Animal Reproduction

1. Animal Reproduction Chapter 41

2. How Do Animals Reproduce? • Sexually or asexually • Asexual reproduction -a single animal produces offspring, usually through repeated mitotic cell division • The offspring are genetically identical to the parent • Sexual reproduction - gonads produce haploid sperm or eggs through meiotic cell division • A sperm and fuse to produce a diploid fertilized egg, undergoes repeated mitotic cell divisions to produce an offspring • Not genetically identical to either parent

3. Asexual Reproduction • Advantages • Efficient in effort (no need to search for mates, courting, or battle rivals), materials (no wasted gametes), and genes (the offspring have all the genes of their parent) • Disadvantages • Genetically identical

4. Types of Asexual Reproduction • Budding produces a miniature version of the adult • The bud grows directly on the body of the adult • When large enough, it breaks off • Sponges and cnidarians

5. Types of Asexual Reproduction • Fission followed by regeneration can produce a new individual • Regeneration - the ability to re-grow lost body parts • Regeneration is part of reproduction in species that reproduce by fission • Annelids, flatworms, corals, sea jellies, brittle stars divide into pieces, each regenerates the missing parts of a complete body

6. Types of Asexual Reproduction • Parthenogenesis - eggs develop without fertilization • Some parthenogenetically produced offspring are haploid • Male honeybees develop from unfertilized haploid eggs and remain haploid; their diploid sisters develop from fertilized eggs • Some fish, amphibians, reptiles produce diploid offspring by doubling the number of chromosomes in the eggs, either before or after meiosis • The whiptail of the Southwestern US and Mexico, have populations consisting entirely of parthenogenetically reproducing females • Aphids, can reproduce either sexually or parthenogenetically, depending on environmental factors such as the season and availability of food

7. The Whiptail Lizard

8. A Female Aphid Gives Birth adult female aphid offspring being born

9. Sexual Reproduction • Requires the union of sperm and egg • No one is certain why sexual reproduction evolved or why natural selection has made it the most common form of reproduction in animals • Advantage - The genetic recombination creates new genotypes and phenotypes that are an important source of variation upon which natural selection may act • Disadvantage - not efficient in effort (need to search for mates, courting,& battle rivals), materials (wasted gametes)

10. Defining Male and Female • In most animals species, an individual is either male or female, defined by the type of gamete produced • Female gonad - ovary, produces eggs • Large, nonmotile haploid cells containing food reserves to provide nourishment for the embryo – an offspring in its early stages of development before birth or hatching • Male gonad - testis (plural, testes), produces small, motile haploid sperm, which have little cytoplasm and no food reserves • Fertilization - union of sperm and egg, forms a diploid zygote

11. Hermaphrodites • Hermaphrodites produce both sperm and eggs - still engage in sex, two individuals exchange sperm • Earthworms and snails • Some hermaphrodites fertilize their own eggs • Tapeworms and some snails are relatively immobile and find themselves isolated from others of the species • Self-fertilization is an advantage • For species with two separate sexes and hermaphrodites that cannot self-fertilize, successful reproduction requires that sperm and eggs from different animals join for fertilization

12. Hermaphroditic Earthworms Exchange Sperm

13. External Fertilization • Occurs outside the parents’ bodies • Spawning - sperm and eggs are released into water and the sperm swim to the eggs • Spawning animals must synchronize their reproductive behaviors both temporally and spatially • Synchronization is achieved through environmental cues like chemical signals, courtship behaviors, or some combination • Seasonal changes in day length stimulate the physiological changes required for breeding • More precise synchrony is required to coordinate the actual release of sperm and eggs • Coral species of Australia’s Great Barrier Reef synchronize spawning by the phase of the moon, simultaneously releasing sperm and eggs

14. Coral Spawning

15. It is about Chemistry… • Some animals communicate sexual readiness by sending visual, acoustic, or chemical signals • Female simultaneously releases eggs and a sex pheromone into the water • Nearby male detect the sex pheromone and release sperm • In some animals, the sperm are also lured by a chemical attractant produced by the eggs

16. Courtship Rituals • Synchronized timing alone does not guarantee efficient reproduction • In mobile animals, both temporal and spatial synchrony can be ensured by mating behaviors • Most fish have some form of courtship ritual in which the male and female come close together and release their gametes in the same place, at the same time • Frogs and toads assume a characteristic mating pose called amplexus • In shallow water male mounts female and prods the sides of her abdomen • This stimulates her to extrude her eggs, which he fertilizes by releasing sperm from above

