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REPRODUCTION IN ORGANISMS 3 12th|II PUC NEET|BIOLOGY:VOL-V REPRODUCTION IN ORGANISM-I The period from birth to the natural death of an organism represents its life span. 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. Elephant (65-90 years) Rose (5-7 years) Dog (20-30 years) Butterfly (1-2 weeks) Crow (15 years) Banana tree (20-25 years) Cow (15-25 years) Parrot (140 years) Crocodile (60 years)
4 REPRODUCTION IN ORGANISMS 12th|II PUC NEET|BIOLOGY:VOL-V Horse (62 years) Fruit fly (about 20-30 days) Rice plant (3-4 months) Tortoise (100-150 years) Banyan tree (200-300 years) Approximate life spans of some organisms Reproduction is defined as a biological process in which an organism gives rise to young ones (off- spring) 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. The organism's habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. When offspring is produced by a single parent with or without the involvement of gamete forma- tion, 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. 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? Answer will be genetically identical. The term clone is used to morphologically and genetically similar individuals. Many single-celled organisms (monerans and protists) reproduce by binary fission. Member of kingdom fungi and algae reproduce through special asexual reproductive structures called zoospores. Conidia (Penicillum) buds (Hydra) and gemmules (sponge) are other common asexual reprodutive structures. Eyes Germinating Eye buds Nodes Buds Adventitious Root a) Potato b) Ginger
REPRODUCTION IN ORGANISMS 5 12th|II PUC NEET|BIOLOGY:VOL-V Adventitious buds c) Agave d) Bryophyllum e) Eichhornia Vegetative propagules in angiosperms : (a) Fyes of potato; (b) Rhizome of ginger; (c) Bulbil of Agave; (d) Leaf buds of Btyophyllum; (e) Offset of water hyacinth (Eichhornia) The fleshy bud which produces new plants in hydrophytes is called Turion. Is vegetative reproduction also a type of asexual reproduction? Answer will be yes. Is the term clone applicable to the offspring formed by vegetative reproduction? Answer will be yes. 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. These structures are called vegetative propagules. 'Water hyacinth' (Terror of Bengal) which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water, which leads to death of fishes. This plant was introduced in India because of its beautiful flowers and shape of leaves. Since it can propagate vegetatively at a phenomenal rate and spread all over the water body in a short period of time, it is very difficult to get rid off them. SEXUAL REPRODUCTION Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. It is an elaborate, complex and slow process as compared to asexual reproduction. Because of the fusion of male and female gametes, sexual reproduction results in offspring that are not identical to the parents or amongst themselves. A study of diverse organisms-plants, animals or fungi-show that though they differ so greatly in external morphology, internal structure and physiology, when it comes to sexual mode of reproduction, surprisingly, they share a similar pattern. Let us first discuss what features are common to these diverse organisms. All organisms have to reach a certain stage of growth and maturity in their life, before they can repro- duce sexually. That period of growth is called the Juvenile phase. It is known as vegetative phase in plants. This phase is of variable durations in different organisms. In some plants, where flowering occurs more than once, what would you call the inter-flowering period - juvenile or mature? Answer will be mature. Plants-the annual and biennial types, show clear cut vegetative, reproductive and senescent phases, but in the perennial species it is very difficult to clearly define these phases.
6 REPRODUCTION IN ORGANISMS 12th|II PUC NEET|BIOLOGY:VOL-V A few plants exhibit unusual flowering phenomenon; some of them such as bamboo species flower only once in their life time, generally after 50-100 years, produce large number of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji), flowers once in 12 years. As many of you would know, this plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches and attracted a large number of tourists. The end of reproductive 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. PRE-FERTILISATION EVENTS Gametogenesis Gamete transfer Gametogenesis Gametogenesis refers to the process of formation of the two types of gametes - male and female. Gametes are 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. They are hence called homogametes (isogametes). However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). (a) (b) (c) Types of gametes : (a) Isogametes of Cladophora (an alga); (b) Heterogametes of Fucus (an alga); (c) Heterogametes of Homo sapiens (Human beings) Sexual reproduction in organisms generally involves the fusion of gametes from two different indi- viduals. But this is not always true. In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual