed do not separate at once and as a result, small groups of attached cells may some times be seen.
“Practical work: observation of budding in yeast using prepared slides”
- How many buds are present on a single yeast cell?
- Can you find nucleus in any bud?
Spore formation in Rhizopus
You have already studied the structure of Rhizopus. Asexual reproduction in Rhizopus is by the formation of spores in sporangia which are borne on the tips of sporangiophores. Spores are single celled, thick walled, and are produced in sporangium without sexual process. These spores are released from the sporangium and are carried by wind. Each spore when exposed to suitable conditions forms new Rhizopus mycelium.
(Fig.16.3 Sporangium and spore formation in Rhizopus)
“Practical work examination of sporangia and spores of Rhizopus, from prepared slides and fresh material”
Study the prepared slides of Rhizopus showing sporangia and spores under the microscope.
In chapter 5, you have already learned to prepare slide of Rhizopus from fresh material. Make a slide and study it under microscope.
Draw labeled diagrams of sporangia and spores.
- What is the shape of sporangium?
- What is the shape of spores?
- How many spores are present within a sporangium?
Most fungi can reproduce asexually as well as sexually (except imperfect fungi in which sexual reproduction has not been observed).
Asexual reproduction
Asexual reproduction takes place by spores, conidia,fragmentation, and budding. Spores are produced inside the reproductive structures called sporangia, which are cut off from the hyphae by complete septa. Spores may be produced by sexual or asexual process, are haploid, non-motile and not needing water for their dispersal, are small, produced in very large number and dispersed by wind to great distances and cause wide distribution of many kinds of fungi, including many plant pathogens. When spores land in a suitable place, they germinate, giving rise to new fungal hyphae. Spores may also be dispersed by insects and other small animals and by rain splashes. Spores are a common means of reproduction in fungi.
Conidia (singular conidium) are non-motile, asexual spores which are cut off at the end of modified hyphae called conidiophores, and not inside the sporangia, usually in chains or clusters. These may be produced in a very large number, can survive for weeks and cause rapid colonization of new food.
Fig. 8.5 :spores are released from sporangia and germinate to produce new hyphae.
Fig . 8.6Conidia cut of at the tip of conidiophores in clusters chains.
Fragmentation is simple breaking of mycelium of some hyphal fungi, each broken fragment giving rise to a new mycelium.
Unicellular yeasts reproduce by budding (an asymmetric division in which tiny outgrowth or bud is produced which may separate and grow (Fig 8.7), or by simple, relatively equal cell division.
Fig. 8.7: Micrograph shows yeast (Saccharomyces cerevisiae) in various stages of budding.Sexual Reproduction
Details of sexual reproduction vary in different groups of fungi but fusion of haploid nuclei ad meiosis are common to all. When fungi reproduce sexually, hyphae of two genetically different but compatible mating types come together, their cytoplasm fuse followed by nuclear fusion. In two of the three main groups of fungi (gasidiomycetes, Ascomycetes), fusion of nuclei (karyogamy) does not take place immediately after the fusion of cytoplasm (plasmogamy); instead the two genetic types of hapoid nuclei from two individuals may coexist and divide in the same hyphae for most of the life of the fungus. Such a fungal hypha/cell having 2 nuclei of different genetic types is called dikaryotic (also heterokaryotic) hypha/cell (Fig. 8.1).
Different groups of fungi produce different types of haploid sexual spores, such as basidiospores and ascospores, subsequent upon meiosis in zygote. These spores may be produced by their charactersitc structure/fruiting bodies such as basidia/basidio carps and asci/ascocarps.
One of the basic characteristics of living things is their ability to reproduce. Reproduction is a biological process by which living things produce their young ones which are similar to the parents. Reproduction ensures the continuity of a species and also helps the organism to adapt itself to the changing environment.
Sexual Reproduction in Flowering Plants
The reproductive part of flowering plants is the flower. You have already studied the structure and functions of floral parts of Brassica in chapter.
Pollination
The transfer of pollen grains (also called pollen) from anthers of a stamen to the stigma of a carpel of a flower is called pollination.
Types of pollination
There are two types of pollination, self pollination and cross pollination.
“Hay fever, is an allergic response to pollen; which are in the air, and are inhaled by us during inhalation”
The transfer of pollen in the same flower or between two flowers of the same plant is called self pollination.
Cross pollination is the transfer of pollens from one plant to another plant of the same species. The transfer of pollens may be by wind, water or by insects ( or other animals).
The characteristics of wind pollinated, insect pollinated and water pollinated flowers is given in the table 16.1.
