Biology Reproduction PDF

Living organisms grow, develop, respire, feed, reproduce, excrete and eventually age and die. For continuity of life, the most critical characteristic of living organisms is reproduction. Organisms must be able to reproduce to pass on...

Living organisms grow, develop, respire, feed, reproduce, excrete and eventually age and die. For continuity of life, the most critical characteristic of living organisms is reproduction. Organisms must be able to reproduce to pass on their genes, whether by replicating themselves or by mating with another individual to produce fertile offspring. Offspring carry the same genetic traits or a mix of traits from their parents into the next generation (Fig. 2.1), ensuring that, even though individuals die, the gene pool and the species continue. Shutterstock.com/LightField Studios Shutterstock.com/Claudia Paulussen FIGURE 2.1 Offspring resemble their parents, physical evidence of genetic material being passed on from one generation to the next to ensure the continuation of the species. Asexual and sexual reproduction – 2.1 one parent or two? Reproduction is a fundamental evolutionary process ensuring the continuity of life. Considering the origin of life and the evolution of living organisms, reproduction must have been one of the first characteristics of life to arise. The ability of a chemical system to make copies of itself and, later, the ability of an organism to make copies of itself, are of primary importance in ensuring the continuity of species. The reproductive success of an organism is determined by its ability to produce fertile offspring that survive to reproductive maturity and produce offspring of their own, in this way replacing the parent. Biological fitness is a measure of an individual’s reproductive success. Biological fitness is calculated as the average contribution to the gene pool made by a certain genotype within a population and the relative likelihood that those alleles (variants of a gene) will be represented in future generations. An allele with higher fitness is more likely to be represented in future generations than an allele of the same gene with lower fitness. Reproductive success is a feature of an individual, while biological fitness is a feature of an allele in a population. There are two main methods of reproduction. Asexual reproduction involves only one parent and gives rise to offspring that are genetically identical to each other and to the original parent. Sexual reproduction usually involves two parents who produce offspring that have a mix of the parents’ genes and therefore differ from each other and from the parents. (Occasionally, organisms are bisexual and if self-fertilisation occurs, the offspring would arise by sexual reproduction from one parent.) KEY CONCEPTS Reproduction means making a copy or a likeness. For living organisms, this means producing offspring that are identical to the parent or resemble the two parents that gave rise to them. Individuals have a finite lifespan, so in order for a population or a species to survive, genetic material must be passed from one generation to the next. This ability to reproduce is known as the reproductive success of an individual. The genetic material of all organisms in a population makes up the gene pool. The likelihood of genes appearing in the next generation and being passed on is known as biological fitness. In evolutionary terms, reproduction is less significant for individual success and more important for the continuation of the species. 34 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 34 08/08/18 6:28 pm  Advantages of sexual reproduction Sexual reproduction involves the meeting of special sex cells called gametes, which carry genetic information from both parents to the offspring. As a result, the offspring contain a mix of parental genes and are not genetically identical to the parents or to other offspring, and this introduces genetic variation into the population. The greatest advantage that sexual reproduction is thought to provide, in terms of continuity of life at the species level, is genetic diversity. Some offspring may possess random variations that make them better suited to new and changing environmental conditions. They may out-compete their parents and/or other individuals in the population, thereby gaining a selective advantage. In the face of environmental change, the survival of some individuals gives the overall population or species a better chance of survival. The disadvantage of sexual reproduction is that this process demands a greater expenditure of time and energy, involving processes such as finding a mate, courtship behaviour, gamete production and mating, before the production of young. These processes may also make organisms more vulnerable to predators. Because sexual reproduction requires far more investment by an individual than asexual reproduction, it tends to be the first functional process that is sacrificed in times of hardship. Sexual reproduction – the meeting of two gametes During sexual reproduction, a combination of genetic material from two parents is passed on to offspring. Every species has a characteristic number of chromosomes per cell. For example, humans have 46 chromosomes, camels have 70, tomatoes have 24 and chickens have 78. Each species usually has two sets of chromosomes, arranged in homologous pairs. The number of chromosomes does not necessarily reflect the complexity of the organism and even varies among closely related species. For example, the housefly has 12 chromosomes, whereas the fruit fly has only 8. The important thing to remember for studies of genetics is that the chromosome number is constant for each species and does not change from one generation to the next. In sexual reproduction, to prevent the chromosome number from doubling in each successive generation, a mechanism to ensure that each parent contributes only half of his or her chromosomes to their offspring is necessary. Meiosis, a type of cell division that takes place in the reproductive organs of plants and animals, is important to maintain the characteristic chromosome number during sexual reproduction. When a cell involved in sexual reproduction divides by meiosis to produce gametes (sex cells), the chromosome number halves – that is, each resulting gamete contains only one set of chromosomes. The terms diploid and haploid refer to the number of sets of chromosomes within any cell. In most organisms, the somatic cells (body or non-reproductive cells) contain two sets of chromosomes – that is, the diploid number of chromosomes (in humans this number is 46 or 23 pairs). Offspring inherit one set of chromosomes from the mother (maternal chromosomes) and one set from the father (paternal chromosomes) (Fig. 2.2). A fertilised egg or zygote arises as a result of the fusion of haploid gametes, when the chromosome number changes from haploid to diploid. 1 The diploid zygote divides by mitosis to become an embryo with identical Sperm (n) Egg (n) Offspring (2n) body cells that all have the diploid number of chromosomes. If n = 1 set of chromosomes, then in humans, n = 23. Human somatic cells are diploid (2n) and have 46 chromosomes, whereas human gametes are FIGURE 2.2 The sperm and the egg are haploid haploid (n) and have 23 chromosomes (as a result of meiosis). Fertilisation gametes that give rise to diploid offspring in sexual reproduction. of an egg by a sperm restores the diploid number (Fig. 2.3). 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 35 02_bio_InFocus12_2e_sb_08851_txt.indd 35 08/08/18 6:28 pm  Mother’s Father’s chromosomes chromosomes 2n 5 46 2n 5 46 Meiosis Specialised cells in the ovaries Specialised cells in the testes Egg cells n 5 23 Sperm cells n 5 23 Fertilisation Zygote (2n) 5 46 (fertilised egg cell) or Mitosis Embryo Each cell is 2n 5 46 chromosomes Female child Male child FIGURE 2.3 Maintenance of the diploid number during sexual reproduction in humans KEY CONCEPTS Sexual reproduction requires the production of male and female gametes (sperm and ova) by the process of meiosis (reduction division). Each gamete is haploid (n) – that is, it has half the normal number of chromosomes. The gametes fuse during the process of fertilisation to create a zygote (fertilised egg) with the full diploid (2n) complement of chromosomes. In offspring, 50% of the chromosomes come from the mother and 50% from the father. The cells of the zygote divide by mitosis, keeping the chromosome number constant, and the resulting embryo continues to grow and mature into a new individual. Fertilisation and meiosis are reciprocal processes – that is, one is a fusion from haploid to diploid, and the other is a reduction from diploid to haploid. 36 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 36 08/08/18 6:28 pm  CHECK YOUR 1 Explain what is meant by each of the following terms: diploid, haploid, gamete, somatic cell, paternal, UNDERSTANDING maternal. 2 Distinguish between asexual and sexual reproduction. 2.1a 3 Define ‘reproductive success’ and ‘biological fitness’. What is the difference between these two terms? 4 Give one advantage and one disadvantage of sexual reproduction. 5 What is meiosis? Why is this process important to a species? 6 What is the importance of variation in a population? Sexual reproduction in animals Sexual reproduction is a mechanism that has evolved to ensure continuity of species. In animals, a number of sexual reproductive strategies ensure that reproduction occurs effectively in the environment in which an organism lives. Most animals are unisexual – there are separate male and female individuals. However, a small range of animals are bisexual or hermaphrodites, where each individual has both male and female reproductive organs. Hermaphroditism can be advantageous to species with low population densities, or in animals that are non-motile (such as coral), where finding a mate is difficult. The disadvantages of hermaphroditism are that individuals must expend larger amounts of energy to grow and maintain two sets of reproductive organs. Then if self-fertilisation occurs, the gametes carry fewer possible combinations of genes and therefore the offspring will have less variation. Other reproductive strategies include the type of fertilisation (internal or external), the number of gametes produced, the timing of gamete release, where the young develop (outside or inside the body) and the nature of parental care. If these strategies are advantageous within the particular environment in which an organism lives, they can increase its reproductive success. Fertilisation – external or internal? In animals, the union of male and female gametes (sperm and ova) can occur outside the body (external Invertebrates are fertilisation) or inside the body (internal fertilisation). The key to successful fertilisation of ova by sperm animals without is that the gametes, each of which is a single haploid cell surrounded by a cell membrane, must meet a backbone, such as coral polyps, and not dehydrate in the process. Therefore, external fertilisation is better suited to organisms that insects and snails. reproduce in an aquatic environment (such as marine creatures) or a very moist environment (such as earthworms), whereas internal fertilisation is typical of many terrestrial organisms (such as insects, Vertebrates are lizards and kangaroos). Vertebrate sexual reproduction is thought to have started in the ocean (fish) animals with a and freshwater environments (amphibians) and then evolved once vertebrates such as reptiles, birds backbone, such as fish, amphibians, and mammals colonised the land and the air. The change in type of fertilisation (external or internal) is reptiles, birds and consistent with the accepted sequence of species’ evolution. mammals. The chances of successful external fertilisation are increased by synchronisation of reproductive cycles, mating behaviours and the release of gametes. When fertilisation and development of the young take place externally, there is little or no parental care. This means that less time and energy are required of the parents, but a larger number of gametes must be produced to ensure that some young survive. The advantage of external fertilisation is the wide dispersal of young. Some marine animals release their gametes into the sea, and fertilised eggs are carried away to settle in an area far from their parents. This reduces competition for food and living space, and also allows rapid recovery of populations away from damaged areas. 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 37 02_bio_InFocus12_2e_sb_08851_txt.indd 37 08/08/18 6:28 pm  Pheromones are Staghorn coral chemical substances Staghorn coral is an example of a colony of invertebrate marine animals (polyps) that achieve fertilisation released by one organism that have by simply shedding millions of gametes into the sea (Fig. 2.4). Environmental cues, such as water an effect on another temperature, tides and day length, help synchronise the reproductive cycle. When polyps in one coral organism. colony start to spawn, pheromones released along with gametes stimulate nearby individuals to spawn, resulting in coordinated spawning over a wide area. AUSCAPE All rights reserved/© Nature Production FIGURE 2.4 Staghorn coral (Acropora yongei sp.) releases bundles containing sperm and eggs. During the mass spawnings of coral on Australia’s Great Barrier Reef, the number of gametes shed is so great that, for a time, the sea turns milky. Within one day, fertilised eggs develop into swimming larvae. After a few days at the surface, the larvae descend to find a suitable site to form a new colony. Although millions of staghorn coral larvae are produced, almost all are eaten by predators. Of the few remaining, only a tiny proportion reach adulthood. Bony fish The females of most species of marine bony fish produce eggs (ova) in large batches and release them into the water, where they fuse with sperm outside the body of the female. Because the gametes disperse quickly, the release of large numbers of eggs and sperm from the females and males must occur almost simultaneously. In most marine fish, the release of gametes is restricted to a few brief and clearly determined periods. Getty Images/Derek Middleton/FLPA/Minden Pictures Although thousands of eggs are fertilised in a single mating of bony fish, many of the resulting offspring succumb to microbial infections or predation, and few survive to maturity. Amphibians Amphibians invaded the land without fully adapting to the terrestrial environment, and so their life cycles still involve stages in water. Gametes from both males and females are released in fresh water, such as ponds or streams. In frog and toad copulation, the male grasps the female and straddles her back, discharging fluid containing sperm onto the eggs FIGURE 2.5 Copulation in frogs, where eggs are fertilised externally as they are released by the female into the water (Fig. 2.5). 38 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 38 08/08/18 6:28 pm  An enormous number of gametes are Science Source/Michael J. Tyler produced by amphibians, first to ensure that many undergo fertilisation, and second to ensure the production of a large number of offspring. Because most amphibians provide no parental care, the young tadpoles are easy prey and not many survive to reproductive age. Some frogs, such as the Southern gastric brooding frog (discovered in forests north of Brisbane in 1974 and recently registered as extinct), evolved special FIGURE 2.6 A young froglet emerging from the mouth of a adaptations to ensure survival of the female Southern gastric brooding frog (Rheobatrachus silus) after developing in its mother’s stomach young. The eggs of the terrestrial frog were fertilised by sperm externally in a watery environment, after which the female would swallow the eggs, and the young developed internally in the female’s stomach. In the stomach, digestive secretions ceased and the eggs settled into the stomach wall, where they were protected and absorbed nutrients from the mother for about 6–7 weeks (during which time the female did not eat). Young frogs were then regurgitated through the mouth (Fig. 2.6). This mechanism provided some protection for the underdeveloped young from predation, infection and dispersal, significantly increasing the chance of successful survival of the offspring. These animals were unusual in having external fertilisation but internal development, and provide an extreme example of parental care. Internal fertilisation and parental care in animals Organisms that undergo internal fertilisation tend to be adapted to terrestrial environments and reproduce successfully on land. The internal environment for fertilisation not only protects gametes from dehydration and loss to external elements, but also protects the fertilised eggs and developing young from immediate predation. Therefore, with internal fertilisation, fewer eggs are required for the survival of a sufficient number of offspring. The internally fertilised egg may develop a shell and be laid in the external environment (oviparous) to complete its development (in reptiles and birds, for example), or it may continue to develop inside the female’s body. In most mammals, the fertilised egg becomes an embryo that is nurtured inside the female parent’s body, obtaining nutrients through a placenta, and is born alive (viviparous development). In rare instances, a combination of the above Shutterstock.com/Webitect occurs and eggs with yolk for nourishment are retained inside the mother’s body until they are ready to hatch. Newly hatched young are born alive (ovo-viviparous, for example in some snakes and sharks). Reptiles Most reptile eggs are fertilised internally and then deposited outside the mother’s body for development. During copulation (Fig. 2.7), male reptiles use a tubular penis FIGURE 2.7 Reptiles such as tortoises copulate, so the eggs are to introduce sperm into the female. fertilised internally. 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 39 02_bio_InFocus12_2e_sb_08851_txt.indd 39 08/08/18 6:29 pm  In crocodiles, fertilisation occurs Getty Images/hphimagelibrary internally. The female crocodile (Crocodylus porosus) lays small numbers of large yolky eggs in clutches along the sandbanks beside the sea or a river (Fig. 2.8). The eggs of most reptiles are covered in a soft but tough leathery shell. Exceptions are tortoises, geckos and crocodiles, which lay hard-shelled eggs. The eggs contain sufficient food reserves to last until the eggs hatch. The offspring FIGURE 2.8 After developing in a yolky egg, tiny crocodiles (Crocodylus porosus) hatch. resemble miniature adults and are able to crawl from the buried nest to the surface and make their way to water, a journey that makes them vulnerable to predation. There is no parental care of these young. It interesting to note that the temperature at which reptile eggs are incubated often determines whether the resulting individuals are male or female. At high temperatures, females hatch, even from eggs in which the individual has male chromosomes. These females are able to lay eggs and reproduce like normal females, despite being genetically male. Birds Courtship behaviour by birds may take place in flight or on the ground, but copulation takes place on the ground, making the birds vulnerable to predators. Fertilisation is internal. Most male birds do not have a penis, so during copulation, male and female birds rub the openings of their cloacas together and sperm are transferred to the female’s body. In some larger birds (such as swans) the male cloaca extends to form a ‘false’ penis. Once fertilised, the ovum passes along the oviduct and successive glands secrete yolk, followed by protein (albumen, commonly called egg white) around it. A calcium carbonate shell is then secreted and this hardens when the egg comes into contact with the air, directly after it is laid. The hard shell distinguishes bird eggs from soft-shelled reptilian eggs and gives more protection. Most birds incubate their eggs after laying them, to keep them warm, and exhibit parental care once the young have hatched. Mammals Imagefolk/D.Parer & E.Parer-Cook/ardea.com Monotremes are mammals, such as The gametes of all mammals undergo the platypus and the echidna, that lay fertilisation internally. Mammals are divided eggs but still suckle into three subclasses – monotremes, their young. marsupials and eutherians – based on the subsequent development of their embryos. Monotremes, such as the platypus and the echidna, are oviparous. After internal fertilisation, they lay eggs that develop outside the mother’s body. Platypus parents incubate their eggs in a nest, whereas echidnas place their eggs into an abdominal FIGURE 2.9 A young echidna with its mother pouch where they stay for about seven weeks. The young hatchlings (puggles) obtain milk from their mother’s mammary glands by licking her abdominal skin (Fig. 2.9). 40 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 40 08/08/18 6:29 pm  Young marsupials develop internally Alamy Stock Photo/Auscape International Pty Ltd for a short time after fertilisation and then continue their embryonic development in a pouch. Offspring are born at a very young age and crawl up the mother’s abdomen to the pouch (Fig. 2.10). In favourable environmental conditions, marsupials such as the kangaroo can have three offspring at different stages of development at any one time – one out of the pouch but still drinking milk, one in the pouch attached to a nipple, and a fertilised ovum at the blastocyst FIGURE 2.10 Kangaroos give birth to small foetuses, which complete (ball of cells) stage in the uterus. The their development in a pouch. development of the youngest is triggered when the second-youngest detaches from the nipple and leaves the pouch. Because milk production in kangaroos lasts much longer than a pregnancy, it is necessary to delay development of the new embryo until the older one is no longer suckling. This delay in development, termed embryonic diapause, is another strategy to increase chances of survival. The red kangaroo has a special reproductive adaptation to ensure survival of the young in times of drought. If the mother is unable to produce sufficient milk to sustain a joey attached to a nipple in the pouch, this joey dies and a newborn individual will enter the pouch a month later. The tiny newborn requires far less milk for the first few weeks in the pouch. This strategy ensures the continuity of young who are ready for development when the drought ends and allows very rapid population growth when conditions are good. However, in prolonged drought conditions, kangaroos stop breeding and only begin again when rain triggers a hormonal response in the female. This very effective mechanism restricts reproduction to times when conditions favour survival of the young. Eutherians, otherwise known as placental mammals, include dingoes, rodents (such as rabbits, rats and mice), domesticated animals (such as dogs, sheep and cattle) and humans. Following internal fertilisation, the young completes its embryonic development inside the body of the mother in a special organ, the uterus, which nurtures and protects the embryo. Once one or more fertilised eggs implant into the uterine wall, a placenta develops, connecting the young to a supply of nutrients and oxygen that passes from the bloodstream of the mother to the developing young (Fig. 2.11). Excretory wastes such as nitrogenous wastes and carbon dioxide from the embryo diffuse Getty Images/Joe Lee Brain across the placenta to the mother’s Amniotic sac body, where they are excreted along with Placenta the mother’s own wastes. The mother gives birth to live young that are mature and therefore have a greater chance Eye of survival. This type of development, where live young are born, is described as viviparous. Placental mammals Umbilical produce one to a few young at a given cord time and they invest a large amount of Liver energy in parental care, increasing the chance of survival of the young. FIGURE 2.11 Human embryo developing in a uterus attached to a placenta 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 41 02_bio_InFocus12_2e_sb_08851_txt.indd 41 08/08/18 6:29 pm  Internal and external fertilisation in animals are compared in Table 2.1. TABLE 2.1 Comparison of internal and external fertilisation in animals DIFFERENCES CHARACTERISTICS EXTERNAL FERTILISATION INTERNAL FERTILISATION SIMILARITIES Gametes Large numbers of male and female Large number of male gametes and Male and female gametes gametes produced fewer female gametes produced required – sperm and eggs (ova) Union Occurs in open water Occurs inside the reproductive tract Sperm fertilise the eggs when they environments of the female in organisms that live unite mostly or completely on land Conception Simultaneous release of gametes Copulation: the male inserts sperm Sperm will fertilise eggs when mechanism into the female’s reproductive tract in very close proximity to each via penis or cloaca other; gametes require a watery environment for this to occur Chance of Low, because male gametes are High, because male gametes are If male and female gametes are fertilisation released into a large open area released into a confined space in close proximity to each other, where there is less chance of where there is more chance of fertilisation will usually occur successfully uniting with female successfully uniting with female gametes gametes Environment for Usually external, in a watery Usually internal, in a very protected Zygote requires a watery zygote environment that is vulnerable to environment inside the female’s environment for development environmental elements such as body. Temperature is controlled and temperature, predation, infection there is less chance of predation, and rapid dispersal from the area infection and loss of zygote from the area Number of Usually a larger number than in A smaller number of offspring than Zygote number is determined by offspring/zygotes internal fertilisation, but many in external fertilisation, because the number of sperm and ova that zygotes perish and so a smaller very few perish (higher success rate) successfully fuse number of offspring survive Breeding More frequent than in internal Seasonal and less frequent than in Breeding frequency depends frequency fertilisation due to the lower external fertilisation due to higher on the requirements of the fertilisation success rate fertilisation success rate and greater species and the favourability of energy costs environmental conditions Parental Usually no parental care Parental care of eggs and/or Parental investment is indirectly investment developing young is more common proportional to the number of gametes produced KEY CONCEPTS Fertilisation is the union of male and female gametes and may take place externally or internally. External fertilisation occurs in aquatic or moist terrestrial environments, to prevent dehydration of gametes. Gametes must be produced in large numbers to ensure success. Internal fertilisation takes place inside the body of the female and involves mate attraction and copulation, which require energy investment and put the organisms at risk of predation, but fewer eggs need to be produced. External fertilisation occurs in most invertebrates and some vertebrates (fish and amphibians). Internal fertilisation occurs in some invertebrates (insects and snails) and most vertebrates (reptiles, mammals and birds). Other mechanisms that increase the chances of survival and continuity of species include nourishment for the developing young and parental care. 42 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 42 08/08/18 6:29 pm  CHECK YOUR 1 Describe the conditions under which asexual reproduction is advantageous, using a named example. UNDERSTANDING 2 What are the advantages and disadvantages of sexual reproduction? 3 Give examples of one vertebrate and one invertebrate that have external fertilisation. Explain how they 2.1b ensure the gametes meet. 4 Give examples of one vertebrate and one invertebrate that have internal fertilisation. Describe the habitat in which they live. 5 Describe the features of internal and external fertilisation that are similar and those that are different. 6 Give two advantages and two disadvantages of internal and external fertilisation. Sexual reproduction in plants Sexual reproduction in plants also relies on the successful fusion of male and female gametes. However, in plants, this fusion is more difficult because plants grow in the ground and cannot move. The broad range of plants on Earth, from less advanced plants such as mosses and ferns to cone-bearing plants (gymnosperms) and flowering plants (angiosperms), have developed a range of reproductive strategies to ensure the continuity of species. These strategies include relying on external agents to carry the gametes from one parent to another, commonly called pollinating agents. Plants also rely on external agents to disperse their seeds (wind, water and animals, for example) and, because they do not have the ability to move away from extreme heat or cold, they have other survival strategies for seeds once they land. As early angiosperms evolved, advantageous features in flowers that resulted in more successful pollination would have been selected by natural means, and these features would have been retained within populations, leading to the great diversity of flowering plants today. To understand the process of sexual reproduction in plants, it is necessary to examine the structure of flowers, which are the reproductive organs of plants. A flower contains either female reproductive parts (carpel or gynoecium), male reproductive parts (stamens) or both, in addition to their non-sexual parts (petals and sepals) (Fig. 2.12). Anther – where pollen grains are formed Stigma – sticky top surface of the flower, to which Filament – stalk that carries pollen adheres. May be Stamen the anther. The length relatively small and smooth (male parts determines whether the (in insect-pollinated plants) of the flower) anthers are contained inside or large and feathered the petals for insect (wind-pollinated plants) Carpel pollination or hang outside (female parts for wind pollination of the flower) Style – joins the stigma to the ovary Petals – a whorl of leaves modified to increase the Ovary–where ovules likelihood of pollination. They are are formed often brightly coloured and scented to attract pollinators, and may have complex shapes to facilitate entry of particular Sepals – a whorl of modified pollinators to the flower leaves, often green. Main function is to protect the Receptacle – reinforced base of unopened bud the flower, which supports the weight of the reproductive structures FIGURE 2.12 Top view and longitudinal section through a flower (male parts labelled in blue and female parts in red) 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 43 02_bio_InFocus12_2e_sb_08851_txt.indd 43 08/08/18 6:29 pm  Stigma In order for fertilisation to occur, the male gametes inside pollen Pollen tube must be carried from the anthers to the female part of a flower, called the stigma. This process of gamete transfer is called pollination. Once pollen has been deposited on the stigma, a pollen tube germinates and grows Style down the style, carrying inside it the male gamete (sperm cell) to an ovule contained in the ovary (Fig. 2.13). In flowering plants, fertilisation occurs internally inside the ovary. Pollination, fertilisation and seed production Ovary Plants are dependent on agents such as wind, water and animals to carry their pollen, from the anthers of one flower to the stigma of a flower either on another plant (cross-pollination), or on the same plant (self-pollination). Cross-pollination ensures greater variation in the offspring. A strategy that is seen in some plants to favour cross- Ovule Placenta pollination rather than self-pollination is for the plant’s pollen to mature at a different time than its stigma. FIGURE 2.13 The pathway During fertilisation, the sperm cell that was transferred by the pollen taken by a pollen tube as it grows from the stigma to the tube fuses with the egg cell (ovum) inside the ovule in the female part of ovary the flower. The fertilised ovule develops, protected within the ovary. The ovule containing an embryo is now termed a seed and the surrounding ovary grows to become a fruit (Fig 2.14). FIGURE 2.14 Meiosis Sexual reproduction life cycle in flowering plants Anther of Pollen stamen grain Egg cell (haploid) Male gametes Ovule Stigma (haploid) of Petal gynoecium Sepal Fertilisation Flower stalk Embryo (diploid) Leaf Seed Seed is dispersed Mitosis Stem Germination of diploid embryo 44 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 44 08/08/18 6:29 pm  Self-pollination versus cross-pollination In plants, self-pollination requires less energy as there is no requirement for the plant to produce structures to attract pollinators, such as brightly coloured petals or nectar. These plants can grow in areas where insects or other animals that visit plants are absent or very few in number. Plants that undergo cross-pollination rely on outside agents to transfer pollen from anthers to stigmas. These may be abiotic agents such as wind or water, or biotic agents such as insects, birds and mammals. As flowers become increasingly specialised, so do their relationships with pollinating agents. Pollination by wind Getty Images/Nnehring Many angiosperms are wind pollinated and their flowers are small, greenish and odourless, with reduced or absent petals. The flowers are grouped together in fairly large numbers and may hang down in tassels that blow around in the wind and shed pollen freely (Fig. 2.15). Many Australian grasses are wind pollinated. In these species the anthers are very long and produce large amounts of light pollen, which is easily picked up by the wind passing over the flowers. Usually the stigmas are also very large and spread out in a feathery manner to trap pollen carried by wind. Wind pollination is very inefficient, so large quantities of pollen are produced. FIGURE 2.15 Small, wind-pollinated grass flowers (Lolium perenne) Different pollen grain structures between species ensure compatibility within the same species. Pollination by animals Flowers that attract animals are more effective in ensuring the transfer of pollen and, therefore, the reproductive success of that species. Special adaptations in the flower are of considerable advantage, because a one-to-one relationship between a plant and an animal species reduces wastage of pollen by ensuring that it is deposited on the correct flower. Animals such as insects, birds and mammals that act as pollinators search in flowers for a reward, such as a meal of nectar (a sugary liquid secreted by nectaries in the flower) or pollen. During this search, pollen rubs onto their bodies and is then transferred to the next flower they visit. Flower scent, colour, markings, shape and nectar are important in attracting animals, all differing between each flower species and adapted to the type of animal they attract (Table 2.2, Fig. 2.16). TABLE 2.2 Comparison of wind-, bird- and insect-pollinated flowers FEATURE OF FLOWER WIND-POLLINATED FLOWERS BIRD-POLLINATED FLOWERS INSECT-POLLINATED FLOWERS Petals Small and inconspicuous, usually Usually large and colourful, red Usually large and colourful (yellow or green or dull in colour or orange, often a tubular shape, blue); may be shaped to encourage sometimes no petals at all specific pollinators Scent Usually absent Rarely fragrant because birds have Often present because insects are little sense of smell highly attracted to scents Nectar None Large amounts of nectar produced in Sometimes produced at base of petals, nectary at base of flower so insect must enter the flower to reach the nectar Anthers Anthers protrude outside the flower, Anthers are commonly lower than Enclosed within flower, commonly so pollen is easily blown off by the stigma, colourful, and may not be lower than stigma wind; abundant pollen is produced enclosed by petals Stigma Stigma protrudes from the flower, Higher than anthers, sometimes Enclosed within flower, sticky, and is often long, feathery and sticky, to not enclosed by petals and often commonly higher than the anthers increase surface area for trapping colourful wind-borne pollen Pollen Very small grains, light and powdery; Sticky or powdery pollen; small Relatively large grains and often sticky; large amounts produced amount produced small amount produced 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 45 02_bio_InFocus12_2e_sb_08851_txt.indd 45 08/08/18 6:29 pm  Some Australian flowers (Callistemon – bottlebrush, Banksia and Grevillea species) are pollinated by small mammals such as bats, possums, pygmy possums and small rodents (Fig. 2.16). Getty Images/Photograph by David Messent Getty Images/AUSCAPE FIGURE 2.16 a b Examples of native animals and insects pollinating flowers; a the red colour of the New South Wales waratah (Telopea speciosissima) attracts birds as pollinators; b a blue-banded bee pollinating a purple flower (bees cannot see red; they are attracted to flowers in the blue and yellow range); Getty Images/Oxford Scientific Getty Images/Oxford Scientific c a hammer orchid c d (Drakaea glyptodon) mimics a female wasp; male wasps act out mating behaviour with the orchid flower, effecting pollination; d the Australian honey possum (Tarsipes rostratus) pollinates flowers while feeding on nectar. Seed dispersal After pollination and fertilisation of the flowers of a plant, seeds (fertilised ovules) from inside the ovary are dispersed (Fig. 2.17). It is an advantage for seeds to be dispersed over a wide distance, as this helps prevent overcrowding and competition for light, water and soil nutrients. Widespread distribution also increases the chances of continuity of the species in other locations, in case there is a sudden change in the local environment, such as fire or disease. Australian native plants have evolved a variety of adaptations to increase the chance of successful dispersal of their seeds. Dispersed by animals Dispersed by wind Self-dispersed Adherent fruits Fleshy fruits Medicago Solanum Asclepias polycarpa dulcamara syriaca Papaver (poppies) Acer Bidens Juniperus saccharum frondosa chinensis Impatiens (balsam) Ranunculus Terminalia Rubus sp. muricatus calamansanai FIGURE 2.17 Adaptations of different types of seeds to facilitate their dispersal 46 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 46 08/08/18 6:29 pm  The success of seed dispersal depends on the type of agent that the plant relies on. Fruits may be dry (such as banksia pods, gum nuts) or fleshy (such as apples, dragon fruit) (Fig. 2.18). Dry fruits often have inbuilt ‘explosive’ mechanisms for dispersal by air, wind or water (abiotic agents). They are usually light so that they can float on air or water. Fleshy fruits often rely on insects, birds or mammals for dispersal (biotic agents) – the animals eat the fruit, move along and then egest the seeds, usually some distance away from the parent plant. a b c d Pod Capsule Follicle Follicle (legume) Separator Seed Winged seed e Shutterstock.com/Africa Studio FIGURE 2.18 Examples of dry and fleshy fruits: a pods or legumes of Acacia; b capsules of Eucalyptus; c follicles of Banksia; d follicle separator (Banksia) – when wet it pulls the two seeds out of the fruit; e the seeds of fleshy fruits such as strawberries, blueberries, kiwifruit and dragonfruit are contained within a fleshy ovary (fruit). Germination The plant embryo inside a seed is in a dehydrated form and is dormant, to allow the seed to survive adverse conditions. If the seed lands in suitable soil that provides sufficient water, oxygen and warmth, it germinates – that is, the embryo begins to grow, producing a radicle or young root to absorb water and soil nutrients, as well as a plumule or young stem, which develops green leaves for food production by photosynthesis. Once the seedling becomes established, it grows and develops into an adult plant that can begin the reproductive cycle once again. KEY CONCEPTS Sexual reproduction in plants involves external agents of pollination and seed dispersal, such as wind, water, fire (abiotic) and animals (biotic). Pollination mechanisms in plants include self-pollination (to ensure survival if reproductive partners are scarce) and cross-pollination (to increase genetic variation and ensure survival if a sudden environmental change such as disease or drought occurs). The life cycle of a plant involves pollination, fertilisation, seed dispersal and germination. Seed dispersal relies on the type of fruit in which the seeds occur matching the type of dispersal agent available in that environment. 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 47 02_bio_InFocus12_2e_sb_08851_txt.indd 47 08/08/18 6:29 pm  Advantages of sexual reproduction: – New gene combinations are created and so, by selection, some individuals may survive in the face of sudden environmental change. – Harmful mutations may be removed from the population. – Offspring may differ in their requirements due to the variation in the population and so there may be less competition for the same resources among offspring. Disadvantages of sexual reproduction: – It is costly in terms of time, energy and bodily resources. – Two organisms are required. CHECK YOUR UNDERSTANDING 1 List the male reproductive parts of a flower and describe the function of each part. 2 Draw and label the female reproductive parts of a flower and describe the function of each part. 2.1c 3 Distinguish between pollination and fertilisation in plant reproduction. 4 Describe the pollination mechanism of two named Australian plants. 5 Identify two ways in which the reproductive structures differ between wind-pollinated and insect- pollinated plants. 6 Name the structure that each of the following develops into after fertilisation: ovum, seed, ovary wall. 7 Identify two methods of seed dispersal in named Australian plants. Asexual reproduction – 2.2 only one parent Asexual reproduction means not sexual reproduction. The name tells us that it does not involve gametes (sex cells). Only one parent is required and all the genetic material in the offspring is passed down from this single parent. The result is that offspring are genetically identical to the parent and to each other. There is no production or fusion of gametes, and no mixing of genetic information to introduce variation. This has advantages and disadvantages. In unicellular organisms, asexual reproduction is the main form of reproduction. In multicellular organisms, sexual reproduction is more common. The advantage of asexual reproduction is that it enables organisms to reproduce quickly without having to find a mating partner. For plants, which are immobile, finding a mate is complicated. Being genetically identical may give organisms a competitive advantage if they live in an environment to which they are particularly well adapted. Asexual reproduction among plants is more common in harsh environments where organisms are so specific that there is little benefit in having variation within the population. In these habitats, when favourable conditions arise suddenly, organisms can reproduce quickly and effectively. Selective pressures that make asexual reproduction more effective than sexual reproduction include: ◗ a shortage of food and/or other resources – asexual reproduction uses less energy to produce offspring ◗ a small mating population and/or time and other constraints on finding a mate – only one parent is required for asexual reproduction. The main disadvantage of asexual reproduction is that, with little or no variation in the population, the whole group (or species) is particularly vulnerable to sudden changes in the environment, such as drought, disease, or a new parasite or predator. Changes such as these may result in the survival of few, if any, individuals. Many organisms, including protists, fungi and plants, alternate between sexual and asexual reproduction as a normal part of their life cycle. This mechanism, known as an alternation of generations, Alternation of generations in involves a sexually reproducing, gamete-bearing generation alternating with an asexually reproducing, plants spore-bearing generation. 48 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 48 08/08/18 6:29 pm  Asexual reproduction in plants Have you ever planted the cut-off tops of carrots, beetroot or strawberries and discovered that they produce roots and grow into new plants? Or perhaps you have seen the runners that grow out of the ground in strawberry patches or around raspberry bushes. These are examples of asexual reproduction. New individuals arise from portions of the roots, stems, leaves or buds of adult individuals and are genetically identical to their parent. This type of asexual reproductive process is referred to as vegetative propagation. Vegetative propagation Some adult plants produce vegetative organs, such as Getty Images/Studeo-Chase bulbs, tubers, rhizomes and suckers, from which new plants can arise. This is equivalent to cloning an adult plant, as the offspring are genetically identical to the parent. Perennating organs (from the word perennial, meaning returning year after year) are underground organs such as roots or stems that contain enough stored food to sustain the plant in a dormant state, from one season to the next. These organs allow plants to survive adverse conditions such as extreme cold in winter or drought in summer. Even though the parts of the plant above the ground may die, the remaining underground organs develop buds that begin to grow once favourable conditions return. Buds on deciduous trees are also considered to be organs of perennation. In addition to being a way of surviving from one FIGURE 2.19 Potatoes develop tubers from swollen year to the next, perennating organs, when separated, regions of the stem. New plants may also grow from give rise to new plants and so they are a form of asexual the buds (‘eyes’) of a potato. reproduction. Gardeners often exploit this by splitting bulbs, cutting up rhizomes or runners, and taking cuttings of shoots from stems to grow new plants. Vegetative propagation techniques are often used in agriculture – in growing perennial crops such as seedless grapes, watermelons and mangoes. This increases the production of crops when seeds are unavailable and/or difficult to germinate. Asexual reproduction is also used if plants have specific desirable traits that farmers want to perpetuate in future generations. Some common naturally occurring examples of vegetative propagation are described below. Runners – modified stems Runners are long, thin, modified stems that grow along Dreamstime.com/Anne Bradley the surface of the soil. In the cultivated strawberry, for It is important example, leaves, flowers and roots are produced at every to identify the alternate node on the stem runner. Just beyond each organ that has the modification. second node, the tip of the node turns up and thickens, For example, producing new roots and a new shoot that continues the runner is a modified stem, the runner. Another example is spinifex (Fig. 2.20), a and the sucker, grass that has long stems that grow horizontally along on page 50, is a modified root. the surface of the soil. At each node, leaves and roots are produced and so the runner can be subdivided into FIGURE 2.20 Spinifex is a type of grass (Spinifex new plants. Spinifex ensures its survival by sending out hirsutus) with surface stems (runners) producing new leaves and roots (plantlets) at each node. runners in harsh dune conditions such as high wind erosion, high salinity and high temperatures. 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 49 02_bio_InFocus12_2e_sb_08851_txt.indd 49 08/08/18 6:29 pm  Rhizomes – modified stems Shutterstock.com/postolit Underground horizontal modified stems, or rhizomes, are characteristic of ginger, ferns such as bracken fern, and many grasses. They can give rise to a new shoot at each node. Gardeners often propagate ferns by splitting the rhizomes (Fig. 2.21). Suckers – modified roots The roots of some plants produce modified roots called suckers or sprouts, which give rise to new plants. Trees and shrubs that sucker, such as reeds, wattles and blackberries, can spread quickly into a vacant patch of habitat after disturbance. The colony wattle (Acacia murrayana) (Fig. 2.22) sends up shoots from the outer roots and these grow into separate plants if the parent shrub dies. This allows for rapid regrowth after a decline in numbers (after a bushfire or a drought, for example). Apomixis Some plants are able to produce offspring from special generative tissues, without involving fertilisation FIGURE 2.21 or the production of seeds, a form of reproduction known as apomixis. This ‘generative tissue’ may be New fern plants can grow when in the form of gametes, such as in unfertilised ovules, or non-reproductive tissue such as leaf tissue the rhizome (stem) (Fig. 2.23). It is termed generative tissue because it gives rise to plantlets that can produce asexual seeds. is fragmented at the nodes. Apomixis is seen in plants such as kangaroo grass (Themeda triandra), lemon and orange trees (Citrus) and dandelions. Plantlets such as those that arise on leaves (Fig. 2.23) and their seeds grow into individuals that are genetically identical to their parent. The advantages of apomixis are that multiplication is rapid and the plantlets are able to produce seeds, increasing seed dispersal, an adaptation usually associated only with sexual reproduction. The disadvantage is the lack of variation that is typical of reproduction that involves two parents. Apomixis also includes parthenogenesis in animals, a type of reproduction in which the new individual develops from an unfertilised egg produced by a female. You will have the opportunity to research this further in Investigation 2.2. Shutterstock.com/TIPAKORN MAKORNSEN Alamy Stock Photo/Infinity FIGURE 2.22 The colony wattle (Acacia murrayana) regenerates after FIGURE 2.23 Asexual reproduction by apomixis: Kalanchoe plantlets drought or bushfire. budding along the leaf margins TABLE 2.3 Examples of the advantages of asexual reproduction EXAMPLE ASEXUAL REPRODUCTION MECHANISM WHY IT IS AN ADVANTAGE Spinifex grass Stem runners put out leaves and Enables reproduction in harsh conditions, requires roots at nodes along the ground less energy to reproduce by runner, and is very rapid Colony wattle Shoots grow from outer roots Rapid, and large numbers can be reproduced (suckers) and develop into quickly, an advantage when rapid recovery is needed separate plants after a decline in numbers (e.g. fire or drought) 50 MODULE FIVE » HEREDITY 9780170408851 02_bio_InFocus12_2e_sb_08851_txt.indd 50 08/08/18 6:30 pm  INVESTIGATION 2.1 A practical and secondary-source investigation to examine organs of perennation in plants You are required to: examine organs of perennation in plants growing in your local area and/or in your garden at home Literacy conduct secondary-source research using a variety of sources to identify different types of perennating organs and how the modifications are adaptations to promote the continuity of the species present your findings in the form of a poster. The poster you create will detail two examples of perennating organs that allow asexual reproduction in plants. The examples you select must include no more than one of each organ – root, stem or leaf. You may not use the organ outlined in the worked example below. In the description on your poster, include: type of organ a scientifically named plant in which the organ occurs a labelled diagram of the organ, identifying plant tissues that: – store food – contain buds or other structures that give rise to new plants. a table in which you: – explain how the organ of perennation gives rise to new plants – discuss the advantages and disadvantages of the organ of perennation in terms of the survival of the species. Some organs of perennation that you may wish to consider are: bulbs, corms and tubers epicormic buds organs of apomixis. rhizomes runners and suckers WORKED EXAMPLE 2.1 ANNOTATED DIAGRAM AND DESCRIPTION OF AN ORGAN OF PERENNATION Annotate a diagram of a perennating organ and explain the advantage of the organ of perennation to the survival of the species. ANSWER LOGIC Leaves Draw or insert a diagram. Label all parts of the organ of perennation. Bulb Scale leaves Annotate the part that stores food. Main bud store food Basal plate Axillary buds (stem) give rise to Annotate the part that contains buds. Roots new plants Plant: Allium cepa Name the plant and the organ of FIGURE 2.24 Stem bulb: onion (organ of perennation) perennation. 9780170408851 CHAPTER 2 » SEXUAL AND ASEXUAL REPRODUCTION 51 02_bio_InFocus12_2e_sb_08851_txt.indd 51 08/08/18 6:30 pm  ANSWER LOGIC An onion is a stem bulb with fleshy leaves. The stem is reduced to a Describe the organ of perennation short disc, the basal plate. Roots grow on the lower surface of the disc and leaf bases on the upper surface. Axillary buds occur at nodes where the leaves attach, and these Describe where the buds are. buds can develop into new bulbs. If an onion bulb is planted, within a year or two there will be several Explain the advantage of the organ of bulbs in that place. These can be separated and re-planted to perennation to the survival of the species. produce new plants. Scientists often use Asexual reproduction in other organisms yeast for genome studies, because It is not only members of the plant kingdom that undergo asexual reproduction. This type of reproduction yeast is a simple eukaryote with is common in other organisms where it is advantageous for the parent and young to be genetically similar cellular identical. Mechanisms of asexual reproduction in animals include budding, binary fission, sporogenesis, organisation and processes to fragmentation and parthenogenesis. those of complex multicellular Budding organisms such as humans. The yeast In reproductive budding, an adult organism gives rise to a small bud, which separates from the parent and genome was the first to be sequenced grows into a new individual. (in 1996). Yeast are microscopic unicellular organisms that are a classified as fungi, along with macroscopic moulds and Science Photo Library/Eye of Science mushrooms. There are hundreds of species of yeast; those with which you may be familiar are baker’s and brewer’s yeast (different strains of the same species, Saccharomyces cerevisiae) and the less useful Candida albicans, which causes digestive disorders and thrush in humans. The cells of baker’s yeast are ov