17. Courtship Rituals Synchronize Mating Beta mating Ritual

18. Internal Fertilization Sperm are placed within the female’s reproductive tract, where the egg is fertilized An important adaptation to terrestrial life because sperm quickly die if they dry In aquatic environments internal fertilization may increase the likelihood of success because sperm and eggs are confined to a small space rather than dispersed in a large volume of water

19. Copulation & Spermatophores • Occurs by copulation – male deposits sperm directly into the female’s reproductive tract • Some males package their sperm in a container called a spermatophore • Scorpions, grasshoppers, salamanders • The male drops a spermatophore on the ground, and if a female finds it, she fertilizes herself by inserting it into her reproductive cavity, where the sperm are released

20. Sperm and Eggs are Shortlived • In most animals, neither sperm nor eggs live very long • Successful internal fertilization requires that ovulation, the release of a mature egg cell from the ovary of the female, occurs shortly before or soon after sperm are deposited in the female’s reproductive tract • Most mammals copulate when the female signals readiness to mate, usually occurs about the same time as ovulation • In rabbits, courtship and mating stimulate ovulation, so new healthy sperm and eggs are released • Some female snails and insects have short-lived eggs, but males produce sperm that remain functional for days to months • The female stores the sperm in a special sac inside her body and releases sperm whenever she produces eggs

21. Humans and Mammals • Have separate sexes, copulate, fertilize eggs internally • Although most mammals reproduce only during certain seasons of the year, and produce sperm and eggs only at that time, human reproduction is not restricted by season • Men produce sperm continuously, and women ovulate monthly • The ability to reproduce begins at puberty • Sexual maturation in humans occurs at puberty, a stage of development characterized by rapid growth and appearance of secondary sexual characteristics • Generally begins in the early teens, it can start as early as 8 or as late as 15

22. Puberty and Hormones • Brain maturation causes the hypothalamus to release gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to produce luteinizing hormone (LH) and follicle-stimulating hormone (FSH) • LH and FSH derive their names from functions in females, but equally essential in males • LH and FSH stimulate testes to produce testosterone and ovaries to produce estrogen

23. Testosterone • In response to increases in testosterone, males develop secondary sexual characteristics • The penis and testes enlarge • Pubic, underarm, and facial hair appears • The larynx enlarges, deepening the voice • Muscular development increases

24. Estrogen • In response to estrogen and other hormones that surge at puberty, secondary sexual characteristics develop • The breasts enlarge • The hips become wider • Pubic and underarm hair appears • Menstruation begins • Courtship behaviors start to appear in both sexes

25. Male Reproductive System • The testes and accessory structures • The testes produce testosterone and sperm • Several glands and ducts secrete substances that activate, nourish, store, and conduct sperm to the female reproductive system

26. The Human Male Reproductive System pubic bone urinary bladder ureter rectum seminal vesicle prostate gland urethra penis vas deferens bulbourethral gland epididymis testis urethral opening scrotum

27. Animation: Male Reproductive Anatomy

29. Sperm is produced in the testes • Scrotum - pouch that hangs outside the body cavity • This keeps the testes about 1º - 6º F cooler than the core of the body, depending • Cooler temperatures promote sperm development • Coiled, hollow seminiferous tubules are where sperm are produced, fill each testis • Interstitial cells synthesize testosterone, located in the spaces between the tubules

30. The Anatomy of the Testes vas deferens epididymis uncoiled seminiferous tubule testis (a) A section through the testis spermatogonia daughter cells sperm Sertoli cells interstitial cell (b) Cross-section of a seminiferous tubule

31. Inside the Seminiferous Tubules • Inside the seminiferous tubule lie cells - spermatogonia that give rise to sperm, and larger Sertoli cells that nourish the developing sperm and regulate their growth • Diploid cells undergo mitosis to form two daughter cells • One daughter cell remains a spermatogonium, ensuring a steady supply throughout life • The other daughter cell undergoes spermatogenesis, a developmental process that produces haploid sperm

