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Reproduction Reproduction Unit 8 Reproduction UNIT 8 REPRODUCTION StructureStructureStructure 8.1 Introduction 8.5 Mammalian Male Reproductive Physiology : Objectives Humans 8.2 Mechanisms of Structure of Testes Reproduction Hormones of Human Testis Asexual Reproduction Hormonal Regulation of Testis Sexual Reproduction in Animals 8.6 Mammalian Female 8.3 Sex Determination in Reproductive Physiology : Sexually Reproducing Human Animals Oogenesis Genotypic or Chromosomal Sex Determination Female Hormones Non-Genotypic Sex Hormonal Regulation of Ovary Determination 8.7 Reproductive Cycles 8.4 Reproductive Systems in 8.8 Summary Sexually Reproducing Animals 8.9 Terminal Questions Invertebrates 8.10 Answers Reproductive Systems of Vertebrates 8.1 INTRODUCTION Reproduction is the fundamental biological process of all life forms and the mainstay for the continuance of life and the survival of species. This is because if species of an organism develop, grow and live their normal life span, without reproducing then the evolutionary changes in their populations and the continuity and survival of their future generations both of which are a result of reproduction would come to an end leading to their extinction. Reproduction thus, ensures that the older generations of organisms of a 249 Block 2 Animal Physiology-II species are constantly replaced by younger generations which are similar but somewhat variable thus, ensuring the survival of the species. Successful reproduction in fact is the ultimate objective of all living organisms. In the earlier units of this course (BZYCT-135 Animal Physiology and Biochemistry), you have studied the various physiological processes that are essential for the survival of animals. In this unit you shall study about the physiology of animal reproduction, various reproductive mechanisms, the process of gametogenesis, the role of hormones of reproduction, breeding cycles and regulation of reproduction. Asexual mode of reproduction in animals has also been dealt with briefly in the present unit. ObjectivesObjectivesObjectives After studying this unit you should be able to : explain the necessity for reproduction in animals; describe the reproductive mechanisms in animals; discuss and compare the reproductive physiology in male and females; outline the breeding cycles in animals; and explain the mechanisms that regulate reproduction. 8.2 MECHANISMS OF REPRODUCTION Reproduction is a biological process by which organisms produce their offsprings. Organisms reproduce in two ways by: i) asexual reproduction or ii) sexual reproduction. Let us now study about these two types of reproduction. 8.2.1 Asexual Reproduction Asexual or agamic reproduction (reproduction without fertilization) takes place by mitotic division or mitosis. In this type of reproduction offsprings are produced by only one parent by process of mitotic cell division. As a result the offsprings produced by this process inherit all their genes from the single parent and so are genotypically identical copies of their single parent and of each other. This is because genetic mixing does not occur in these organisms during reproduction. In asexually reproducing organisms special reproductive organs or gametes are absent. Asexually reproducing organisms have successfully persisted in large numbers on earth for 3.5 million years. Their involvement in important events of life such as food chain, clearly demonstrates that these organisms are successful and very important. Major methods of asexual reproduction are: (i) fission (ii) budding (iii) fragmentation, and (iv) parthenogenesis. In this section we shall study about these methods in brief. i) Fission : In this method the organisms divide mitotically into two or more equal sized offsprings also called daughter cells. This method of asexual division of the organism into two or more equal sized daughter cells is known as binary fission. It occurs in prokaryotes and some animals like 250 Unit 8 Reproduction protists (Fig.8.1). Some protists also exhibit sexual reproduction. Multiple fission occurs in sporozoan protists. In multiple fission the nucleus of the parent cell divides by mitosis into many nuclei, each of which gets surrounded by the cytoplasm of the parent cell, producing new individuals. Each new individual or daughter organism which is produced contains cytoplasmic organelles and the genetic material of the single parent individual, and each grows to the size of its parent. Some organelles like mitochondria divide at the time of division, while others like flagella and contractile vacuoles are formed afresh by the daughter organisms.. Fig.8.2: a) Hydra parent hydra with developing buds in various stages of development : (1) newly forming bud at the side of the parent hydra; (2) development and growth of bud; (3) developed bud with Fig 8.1: Fission in the amoeba. mouth and tentacles;(b) fully ii) Budding: In this method of reproduction an organism develops by formed miniature mitotic division an outgrowth or bulge called bud which contains the hydra detaches from identical genetic and cytoplasmic material of the parent organism. The parent and becomes bud detaches from the parent organism to become a new individual. In independent. each hydra, there may be one or more buds. Budding takes place in some protists, coelenterates, platyhelminthes and several groups of annelids. Fig.8.2 shows hydra generating offsprings by budding. iii) Fragmentation: In fragmentation an organism breaks into two or more pieces, each of which grows into a new individual. This method of reproduction is common in animals with a high capacity for regeneration. Sponges, hydroid coelenterates, planarians and annelids reproduce by fragmentation. For example, if a sponge is macerated by pressing it through fine gauge and separating the cells and if the cells are brought back together the cells forms groups and grow into new individuals. Very small fragments of free-living flatworms will regenerate into new individuals under favourable conditions. 251 Block 2 Animal Physiology-II Fig 8.3: Fragmentation in Planaria (onlythe first figure should be redrawn the second is redundant). iv) Parthenogenesis : Parthenogenesis is also called agamogenesis and is a unique form of reproduction where embryonic development occurs in unfertilized eggs without the participation of a gamete from the opposite sex. It is a spontaneous activation of a mature egg, followed by normal egg divisions and subsequent embryonic development. Parthenogenes has been defined as ‘the production of an embryo from a female gamete without any genetic contribution from a male gamete and with or without Fig 8.4: The first eventual development into an adult’. Parthenogenes is considered parthenogenetic birth was observed and distinct from asexual forms of reproduction, such as fission and budding documented In May and fragmentation, as it involves the female gametes. 2006, in a female lizard species of a Komodo Parthenogenic species may be:1) obligate (that is, incapable of dragon named Flora, sexual reproduction) or ii) facultative (that is, capable of switching housed in the Chester Zoo, London. between parthenogenesis and sexual reproduction depending upon Zookeepers observed environmental conditions and availability of food). Most parthenogenetic that though Flora had animals reproduce sexually at some point in their life history and so are never come in contact facultative. Facultative parthenogenesis maybe seasonal and may also with a male Komodo dragon it laid 25 eggs, be influenced by environmental factors such as temperature or food including 11 that were supply or some chemicals. viable. A paternity test confirmed that all the Facultative parthenogenesis may occur at irregular intervals or maybe cyclical. genetic material in the eggs had come from Many facultative parthenogenetic invertebrates have a cyclical alternation of Flora and it was asexual with bisexual reproduction influenced by season. An example is the confirmed that Flora crustacean Daphnia that lives in freshwater throughout the world. In any had reproduced parthenogenitically. particular year Daphnia reproduces exclusively by parthenogenesis producing diploid female offspring that continue to grow in a brood pouch. The eggs are released when the mother moults and sheds her exoskeleton. In late summer or early fall, some of the females start producing male offspring through 252 Unit 8 Reproduction parthenogenesis. Then the males produce haploid sperms and fertilize the haploid eggs produced by the females. The offspring produced through this sexual reproduction again go into the asexual mode of reproduction in spring. So far more than 2,000 species of organisms are known to reproduce parthenogenitically. Parthenogenesis occurs commonly among the lower invertebrate animals (particularly rotifers, arthropods such as aphids, ants, wasps, and bees, spider, mites etc.) and rarely occurs among vertebrates. However though not very common among vertebrates it does occur in some species of sharks (egs: hammer head, zebra shark ), Amazon molly fish ( Poecilia formosa,) and amphibian species such as the common water frog (Pelophylax kl. esculentus ), silvery salamander (Ambystoma platineum), mole salamander (Ambystoma talpoideum) Among reptiles facultative parthenogenesis is seen in Komodo dragon (Varanus komodoensis) (Fig. 8.4) while obligate parthenogenesis occurs in whiptail lizards (Refer to Box 8.1) such as New Mexico whiptail (Cnemidophorus neomexicanus) and species of Aspidoscelis. Parthenogenesis occurs even
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