Chapter 1 Reproduction in Organisms Chapter 2 Sexual Reproduction in flowering Plants Chapter 3 Human Reproduction Chapter 4 Reproductive Health Biology in essence is the story of life on earth. While individual organisms die without fail, species continue to live through millions of years unless threatened by natural or anthropogenic extinction. Reproduction becomes a vital process without which species cannot survive for long. Each individual leaves its progeny by asexual or sexual means. Sexual mode of reproduction enables creation of new variants, so that survival advantage is enhanced. This unit examines the general principles underlying reproductive processes in living organisms and then explains the details of this process in flowering plants and humans as easy to relate representative examples. A related perspective on human reproductive health and how reproductive ill health can be avoided is also presented to complete our understanding of biology of reproduction. CHAPTER 1 REPRODUCTION IN ORGANISMS 1.1 Asexual Reproduction Each and every organism can live only for a certain period 1.2 Sexual of time. The period from birth to the natural death of an Reproduction organism represents its life span. Life spans of a few organisms are given in Figure 1.1. Several other organisms are drawn for which you should find out their life spans and write in the spaces provided. Examine the life spans of organisms represented in the Figure 1.1. Isn’t it both interesting and intriguing to note that it may be as short as a few days or as long as a few thousand years? Between these two extremes are the life spans of most other living organisms. You may note that life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly, a mango tree has a much shorter life span as compared to a peepal tree. Whatever be the life span, death of every individual organism is a certainty, i.e., no individual is immortal, except single-celled organisms. Why do we say there is no natural death in single-celled organisms? Given this reality, have you ever wondered how vast number of plant and animal species have existed on earth for several thousands of years? There must be some processes in living organisms that ensure this continuity. Yes, we are talking about reproduction, something that we take for granted. Figure 1.1 Approximate life spans of some organisms REPRODUCTION IN ORGANISMS Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation. You will study later in Chapter 5 (Principles of Inheritance and Variation) how genetic variation is created and inherited during reproduction. There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction. 1.1 ASEXUAL REPRODUCTION In this method, a single individual (parent) is capable of producing offspring. As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. Are these offspring likely to be genetically identical or different? The term clone is used to describe such morphologically and genetically similar individuals. (b)(a) Figure 1.2 Cell division in unicellular organism: (a) Budding in yeast; (b) Binary fission in Amoeba Let us see how widespread asexual reproduction is, among different groups of organisms. Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals (Figure1.2). Thus, (a) (b) (c) (d) Figure1.3 Asexual reproductive structures: (a) Zoospores of Chlamydomonas; (b) Conidia of Penicillium; (c) Buds in Hydra ; (d) Gemmules in sponge in these organisms cell division is itself a mode of reproduction. Many single-celled organisms reproduce by binary fission, where a cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium). In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which, eventually gets separated and mature into new yeast organisms (cells). Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures (Figure 1.3). The most common of these structures are zoospores that usuallyare microscopic motile structures. Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge). REPRODUCTION IN ORGANISMS Nodes Buds (a) Adventitious Buds (b) Adventitious Root (c) (e) (d) Figure 1.4 Vegetative propagules in angiosperms: (a) Eyes of potato; (b) Rhizome of ginger; (c) Bulbil of Agave; (d) Leaf buds of Bryophyllum; (e)Offset of water hyacinth You have learnt about vegetative reproduction in plants in Class XI. What do you think – Is vegetative reproduction also a type of asexual reproduction? Why do you say so? Is the term clone applicable to the offspring formed by vegetative reproduction? While in animals and other simple organisms the term asexual is used unambiguously, in plants, the term vegetative reproduction is frequently used. In plants, the units of vegetative propagation such as runner, rhizome, sucker, tuber, offset, bulb are all capable of giving rise to new offspring (Figure1.4). These structures are called vegetativepropagules. Obviously, since the formation of these structures does not involve two parents, the process involved is asexual. phase can be considered as one of the parameters of senescence or old age. There are concomitant changes in the body (like slowing of metabolism, etc.) during this last phase of life span. Old age ultimately leads to death. In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms. Events in sexual reproduction : After attainment of maturity, all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different. The events of sexual reproduction though elaborate and complex, follow a regular sequence. Sexual reproduction is characterised by the fusion (or fertilisation) of the male and female gametes, the formation of zygote and embryogenesis. For convenience these sequential events may be grouped into three distinct stages namely, the pre-fertilisation, fertilisation and the post-fertilisation events. 