Asexual Reproduction Miss Henry ☺ Objectives • Explanation of the term asexual reproduction. • Relate binary fission, budding, asexual spore formation, fragmentation to asexual reproduction in plants, for example, ginger, meristems, hormone stimulation, • details of the processes involved in tissue culture and the production of cuttings. Reproduction • Reproduction is one of the seven fundamental characteristics of life. • We can define it simply as the process by which a new generation of individuals of the same species is produced. • As known, there are two main types of reproduction. These are: ❑ Asexual reproduction ❑ Sexual reproduction Asexual and sexual reproduction • Sexual reproduction is the fusion of two gametes to form a zygote which eventually develops into a new individual. It requires two parents. Each parent contributes a gamete - a sex cell that has half of the normal DNA of a regular body cell. In males, the gametes are sperm and in females, the gametes are eggs. Sexual reproduction leads to variation which can be advantageous to a species because it influences natural selection and thereby, evolution. We will discuss this in our sexual reproduction lesson ☺ Asexual and sexual reproduction • Asexual reproduction can be defined as the process by which offspring are produced from a single parent without the use of gametes. It usually occurs by mitosis and so the offspring produced tend to be genetically identical to the parent unless some form of mutation occurred. Offspring identical to their parent are generally referred to as a clone. A clone will only be genetically different if random mutation occurred. For this lesson, our focus will be on asexual reproduction. Asexual reproduction • There are various types of asexual reproduction in living organisms. We will discuss examples where necessary. • • These types include:Binary fission Spore formation Budding Fragmentation Parthenogenesis Vegetative propagation (this is asexual reproduction in plants and it includes different types… we will discuss these soon) Binary fission • In unicellular organisms, namely bacteria and protoctists (amoeba, plasmodium), asexual reproduction occurs by means of binary fission. • Binary fission is the process by which unicellular organisms (and some multicellular) carry out cell division to produce two (2) new identical daughter cells or organisms. • After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual. • Multiple fission is possible in some organisms like the Plasmodium parasite where the nucleus may divide multiple times in the parent cell before it is separated into multiple daughter cells. Binary fission • Single parent cell doubles its DNA, then divides into two cells. Usually occurs in bacteria. Binary fission • In amoeba, the chromosomes replicate first, then the nucleus divides into two after which the cytoplasm follows. Spore formation • Fungi reproduce through the formation of spores. • In fact, some protozoans and many bacteria and plants also reproduce via spores. Spores are small structures containing a nucleus. • They are found at the tips of the hyphae where they can lie free or be enclosed in a sac-like structure called the sporangium. • The hyphae are fine tubes that make up the body structure of a fungi, together the whole mass of hyphae is called the mycelium. Spore formation • Spores are produced in a large quantity and are very light which enables them to be easily carried by wind and water as well as by different types of insects after they are released.. • When conditions are correct, the organism will release its spores, which are each then considered entirely separate and autonomous organisms. Given that an environment suitable for life, the spores will then develop into fully grown organisms and eventually grow their own spores, repeating the cycle Spore formation • Fungal spores germinate under favourable conditions and may cause fruits and breads and other food to rot. • Fungi, that is the Penicillium sp tend to be used in the production of cheeses. • Image shows the Penicillium sp. • Penicillium sp. gives blue cheese its characteristic appearance and flavours Budding • A process by which an entirely new organism grows on an existing one. • Organisms like proteins, yeast, and some viruses reproduce asexually via budding. In fact, some plant reproduce via budding (we’ll discuss this in vegetative propagation). • Budding involves the parent cell producing a small outgrowth which grows until it is approximately the same size as the parent. The nucleus then divides to ensure both parent and daughter cell has a nucleus after which the two cells separate. Budding occurs commonly in some invertebrate animals such as corals and hydras. Budding Budding • Some unspecialized animals like the Cnidaria can undergo budding in which a new individual is produced as an outgrowth of the parent (very similar to yeast). • For example: the Hydra • Image showing Hydra. Fragmentation • Segmented worms, plants (vegetative propagation) and many echinoderms such as starfish reproduce asexually via fragmentation. • In this process, an organism physically splits and develops new, genetically identical organisms out of each segment. The segments rapidly grow new cells to constitute their muscle fiber and internal structure through mitosis. This split can be either intentional or unintentional on the part of the organism. It occurs in two steps: 1. 