Table 16.1 Differences among wind pollinated, insect pollinated and water pollinated flowers.
| Wind pollinated flowers | Insect pollinated flowers | Water pollinated flowers |
| 1. Flowers are small anad without bright coloured petals. | The flowers are large and have brightly coloured petals | The flowers are small and without brightly coloured petals. |
| 2. Pollens produced in enormous numbers and are light in weight. | Pollens are not produced in enormous numbers and are sticky. | The pollen grains neither sink nor float on the surface of water, due to their peculiar specific gravity. They float under water. |
| 3. The flowers have long feathery stigmas. | The stigmas are variously shaped so that pollen grains are attached or entangled there. | The flowers have long stalks. The styles are long and the stigmas are sticky. |
| 4. The filaments of stamens are long and have versatile anthers. | Stamens have short filaments and anthers are not versatile. | Stamens have short filaments and anthers are not versatile. |
| 5. The flowers do not produce nectar. | The flowers produce nectar by glands called nectarines. | Do not produce nectar. |
| 6. Examples are the flowers of maize, wheat and rice. | Examples are the flowers of most dicots e.g. Bougainvillea, rose and calotropis. | Examples are the flowers of plants such as potamogeton and Vallisneria. |
Reproduction in Animals – There are two main types of reproduction in animals, asexual and sexual.
Sexual Reproduction: There are several types of asexual reproduction in animals. The most common are:
- Binary Fission
- Multiple Fission
- Budding
- Regeneration
- Parthenogenesis
Binary fission
It is the simplest method of asexual reproduction at the level of unicellular organisms. In this process, the unicellular organism divides into two identical daughter cells. During the process, the nucleus becomes elongated followed by the constriction of the cytoplasm. Then the nucleus divides into two and the cell afterwards also divides into two. This is common in Amoeba and Paramecium.
Multiple fission
This is a process of repeated divisions of the unicellular organisms into many small cells. In this process nucleus divides into many nuclei divides into many nuclei followed by the division of parent cell into many daughter cells. For example Amoeba becomes enclosed in a covering or cyst under the unfavorable conditions.
Its nucleus divides into many nuclei and each nucleus becomes surrounded by a bit of cytoplasm. When the conditions become favourable, the cyst ruptures and this cause release of a large number of daughter amoebae. Another example is that of malaria parasite, the Plasmodium.
In this case, each plasmodium enters red blood cell and gives rise to 24 daughter cells by multiple fission.
Budding
In this process an outgrowth or bud develops by mitosis on the body of parent organism. It grows to a new individual, which either separates immediately or may remain attached to the parent body for sometime. It is commonly appears in Hydra.
Fig 16.14 Budding in Hydra
Regeneration
Regeneration means formation of the lost parts of the body through the growth of the remaining parts. For example an arm of starfish if lost in accident can regenerate on the body. Some animals use this power of regeneration for reproduction.
For example, if planarian is divided into two halves, each half can regenerate its lost half. Thus each half can become a complete animal. Regeneration is common in annelids, echinoderms and flatworms.
Planaria reproduces by dividing across the middle. Then each offspring regenerates the missing half of its body.
Parthenogenesis
This is a modified form of sexual reproduction in which an unfertilized egg develops into a complete individual. For example in the honey bees, the queen lays eggs which are either fertilized to form diploid workers, or if not fertilized it forms the haploid males called drones. In rotifers, parthenogenesis is a common process of reproduction.
Sexual Reproduction In Animals
The sexual reproduction is based on the fusion of two haploid cells (gametes) to form a diploid zygote. In multi-cellular animals the sexual reproduction is based on meiosis, which results in the formation of specialized sex cells called gametes. The two gametes from two parents, or from one parent (in the case of self fertilization) unite together to form zygote which is the first diploid cell of the new generation.
Fig 16.16 (a) Spermatogenesis
The gametes are of two types: male gametes or sperms and female gametes or eggs. The male gametes are formed in testes while the female gametes are formed in ovaries. Testes and ovaries are known as male and female gonads respectively. The production of sperms is called spermatogenesis while production of ova or eggs is called oogenesis (fig 16.16). Gametes form the physical link between the parent generation and the offspring.
The meiosis plays the fundamental roles in sexual reproduction. It provides the mechanism for reducing the number of chromosomes to one half in gametes and for the exchange of genetic material in chromosomes by the phenomenon of crossing over. The diploid number of chromosomes in the zygote provides the genetic make up for the new individual. The zygote, by simple mitosis changes into embryo, and the embryo develops into a new individual by cell division, differentiation and growth.
Fig 16.16 (b) Oogenesis
Table 16.2 comparison of asexual reproduction with sexual reproduction
| Asexual Reproduction | Sexual Reproduction | |
| 1 | Only one parent in involved | Two Parents, take part |
| 2 | No meiosis, No gamete formation | Meiosis takes place and gametes are formed. |
| 3 | No fertilization | Fertilization takes place |
| 4 | No variations, offspring are identical to their parents | |
| 5 | Rapid production of large number offspring | Less rapid increase in number |
Reproduction in grog
A fertilized egg or zygote of frog passes through a series of events to form an adult animal. The process involving these events is known as development. The developing individual in its early stage is called an embryo and in placental mammals at the later stage it is called a foetus. The study of embryo is called embryology.