1.2.1 Pre-fertilisation Events These include all the events of sexual reproduction prior to the fusion of gametes. The two main pre-fertilisation events are gametogenesis and gamete transfer. 1.2.1.1 Gametogenesis As you are already aware, gametogenesis refers to the process of formation of the two types of gametes – male and female. Gametesare haploid cells. In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes. (a) (b) (c) Figure 1.5 Types of gametes: (a) Isogametes of Cladophora (an alga); (b) Heterogametes of Fucus (an alga); (c) Heterogametes of Homo sapiens (Human beings) Clitellum Testis sac with testis Male Ovary Testis (a) Female Ovary (c) Antheridiophore (b) Archegoniophore Female thallus Male thallus (d) (e) Figure 1.6 Diversity of sexuality in organisms (a) Bisexual animal (Earthworm); (b) Unisexual animal (Cockroach); (c) Monoecious plant (Chara); (d) Dioecious plant (Marchantia); (e) Bisexual flower (sweet potato) be transferred to the stigma before it can lead to fertilisation (Figure 1.7b). In bisexual, self-fertilising plants, e.g., peas, transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other; pollen grains soon after they are shed, come in contact with the stigma. But in cross pollinating plants (including dioecious plants), a specialised event called pollination facilitates transfer of pollen grains to the stigma. Pollen grains germinate (a) on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg. In dioecious animals, since male and female gametes are formed in different individuals, the organism must evolve a special mechanism for gamete transfer. Successful transfer and coming together of gametes is essential for the most critical event in sexual reproduction, the fertilisation. 1.2.2 Fertilisation The most vital event of sexual reproduction is perhaps the fusion of gametes. This process called syngamy results in the formation ofa diploid zygote. The term fertilisation is also often used for this process. The terms syngamy and fertilisation are frequently used though , interchangeably. What would happen if syngamy does not occur? However, it has to be mentioned here that in some organisms like rotifers, honeybees and even some lizards (b) and birds (turkey), the female gamete undergoes Figure 1.7 (a) Homogametic contact in development to form new organisms without fertilisation. alga; (b) Germinating pollen grains on the stigma of a flower This phenomenon is called parthenogenesis. Where does syngamy occur? In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water), i.e., outside the body of the organism. This type of gametic fusion is called externalfertilisation. Organisms exhibiting external fertilisation show great synchrony between the sexes and release a large number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy. This happens in the bony fishes and frogs where a large number of offspring are produced. A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood. In many terrestrial organisms, belonging to fungi, higher animals such as reptiles, birds, mammals and in a majority of plants (bryophytes, pteridophytes, gymnosperms and angiosperms), syngamy occurs inside the fertilised eggs covered by hard calcareous shell are laid in a safe place in the environment; after a period of incubation young ones hatch out. On the other hand, in viviparous animals (majority of mammals including human beings), the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms. In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off. Can you name a plant in which the sepals remain attached? The pistil however, remains attached to the plant. The zygote develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function (Figure 1.8). After dispersal, seeds germinate under favourable conditions to produce new plants. Figure 1.8 A few kinds of fruit showing seeds (S) and protective pericarp (P) SUMMARY Reproduction enables a species to live generation after generation. Reproduction in organisms can be broadly classified into asexual and sexual reproduction. Asexual reproduction does not involve the fusion of gametes. It is common in organisms that have a relatively simple organisation such as the fungi, algae and some invertebrate animals. The offspring formed by asexual reproduction are identical and are referred to as clones. Zoospores, conidia, etc., are the most common asexual structures formed in several algae and fungi. Budding and gemmule formation are the common asexual methods seen in animals. Prokaryotes and unicellular organisms reproduce asexually by cell division or binary fission of the parent cell. In several aquatic and terrestrial species of angiosperms, structures such as runners,