2. The body of an organism is broken into pieces The pieces then regenerate to give a whole new organism identical to the parent. Fragmentation • Examples of fragmentation is shown in regeneration that occurs in certain animals. • Marine sponges have the ability to regenerate as small clusters of cells from the parent sponge can grow into new cells. • Echinoderms are another example of animals that show fragmentation. They include marine animals like sea stars, sea urchins and sea cucumbers among others. If a body part is removed such as the arm, it can simply be regenerated. • Another organism, the Lineus can regenerate new worms if its body is cut into pieces. Parthenogenesis • Parthenogenesis in bees • Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. • The resulting offspring can be either haploid or diploid, depending on the process and the species. • Parthenogenesis occurs in invertebrates such as water flees, rotifers, aphids, stick insects, some ants, wasps, and bees. Bees use parthenogenesis to produce haploid males (drones) and diploid females (workers). If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced. • It can also occur in arthropods and in rare cases in fishes, amphibians and reptiles. Cloning of animals • Cloning is a form of asexual reproduction. It is the production of many genetically identical copies of an individual by asexual reproduction. • A clone is an exact copy of an organism. • In vertebrates, cloning does not occur natural so humans interview to artificially clone an organism. • In animals, clones are really identical twins. Identical twins are generally formed when a fertilised egg called a zygote divides and separate into two (2) distinct cells. These two cells lead to two (2) different organism. • Scientists have utilized TWO (2) different methods of cloning. Cloning of animals Cloning methods 1. Separating the first four cells of the zygote • Scientists have now learnt how to separate the first four (4) cells of a zygote to produce clones of different organisms. This is only possible as these cells have not yet been specialised, therefore they are termed totipotent. Totipotency is the "ability for a single cell to divide and produce all of the differentiated cells in an organism. A totipotent cell has the capacity to form an entire organism. • This is actually practiced in the livestock industry where scientists clone those livestock that possess favourable traits. • After the cells have been separated, they are then placed in a mother who acts as a surrogate to provide the conditions to develop a growing embryo. This is just one way of cloning. Cloning of animals Cloning methods 2. Another way of cloning is simply to take the nucleus of a somatic/body cell from an individual with the desired traits and place it into an unfertilized ovum (the nucleus of this ovum had been previously destroyed by ultraviolet radiation). • The cell is then made to divide as if it was fertilised after which it is then placed in the womb of a surrogate mother. • The first animal successfully cloned was a sheep that was named Dolly. Vegetative propagation Asexual reproduction of plants. Vegetative propagation Vegetative Reproduction Vegetative propagation is a form of asexual reproduction of a plant. As known, only one plant is involved and the offspring is the result of one parent. The new plant is genetically identical to the parent. It occurs when a bud grows and develops into a new plant. At some point, this new plant becomes detached from the parent and starts its own independent life. Specialised organs of propagation often develop but they all have buds. Since they all have buds, it also means they also have stems since buds only arise from stems. • Examples of organs of vegetative propagation in plants include bulbs, corms, rhizomes, stolons, rhizomes, buds and tubers. • These organs of vegetative propagation normally have perennating organs so that they can survive through harsh conditions. Perennating organs are those parts of a plant that store enough nutrients to sustain the organism during the unfavourable season, and develops into one or more new plants the following year. Vegetative propagation Vegetative Reproduction • Many plants have evolved specialized genetic features that allow them to reproduce without the aid of seeds or spores. • This form of specialization is most common in environments with seasonally harsh