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Chapter 46

BIOL 1030 Chapter 46: Chapter 46 Animal Reproduction

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Biological Sciences
BIOL 1030
Scott Kevin

Chapter 46 Animal Reproduction Lecture Outline Overview: Doubling Up for Sexual Reproduction Concept 46.1 Both asexual and sexual reproduction occur in the animal kingdom • Asexual reproduction involves the formation of individuals whose genes come from a single parent. • There is no fusion of sperm and egg. • Sexual reproduction is the formation of offspring by the fusion of haploid gametes to form a diploid zygote. • The female gamete, the unfertilized egg, or ovum, is usually large and nonmotile. • The male gamete is the sperm, which is usually small and motile. • Sexual reproduction increases genetic variation among offspring by generating unique combinations of genes inherited from two parents. Diverse mechanisms of asexual reproduction enable animals to produce identical offspring rapidly. • Many invertebrates can reproduce asexually by fission, in which a parent separates into two or more approximately equal-sized individuals. • Budding is also common among invertebrates. This is a form of asexual reproduction in which new individuals split off from existing ones. • In fragmentation, the body breaks into several pieces, some or all of which develop into complete adults. • Reproducing in this way requires regeneration of lost body parts. • Many animals can also replace new appendages by regeneration. • Asexual reproduction has a number of advantages. • It allows isolated animals to reproduce without needing to find a mate. • It can create numerous offspring in a short period of time. • In stable environments, it allows for the perpetuation of successful genotypes. Reproductive cycles and patterns vary extensively among mammals. • Most animals exhibit cycles in reproductive activity, usually related to changing seasons. • This allows animals to conserve resources and reproduce when more energy is available and when environmental conditions favor the survival of offspring. • Reproductive cycles are controlled by a combination of environmental and hormonal cues. • Environmental cues may include seasonal temperature, rainfall, day length, and lunar cycles. • Animals may reproduce exclusively asexually or sexually or they may alternate between the two modes, depending on environmental conditions. • Daphnia reproduce by parthenogenesis under favorable conditions and sexually during times of environmental stress. • Parthenogenesis is the process by which an unfertilized egg develops without being fertilized. • Parthenogenesis plays a role in the social organization of some bees, wasps, and ants. • Male honeybees (drones) are haploid, and female honeybees (queens and workers) are diploid. • Several genera of fishes, amphibians, and lizards reproduce by a form of parthenogenesis that produces diploid “zygotes.” • Fifteen species of whiptail lizards reproduce exclusively by parthenogenesis. • There are no males in this species, but the lizards imitate courtship and mating behavior typical of sexual species of the same genus. • Sexual reproduction presents a problem for sessile or burrowing animals or parasites that may have difficulty encountering a member of the opposite sex. • One solution is hermaphroditism, in which one individual functions as both a male and a female. • Some hermaphrodites can self-fertilize, but most mate with another member of the same species. • In such matings, each individual receives and donates sperm. • This results in twice as many offspring as would be produced if only one set of eggs were fertilized. • In sequential hermaphroditism, an individual reverses its sex during its lifetime. • In some species, the sequential hermaphrodite is female first. • In other species, the sequential hermaphrodite is male first. Concept 46.2 Fertilization depends on mechanisms that help sperm meet eggs of the same species • The mechanisms of fertilization, the union of sperm and egg, play an important part in sexual reproduction. • In external fertilization, eggs are released by the female into a wet environment, where they are fertilized by the male. • In species with internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract. • A moist habitat is almost always required for external fertilization, both to prevent gametes from drying out and to allow the sperm to swim to the eggs. • In species with external fertilization, timing is crucial to ensure that mature sperm encounter ripe eggs. • Environmental cues such as temperature or day length may cause gamete release by the whole population. • Individuals may engage in courtship behavior that leads to fertilization of the eggs of one female by one male. • Internal fertilization is an adaptation to terrestrial life that enables sperm to reach an egg in a dry environment. • Internal fertilization requires sophisticated reproductive systems, including copulatory organs that deliver sperm and receptacles for their storage and transport to ripe eggs. • Mating animals may use pheromones, chemical signals released by one organism that influence the behavior or physiology of other individuals of the same species. • Pheromones are small, volatile, or water-soluble molecules that disperse into the environment. • Like hormones, pheromones are active in minute amounts. • Many pheromones act as male attractants. • All species produce more offspring than can survive to reproduce. • Internal fertilization usually involves the production of fewer zygotes than does external fertilization. • However, the survival rate is higher for internal fertilization. • Major types of protection include tough eggshells, development of the embryo within the reproductive tract of the mother, and parental care of the eggs and offspring. • Marsupial mammals retain their embryos for only a short period in the uterus. • The embryos crawl out and complete fetal development attached to a mammary gland in the mother’s pouch. • The embryos of