32. Spermatogenesis The daughter cell differentiates into a primary spermatocyte - a large diploid cell that undergoes meiosis At the end of meiosis I, each primary spermatocyte gives rise to two haploid secondary spermatocytes Each secondary spermatocyte undergoes meiosis II, producing two spermatids; each primary spermatocyte generates four spermatids Spermatids differentiate into sperm without further cell division Spermatogonia, spermatocytes, spermatids are enfolded in Sertoli cells As spermatogenesis proceeds, developing sperm migrate to the central cavity of the seminiferous tubule, and are released

33. Animation: Sperm Maturation

34. Spermatogenesis occurs continuously starting at puberty spermatogonium primary spermatocyte secondary spermatocytes spermatids sperm Mitosis Meiosis II Differentiation Meiosis I

35. Human Sperm Very little cytoplasm, the nucleus fills the sperm’s head Acrosome – atop the nucleus - a specialized lysosome containing enzymes that dissolve layers around the egg Midpiece - packed with mitochondria Mitochondria provide the energy needed to move the tail Whiplike movements propel the sperm

36. A Human Sperm Cell acrosome nucleus tail sheath mitochondria Head Midpiece flagellum Tail

37. Hormones that Regulate Spermatogenesis • GnRH from the hypothalamus stimulates the anterior pituitary to produce LH and FSH • LH stimulates the interstitial cells to produce testosterone • In combination with FSH, testosterone stimulates the Sertoli cells and promotes spermatogenesis

38. Testicular function is regulated by negative feedback • Testosterone inhibits the release of GnRH, LH, and FSH • The Sertoli cells are stimulated by FSH and testosterone to secrete inhibin, which inhibits FSH production by the anterior pituitary • This complex feedback process maintains constant levels of testosterone and sperm production

39. Accessory Structures • Seminiferous tubules form the epididymis, a long, continuous, folded tube • Epididymis leads to the vas deferens, a tubule that carries sperm out of the scrotum • Vas deferens joins the urethra, which conducts both urine and sperm • The roughly hundred million sperm produced by a human male each day are stored in the vas deferens and epididymis

40. Semen • Semen is ejaculated from the penis - 5% sperm, mixed with secretions that empty into the vas deferens and urethra: • The seminal vesicles • The prostate gland • The bulbourethral glands

41. Seminal Vesicles • Paired seminal vesicles make up 60% of the semen • Rich in fructose, that provides energy for the sperm • Slightly alkaline pH protects the sperm from the acidity of urine in the man’s urethra and the woman’s vagina • It also contains prostaglandins, which stimulate uterine contractions to transport the sperm up the female reproductive tract

42. Prostate Gland • Produces an alkaline, nutrient-rich secretion, about 30% of the semen volume • Includes enzymes that increase the fluidity of the semen after it is released into the vagina, allowing sperm to swim more freely

43. Bulbourethral Glands Paired bulbourethral glands secrete a small amount of alkaline mucus in the urethra, neutralizing remaining traces of acidic urine

44. Female Reproductive System • The ovaries and accessory structures • Almost entirely contained within the abdominal cavity • It consists of the ovaries and structures that accept sperm, conduct the sperm to the egg, and nourish the developing embryo

45. Animation: Human Female Reproductive Anatomy

46. The Human Female Reproductive System fimbria ovary myometrium uterine tube uterus endometrium urinary bladder pubic bone cervix urethra vagina rectum clitoris anus labia

48. Oogensis • Oogenesis - formation of egg cells • Begins in the developing ovaries before birth • Starts with the formation of diploid cells - oogonia - as early as the 6th week of embryonic development • From the 9th - 20th weeks, oogonia enlarge and differentiate, becoming primary oocytes • So a woman is born with a lifetime’s supply of primary oocytes—1 -2 million— no new are generated • Many die each day, but 400,000 remain at puberty • This is plenty, only a few oocytes resume meiotic cell division during each month of a woman’s reproductive span, from puberty to menopause

49. Oogenesis

50. Follicle • Surrounding each oocyte is a layer of accessory cells and together, they make up a follicle • Every month after puberty, the hormonal changes of the menstrual cycle stimulate the development of a dozen follicles • The small follicle cells multiply, providing nourishment for the developing oocyte • In response to hormones secreted by the anterior pituitary, they also release estrogen into the bloodstream • Only one follicle matures during each menstrual cycle • As the follicle develops, its primary oocyte completes meiosis I, dividing into a single secondary oocyte and a polar body - a small cell that is little more than a discarded set of chromosomes • Meiosis II will not occur unless the egg is fertilized