conditions; it allows plants to survive and thrive in situations where the traditional seeding process is subject to frequent interruption. •There are two main types: natural (man does not intervene) and artificial (man intervenes). Natural Vegetative propagation• Image showing a stolon. Different forms/examples of vegetative propagation: ❑ Stolons are long, creeping, slender stems with elongated internodes that grow horizontally along the surface of the soil. • Few stolons may even grow right beneath the soil • Stolons produce roots and shoots where the nodes touch the soil. • Examples include gooseberry, blackberry and black currant plants. • Stolons do not have any perennating organs (which differentiates it from a rhizome). • Image showing a runner. Natural Vegetative propagation Different forms/examples of vegetative propagation: ❑ Runners- these are stems that grow horizontally above the ground. • Simply a type of stolon that grows rapidly along the soil’s surface • They have nodes where buds are formed. These buds grow into a new plant. • After the new plant is formed, it becomes detached from the runner and exist independently. • Image showing nodes. Natural Vegetative propagation Image showing the nodes and internodes of a plant. • Image showing a bulb (onion) Natural Vegetative propagation More forms of vegetative propagation: ❑ Bulbs- A bulb contains an underground stem. Leaves are attached to the stem. These leaves contain much stored food. • Simply put, bulbs are short, erect, underground stems that are enclosed by thick, fleshy modified leaves or leaf bases. • At the center of the bulb is an apical bud. Also attached are lateral buds. The apical bud will produce leaves and a flower while the lateral buds will produce new shoots. As the plant grows and develops it will form a new bulb underground. E.g.: onion, garlic. • Bulbs are also organs of perennation. • Image showing a potato tuber. Natural Vegetative propagation More forms of vegetative propagation: ❑ Tubers- New plants will grow out of swollen, modified roots called tubers. Buds develop at the base of the stem and then grow into new plants. • Tubers are simply swollen stems or roots that function as underground storage organs. • There are stem tubers and root tubers. Stem tubers produce buds along their stem which can generate new plants (eg: potatoes). • Root tubers do not produce buds but have buds present only where they were attached to the plant (eg: sweet potatoes, yams). Vegetative propagation • Image showing rhizomes. More forms of vegetative propagation: ❑ Rhizomes Rhizomes are simply fleshy underground stems that have scale like leaves. • They grow underground or right at ground level with many growing points or eyes similar to potatoes. • They grow in areas near the parent plant and each node gives rise to a new shoot or plant. • Also is a perennating organ. • Common examples of rhizomes include canna lilies, bearded Iris, ginger and bamboo. • Image showing corm. Vegetative propagation More forms of vegetative propagation: ❑ Corms • Corms are underground stems that produce buds that develop into small plants called suckers. It is an organ of perennation as well as one of vegetative propagation. • The suckers are the organs of propagation. • Simply put, they are short, erect, fleshy underground stem covered with thin, dry, scaly leaves that help to protect the stem. • Leaves are not fleshy like those in the bulbs. In fact, scale leaves are the remains of the previous season’s foliage leaves. At the end of the growing season, contractile roots pull the corm down into the soil, buds on this corm may result in vegetative propagation. Corms are different from bulbs in, when cut open, they are solid and not layered like that of bulbs. Gladiolas and crocuses are great examples of plants with corms • Eg: banana. Advantages of natural asexual reproduction • Only one parent is needed in contrast to sexual reproduction which requires two parents. Therefore, time and energy is not spent finding a mate to produce offspring. For plants, special mechanisms such as pollination may not be necessary. • Produces genetically identical offspring- Where certain traits are favourable, they are preserved which may prove adavantageous to the species. Successful combinations of genes are preserved. • Dispersal and spread- Asexual reproduction methods allow for dispersal of species in an environment. Spores produced by asexual organisms are very light and can easily be disersed across an environment by wind. • Rapid multiplication- Cell division is normally rapid. For example: bacteria can divide as often as every 20 minutes. Disadvantages of natural asexual reproduction • No genetic variation. Therefore, if a trait is not so favourable to an environment, it may cause the eradication of a species. • If spores are produced, many may not find a suitable environment for germination and so the energy and food used to create them are wasted. • May lead to overcrowding if an organism spreads in one area which may lead to increased competition between members of the same species. Advantages and Disadvantages of natural asexual reproduction Artificial Vegetative Propagation Artificial Vegetative Reproduction • Image showing a cutting. Horticulturists and farmers use artificial means to produce plants that are identical to the parent plant. Some of the methods used are: • Cuttings Cuttings are part of the plant that is removed from the parent plant and placed in a suitable medium to grow. Shoots with leaves attached are usually used. New roots and leaves will grow from the cutting. The shoot is cut at an angle. A growth promoter may be used to help with the growth of the roots. • Image showing grafting. Artificial Vegetative Propagation Artificial Vegetative Reproduction (cont’d): Grafting • In grafting, 2 plants are used to develop a new plant with combined traits from the 2 parent plants. • In grafting, a cutting (a shoot) called the scion is inserted into the slit of the stem of another plant called the stock. The joint is then bounded up. The stock plant provides the root system for the scion allowing the scion to grow into a new plant. • In other words, grafting involves joining a stem piece (as in grafting) or a single bud (as in budding) onto the stem of a plant that has roots. The stem piece or bud is called the scion, and the plant with roots is called the rootstock. Grafting is commonly used to produce fruit trees sometimes with more than one variety of the same fruit species growing from the same stem. • Used to grow apple, peach and plum. Artificial Vegetative Propagation Artificial Vegetative Reproduction (cont’d): • Image showing air layering. Air layering • A method used to root a stem while it is still attached to the parent plant. • Tend to be used for plants that produce runners. • Stems that are still attached to their parent plant may form roots where they come in contact with a rooting medium. • It involves the stem being cut just above the node and wedged open to separate the cut surfaces. Sphagnum moss is then wrapped around the cut stem and kept moist until the root forms. A layer is the rooted stem following detachment (removal) from the parent plant. • Some plants propagate naturally by layering, but sometimes plant propagators assist the process. Layering is enhanced by wounding the stem where the roots are to form. The rooting medium should always provide aeration and a constant supply of moisture. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) • Micropropagation is the propagation or cloning of plants by tissue culture. • We say micro because of the tiny plant materials used in the propagation. Plant material can be isolated cells or simply plant tissue. • That is, these cells are removed or excised from a plant. The pieces or cells taken can be termed explants. Explants • These cells or tissues are then grown in special culture mediums, hence the name tissue culture. • The tissue culture normally includes a mixture of mineral ions, nutrients and hormones to provide the right medium for plants to grow. • Image showing tissue culture. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) How were these plant cells able to propagate and grow into new plants? • The answer is Totipotency (the ability of a single cell to grow into a new individual). • In the early 1960s, scientists realized that mature plant cells still contained all the information needed to code for an entire plant. • This meant mature cells could be removed from a plant can actually grow into a new plant when placed in the right condition or most suitable medium. • Hence, the cells are termed ‘totipotent’. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Outline of the technique • It is important that the medium that the plant tissue or cells are placed in contain the right nutrients and hormones to ensure growth of the plant tissue. A typical medium must contain: ✔ The right inorganic ions (nitrogen, magnesium, potassium, iron) to ensure plant growth. ✔ Vitamins ✔ Sucrose as a source of energy ✔ Hormones (auxins and cytokinins; auxin is needed to stimulate root growth and cell elongation, cytokinins stimulate shoot growth and cell division) • These chemicals are usually mixed with agar jelly to make a jelly-like nutrient agar which is placed in a petri-dish or flask. • The tissue culture is grown on the surface of the agar jelly. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Outline of the technique These cultures are then grown in special rooms or cabinets where temperature, light intensity, light quality, humidity, day length and other growth factors may be controlled. It is important that procedures used to create the tissue culture are preformed until sterile condition to ensure that there is no bacterial or fungal growth in the tissue culture medium. In fact, dilute bleach solution is generally used to surface sterilize the plant tissue as well other materials and equipment. This is because these microorganisms may compete with the tissue culture and even outgrow them. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Outline of the technique Things to note: Explant- a cell, organ, or piece of tissue which has been transferred from animals or plants to a nutrient medium. Explants usually include: ✔ meristematic tissue from apical or axillary buds (meristematic tissue is normally used to ensure the plants produced are virus free because viruses usually do not extend to the tips of meristems. These meristems can further be treated with heat to ensure it is indeed virus-free. Therefore, using meristems in tissue culture tend to ensure the production of virus-free plants) ✔ callus Meristem-regions of a plant where cell division is actively taking place. Callus-mass of undifferentiated cells taken from non-meristematic tissue. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Summary Tissue culture includes tiny fragments of plants treated with plant hormones in a sterile growing medium. The hormones stimulate the growth of a callus(new plant cells formed from plant wounds), from which a new seedling can grow. This method is used to produce large numbers of identical seedlings. Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Artificial Vegetative Propagation Tissue Culture (also called micropropagation) Artificial Vegetative Propagation Advantages of Tissue Culture (also called micropropagation) (READ more pg 711 of BS) • Rapid multiplication- Tissue culture can be used to produce shoots which in turn produce buds. These buds can be used to generate more shoots via the same tissue culture technique. In this way, buds are constantly recycled which consequently multiplies the number of potential plants at each stage. This can result in a great number of plants being produced from one shoot over the course of a year (in fact, thousands). • Disease-free plants- As we already know, virus disease can be eliminated by using meristems for our tissue culture. Also, certain techniques are always practiced to ensure bacteria and fungi are eliminated as much as possible. • Genetic uniformity- Plants divide by mitosis and so are clones of the parent or stock plant. Artificial Vegetative Propagation Advantages of Tissue Culture (also called micropropagation) cont’d • Tissue culture also takes up less space than growing plants in green plants. • Tissue culture can be done all year around regardless of seasonal changes. This means that certain plants produced out of season can be sold at higher prices. • Plants such as bananas are sterile and so they have to reproduce asexually. This process allows a large production of banana to meet the demands of a growing population. The reason the bananas we eat don't have seeds is that they are all sterile. A long time ago the Cavendish bananas first came into being when a tetraploid banana mated with a normal diploid banana. • The seeds of certain plants are difficult to germinate. Therefore, tissue culture would ensure a more reliable production of these plants. Eg: orchids • Increases the chances of international trade since tissue cultures are not bulky to travel with. • Plant production can be monitored more closely to ensure product uniformity as well as conformity for customers. Artificial Vegetative Propagation Disadvantages of Tissue Culture (also called micropropagation) (READ Pg 711 of BS) • Labour intensive in comparison to sowing a seed since they have to be monitored. Also, the work is skilled and so staff have be trained which may be problematic especially if it is done on a large scale. • Cost of operation can be great due to the training of a large staff. Therefore, tissue culture tends to be more commercially viable for expensive products and not suitable for low cost products/crops. • Sterile condition must be maintained which only makes the operation more demanding and even adds to its cost. • Plants obtained from callus structures sometimes change genetically. Some changes can still be useful but most tend to be unfavourable. • Since clones are produced, crops can be susceptible to new diseases as well as new environmental conditions. If environmental changes do not favour the crops produced, they could all be wiped out. Questions a. b. c. d. e. Define the terms 'asexual reproduction' and 'vegetative propagation'. Discuss ONE advantage and ONE disadvantage of asexual reproduction. Structures such as bulbs, corms and tubers are suitable for use in vegetative propagation. State ONE characteristic of the tissue of these structures, which facilitates this function (vegetative propagation). State ONE function, other than vegetative propagation, that is often carried out by bulbs, corms and tubers. Distinguish between ‘binary fission’ and ‘budding’.