eutherian mammals develop entirely within the uterus, nourished through the placenta. • Parental care of offspring can occur regardless of whether fertilization is external or internal. Reproductive systems produce gametes and make them available to gametes of the opposite sex. • The least complex reproductive systems lack gonads, the organs that produce gametes in most animals. • Polychaete worms lack gonads. Eggs and sperm develop from undifferentiated cells lining the coelom. • As the gametes mature, they are released from the body wall and fill the coelom. • In some species, the body splits open to release the gametes, killing the parent. • Some reproductive systems, such as those of parasitic flatworms, are very complex. • Most insects have separate sexes with complex reproductive systems. • In many species, the female reproductive system includes a spermatheca, a sac in which sperm may be stored for a year or more. • The basic plan of all vertebrate reproductive systems is very similar. • However, there are variations. • In many nonmammalian vertebrates, the digestive, excretory, and reproductive systems share a common opening to the outside, the cloaca. • Mammals have separate openings for the digestive and reproductive systems. • Female mammals also have separate openings for the excretory and reproductive systems. • The uterus of most vertebrates is partly or completely divided into two chambers. • Male reproductive systems differ mainly in copulatory organs. • Many mammalian vertebrates do not have a well- developed penis and simply turn the cloaca inside out to ejaculate. Concept 46.3 Reproductive organs produce and transport gametes: focus on humans Human reproduction involves intricate anatomy and complex behavior. • The reproductive anatomy of the human female includes external and internal reproductive structures. • External reproductive structures consist of two sets of labia surrounding the clitoris and vaginal opening. • Internal reproductive organs consist of a pair of gonads and a system of ducts and chambers. • The role of the ducts and chambers is to conduct the gametes and house the embryo and fetus. • The ovaries, the female gonads, lie in the abdominal cavity, attached to the uterus by a mesentery. • Each ovary is enclosed in a tough protective capsule and contains many follicles. • Each follicle consists of one egg cell surrounded by one or more layers of follicle cells. • A woman is born with about 400,000 follicles. • Only several hundred of these will release eggs during a female’s reproductive years. • Follicles produce the primary female sex hormones, estrogens. • Usually one follicle matures and releases its egg during each menstrual cycle in the process of ovulation. • After ovulation, the remaining follicular tissue develops into the corpus luteum. • The corpus luteum secretes additional estrogens and progesterone, hormones that help maintain the uterine lining during pregnancy. • If pregnancy does not occur, the corpus luteum disintegrates and a new follicle matures during the next cycle. • At ovulation, the egg is released into the abdominal cavity near the opening of the oviduct. • The cilia-lined funnel-like opening of the oviduct draws in the egg. • Cilia convey the egg through the oviduct to the uterus. • The highly vascularized inner lining of the uterus is called the endometrium. • The neck of the uterus, the cervix, opens into the vagina. • The vagina is a thin-walled chamber that forms the birth canal and is the repository for sperm during copulation. • It opens to the outside at the vulva, the collective term for the external female genitalia. • The vaginal opening is partially covered by a thin sheet of tissue called the hymen. • The vaginal and urethral openings are located within a recess called the vestibule. • The vestibule is surrounded by a pair of slender folds called the labia minora. • The labia majora enclose and protect the labia minora and vestibule. • The clitoris is found at the front edge of the vestibule. • During sexual arousal, the clitoris, vagina, and labia engorge with blood and enlarge. • During sexual arousal, Bartholin’s glands secrete mucus into the vestibule, providing lubrication and facilitating intercourse. • Mammary glands are present in both males and females but normally function only in females. • They are not a component of the human reproductive system but are important to mammalian reproduction. • Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts opening at the nipple. • Adipose tissue forms the main mass of the mammary gland of a nonlactating mammal. • The low estrogen level in males prevents the development of the sensory apparatus and fat deposits, so that male breasts remain small, with nipples unconnected to the ducts. • The male’s external reproductive organs consist of the scrotum and penis. • The internal reproductive organs consist of gonads that produce sperm and hormones, accessory glands that secrete products essential to sperm movement, and ducts to carry the sperm and glandular secretions. • The male gonads, or testes, consist of highly coiled tubes surrounded by layers of connective tissue. • The tubes are seminiferous tubules, where sperm are produced. • Leydig cells scattered between the seminiferous tubules produce testosterone and other androgens. • The scrotum, a fold in the body wall, holds the testes outside the body cavity at a temperature about 2°C below that of the abdomen. • This keeps testicular temperature cooler than that in the body cavity. • The testes develop in the body cavity and descend into the scrotum just before birth. • From the seminiferous tubules of the testes, the sperm pass through the coiled tubules of the epididymis. • As they pass through this duct, sperm become motile and gain the ability to fertilize an egg. • Ejaculation propels sperm from the epididymis to the vas deferens. • The vas deferens run from the scrotum and behind the urinary bladder. • Each vas deferens joins with a duct from the seminal vesicle to form an ejaculatory duct. • The ejaculatory ducts open into the urethra. • The urethra drains both the excretory and reproductive systems. • Accessory sex glands add secretions to semen. • A pair of seminal vesicles contributes about 60% of total semen volume. • Seminal fluid is thick, yellowish, and alkaline. • It contains mucus, fructose, a coagulating enzyme, ascorbic acid, and prostaglandins. • The prostate gland secretes directly into the urethra. • Prostatic fluid is thin and milky. • This fluid contains anticoagulant enzymes and citrate. • Prostate problems are common in males older than 40. • Benign prostate enlargement occurs in virtually all males older than 70. • Prostate cancer is one of the most common cancers in men. • The bulbourethral glands are a pair of small glands along the urethra below the prostate. • Prior to ejaculation, they secrete clear mucus that neutralizes any acidic urine remaining in the urethra. • Bulbourethral fluid also carries some sperm released before ejaculation. • This is one of the reasons the withdrawal method of birth control has a high failure rate. • A male usually ejaculates about 2–5 mL of semen, with each milliliter containing about 50–130 million sperm. • Once in the female reproductive tract, prostaglandins in semen thin the mucus at the opening of the uterus and stimulate uterine contractions that help move the semen. • When ejaculated, semen coagulates, making it easier for uterine contractions to move it along. • Anticoagulants then liquefy the semen, and the sperm begin swimming. • The alkalinity of semen helps neutralize the acidic environment of the vagina, protecting the sperm and increasing their motility. • The human penis is composed of three layers of spongy erectile tissue. • During sexual arousal, the erectile tissue fills with blood from arteries. • The resultant increased pressure seals off the veins that drain the penis, causing it to engorge with blood. • The engorgement of the penis with blood causes an erection, which is essential for the insertion of the penis into the vagina. • The penis of some mammals possesses a baculum, a bone that helps stiffen the penis. • Temporary impotence can result from the consumption of alcohol or other drugs, and from emotional problems. • Irreversible impotence due to nervous system or circulatory problems can be treated with drugs and penile implant devices. • The oral drug Viagra acts by promoting the action of nitric oxide, enhancing relaxation of smooth muscles in the blood vessels of the penis. • This allows blood to enter the erectile tissue and sustain an erection. • The main shaft of the penis is covered by relatively thick skin. • The sensitive head, or glans penis, is covered by thinner skin. • The glans is covered by the foreskin, or prepuce, which may be removed by circumcision. • There is no verifiable health benefit to circumcision, which arose from religious tradition. Human sexual response is very complex. • Human arousal involves a variety of psychological and physical factors. • Human sexual response is characterized by a common physiological pattern. • Two types of physiological reaction predominate in both sexes: 1. Vasocongestion, filling of tissue with blood, is caused by increased blood flow. 2. Myotonia is increased muscle tension. • Both smooth and skeletal muscle may show sustained or rhythmic contractions. • The sexual response can be divided into four phases: excitement, plateau, orgasm, and resolution. • Excitement prepares the vagina and penis for coitus. • Vasocongestion is evident in the erection of the penis and clitoris; the enlargement of the testes, labia, and breasts; and vaginal lubrication. • Myotonia may result in nipple erection or tension in the arms and legs. • In the plateau phase, these responses continue. • Stimulation by the autonomic nervous system increases breathing and heart rate. • In females, plateau includes vasocongestion of the outer third of the vagina, expansion of the inner two-thirds of the vagina, and elevation of the uterus to form a depression that receives sperm at the back of the vagina. • Orgasm is the shortest phase of the sexual response cycle. • It is characterized by rhythmic, involuntary contractions of the reproductive structures in both sexes. • In male orgasm, emission is the contraction of the glands and ducts of the reproductive tract, which forces semen into the urethra. • Ejaculation occurs with the contraction of the urethra and expulsion of semen. • In female orgasm, the uterus and outer vagina contract. • Resolution completes the cycle and reverses the responses of earlier stages. • Vasocongested organs return to their normal sizes and colors; muscles relax. Concept 46.4 In humans and other mammals, a complex interplay of hormones regulates gametogenesis Spermatogenesis and oogenesis both involve meiosis but differ in three significant ways. • Gametogenesis is based on meiosis. • Spermatogenesis is the production of mature sperm cells from spermatogonia. • Spermatogenesis is a continuous and prolific process in the adult male. • Each ejaculation contains 100–650 million sperm. • Spermatogenesis occurs in seminiferous tubules. • Primordial germ cells of the embryonic testes differentiate into spermatogonia, the stem cells that give rise to sperm. • As spermatogonia differentiate into spermatocytes and then into spermatids, meiosis reduces the chromosome number from diploid to haploid. • As spermatogenesis progresses, the developing sperm cells move from the wall to the lumen of a seminiferous tubule and then to the epididymis, where they become motile. • The structure of sperm fits its function. • A head containing the haploid nucleus is tipped with an acrosome, which contains enzymes that help the sperm penetrate to the egg. • Behind the head are a large number of mitochondria (or a single large one) that provide ATP to power the flagellum. • Oogenesis is the production of ova from oogonia. • Oogenesis differs from spermatogenesis in three major ways. 1. At birth an ovary may contain all
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