Fertilization
The eggs are produced in paired thin walled sacs called ovaries located in the abdomen. Each egg is a small ball spherical structure having one half pigmented, called animal hemispheres, the other half yellowish due to the storage of food or yolk, and is called vegetal hemisphere. The middle of these two halves is called equator. The animal hemisphere or the upper half contains the nucleus of the egg cell. After the sperm enters the egg a small grayish crescent shaped area called grey crescent is formed on one side of the animal hemisphere close to the equator.
The male sex cells or sperms are produced in a pair of testes, during their breeding season. Female releases eggs and male release sperms in water, where fertilization takes place this is called external fertilization. Fertilization results in the formation of zygote.
Reproduction and Life History
The sexes are separate. The testes and ovaries produce sperms, and eggs respectively.
Metamorphosis
Life history of insects is characterized by metamorphosis (Meta = change + Morphe = form). This is an abrupt change of form or structure during the life cycle. There are three morphologically distinct stages in the life cycle; the egg “finally” develops into larva which is converted into motionless pupa that finally develops into an adult. It some primitive insects the metamorphosis is incomplete. The larva resembles adult and is called nymph or instars’. It lives in the same habitat as adult.
Classification
Phylum Arthropoda is a large group consisting of great variety among them. Some of its important classes are as follows.
Class Crustacea
These arthropods are aquatic and have gills for respiration. On the dorsal side of the cephalothorax the exoskeleton is in the form of carapace. In the exoskeleton deposition of salts in addition to chitin makes it more firms. The appendages are modified for capturing food, walking, swimming, respiration and reproduction. Coelom is reduced and is in the form of hemocoel. Head has two pairs of antennal appendages, one pair of mandibles (jaws) and two pairs of maxillae. Sexes are mostly separate e.g. Daphnia, Cyclops, Crabs, lobsters, prawn, wood louse etc. (Fig. 10.8)
Class Insects
This is the largest group not only of arthropoda of all the animal kingdom and has great variety. Insects are found everywhere, many show social behavior. The body in insects has three distinct regions head, thorax and abdomen. There is a pair of antennae and compound eyes on the head. The head is usually vertical to the body and jaws are ventrally placed. The thorax has three segments in which are present three pairs of jointed legs and in many one or two pairs of wings. Abdomen has varying number of segments. Brain is formed of fused ganglia and double nerve cord is ventral. Sexes are separate and animals are oviparous. Metamorphosis takes place during development e.g. dragonfly, mosquito, butterflies, moths, wasps, and beetles etc. (Fig. 10.9).
| Most spiders have eight eyes placed in such a way as to give them panoramic view of the predators and prey. |
Class Arachnida
Body has the anterior segments that are fused to form a combined cephalothorax, with a pair of appendages called chelicerae with claws, two pairs as pedipalps and four pairs of legs. There are no antennae and no true jaws. Abdomen may be segmented or un-segmented with or without appendages. Respiration is by gills lungs. Or special structures called book lungs; excretion is by the malpighian tubules. Eyes simple, sexes are separate. They are oviparous (lay eggs). No true metamorphosis e.g. scorpions spiders, mites and ticks.
Class myriapoda
The body is divided into large number of segments each having a pair of legs. A pair of antennae and a pair of eyes are present on head e.g. centipedes and millipedes
Key Points
- Reproduction is a biological process leading to the production of new individuals, of ones own kind.
- There are two main types of reproduction in organisms asexual and sexual.
- Some of the different types of asexual reproduction in organisms are binary fission, multiple fission, spore formation, budding, parthenogenesis, regeneration and vegetative propagation in plants.
- During sexual reproduction of flowering plant pollination plays an important role.
- Double fertilization in flowering plants, leads to the formation of seed and fruit.
- Seed contains embryo and also contains food for the development of embryo.
- Seeds and fruits help in dispersal of plants.
- There are two types of seed germination, hypogeal and epigeal – and following conditions are necessary for germination, viability of seeds, water, air and suitable temperature.
- In most animals sperms are produced by spermatogenesis and eggs are formed by oogenesis.
- In frog fertilization is external and is in water.
- Cleavage is rapid cell division by which zygote changes into a sold ball of cells called morula
- Blastula is formed when ball of cell contains a cavity called blastocoele.
- Gastrula is characterized by the formation of three germinal (embryonic) layers.
- Neurula is a developmental stage which has developed nervous system in the form of neural tube.
- Metam0r-hosis is a process by which the larva of frog, called tadpole, changes into a frog.