How Do Animals Reproduce Without Sex?

1. How Do Animals Reproduce Without Sex? Asexual reproduction requires no mating but does not result in genetic diversity. Asexually reproducing species are mostly invertebrates, sessile, and live in constant environments. Three types of asexual reproduction are budding, regeneration, and parthenogenesis.

2. Budding produces new individuals that form from the bodies of older animals. A bud grows by mitotic cell division and cells differentiate before the bud breaks away. The bud is genetically identical to the parent.

3. Figure 42.1 Asexual Reproduction in Animals (A)

4. Regeneration can replace damaged tissue or may form a complete individual. Example: Echinoderms

5. Figure 42.1 Asexual Reproduction in Animals (B)

6. Parthenogenesis is the development of offspring from unfertilized eggs. Parthenogenesis may determine the sex of the offspring. In some species, females can act as males depending on cyclic states of estrogen and progesterone.

7. Figure 42.2 Sexual Behavior May Be Required for Asexual Reproduction (Part 1)

8. Figure 42.2 Sexual Behavior May Be Required for Asexual Reproduction (Part 2)

9. In sexual reproduction two haploid cells form a diploid individual. Gametogenesis produces haploid gametes through meiotic cell division. Meiosis allows genetic diversity through crossing over, between homologous chromosomes, and independent assortment.

10. Three fundamental steps of sexual reproduction: • Gametogenesis: making gametes • Mating: getting gametes together • Fertilization: fusing gametes

11. Gametogenesis occurs in the gonads. In males the gonads are the testes and the gametes are small, motile sperm. In females the gonads are the ovaries and the gametes are nonmotile ova,or eggs.

12. Figure 42.3 Gametogenesis (A)

13. Figure 42.3 Gametogenesis (B)

14. Fertilization is the union of a haploid sperm and a haploid egg. It creates a single diploid cell, called a zygote, which will develop into an embryo.

15. Steps in fertilization: • Recognition of sperm and egg • Activation of sperm • Plasma membranes fuse • Additional sperm entry blocked • Activation of egg • Egg and sperm nuclei fuse

16. Species-specific sperm and egg interactions are controlled by specific recognition molecules. Eggs of aquatic species release chemical attractants to cause sperm to swim toward the egg. Sperm must go through two protective layers to reach an egg: a jelly coat and the vitelline envelope.

17. Figure 42.4 The Sea Urchin Egg

18. The acrosome is a membrane-enclosed structure on the sperm head. Egg and sperm contact causes substances in the jelly coat to trigger an acrosomal reaction. Membranes in the sperm head and acrosome break down, enzymes are released and digest the jelly coat.

19. Figure 42.5 Fertilization of the Sea Urchin Egg (Part 1)

20. Figure 42.6 A Mammalian Egg is Surrounded by Barriers to Sperm

21. Sexual reproduction requires: • Production of haploid gametes • Formation of a diploid zygote Mating brings eggs and sperm together so that fertilization occurs.

22. External fertilization occurs in an aquatic environment when gametes are released into the water. Reproductive behaviors to ensure fertilization include: • Release of gametes synchronized by day length, temperature, or weather • Congregating with potential mates

23. Internal fertilization occurs when sperm is released directly into the female reproductive tract. Gonads are the primary sex organs. Accessory sex organs include the penis and vagina, and copulation is the joining of the male and female accessory organs.

24. Dioecious species have separate male and female members. In a monoecious, or hermaphroditic, species, a single individual may produce sperm and eggs.

25. An earthworm is an example of a simultaneous hermaphrodite; male and female at the same time. An anemone fish is a sequential hermaphrodite and may function as a male or a female at different times.

26. Figure 42.7 When Size Determines Sex

27. Reptiles and birds evolved an amniote egg to overcome a dry environment. An egg contains food and water and the hard shell reduces water loss. Sperm must reach the egg before the shell forms, requiring internal fertilization and accessory sex organs.

28. Snakes and lizards have paired hemipenes that fill with blood and protrude from the male’s body. Birds may have erectile penises or bring their genital openings close enough to transfer sperm. All mammals practice internal fertilization and the embryo develops in the female reproductive tract.

29. Figure 42.8 The Shelled Egg (A)

30. Figure 42.8 The Shelled Egg (B)

31. Oviparity: egg-laying • Oviparous animals lay eggs in the environment and embryos develop outside the mother’s body. Viviparity: live-bearing • Viviparous animals retain the embryo in the mother’s body during early development.

32. Viviparity differs in mammals as they have a specialized female reproductive tract: • Uterus(or womb): holds the embryo • Placenta: develops in the uterus and enables exchange of nutrients and waste

33. Nonmammalian viviparous animals retain fertilized eggs in the mother’s body until they hatch. Ovoviviparity means that the embryo still receives nutrients from the egg.

34. Human Reproduction Semen is made up of sperm and other fluids and molecules. Sperm are produced in the paired male gonads, or testes. The testes are located in the scrotum, outside of the body, to maintain optimal temperature for sperm production.

35. Figure 42.9 The Reproductive Tract of the Human Male (Part 1)

36. Figure 42.9 The Reproductive Tract of the Human Male (Part 2)

37. Figure 42.10 Seminiferous Tubules Are the Site of Spermatogenesis (A)

38. Figure 42.10 Seminiferous Tubules Are the Site of Spermatogenesis (C)

39. Sperm are stored in the epididymis, where they mature and become motile. The urethra is the common duct for the reproductive and urinary systems—connected to the epididymis by the vas deferens.

40. Semen is secreted mostly by the paired seminal vesicles, which empty into the vas deferens. The prostate gland surrounds the urethra and contributes to semen. • Prostate fluid is alkaline and reduces acidity in male and female reproductive tracts.

41. The bulbourethral glands produce an alkaline secretion that: • Neutralizes acidity in the urethra. • Provides lubrication and facilitates sperm movement during climax. These secretions precede climax and can carry residual sperm that are capable of fertilization.

42. Semen is expelled through the vasa deferentia and urethra in two steps: Emission: rhythmic contractions move the semen into the urethra at the base of the penis. Ejaculation: contractions by muscles at the base of the penis that force the semen out.

43. Figure 42.11 Hormones Control the Male Reproductive System

44. In females, the ovary releases an egg into the oviduct, or Fallopian tube, where it may be fertilized. The egg is propelled towards the uterus where it will develop if fertilized. The bottom of the uterus is the narrow cervix, which opens into the vagina.

45. Figure 42.12 The Reproductive Tract of the Human Female (Part 1)

46. Humans have two sets of skin folds surrounding the opening of the vagina and the urethra: The inner folds are the labia minora. The outer folds are the labia majora: at the anterior tip is the clitoris, erectile tissue that is important in sexual response.

47. Figure 42.12 The Reproductive Tract of the Human Female (Part 2)

48. Sperm deposited in the vagina must swim to the upper oviduct to fertilize an egg. Fertilization results in the fusion of the haploid sperm and egg nuclei and produces a diploid zygote.

49. The zygote undergoes cell division and becomes a blastocyst. The blastocyst moves down the oviduct and attaches to the lining of the uterus, the endometrium. Implantation occurs when the blastocyst burrows into the endometrium.

50. The female reproductive cycle consists of two linked cycles: • The ovarian cycle that produces eggs and hormones. • The uterine cycle that prepares the endometrium for a blastocyst. If a blastocyst does not arrive, the thickened endometrium sloughs off.