1. Reproduction and Development
Reproduction and Development

2. Concept 36.1: Both asexual and sexual reproduction occur in the animal kingdom
Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote
Asexual reproduction is the creation of offspring without the fusion of egg and sperm 2

3. Mechanisms of Asexual Reproduction
Many invertebrates reproduce asexually by budding, in which new individuals arise from outgrowths of existing ones
Also common among invertebrates is fission, the separation of a parent organism into two individuals of approximately equal size 3

4. Figure 36.2
Figure 36.2 Asexual reproduction of a sea anemone (Anthopleura elegantissima) 4

5. Fragmentation is breaking of the body into pieces, some or all of which develop into adults
Fragmentation must be accompanied by regeneration, regrowth of lost body parts
Parthenogenesis is the development of a new individual from an unfertilized egg 5

6. (a) A. uniparens females (b) The sexual behavior of A. uniparens is
Figure 36.5
Ovary
size
Ovulation
Ovulation
Progesterone
Estradiol
Hormone
level
Time
Behavior
Female
Male-
like
Female
Male-
like
Figure 36.5 Sexual behavior in parthenogenetic lizards
(a) A. uniparens females
(b) The sexual behavior of A. uniparens is
correlated with the cycle of ovulation. 6

7. Sexual Reproduction: An Evolutionary Enigma
Sexual females have half as many daughters as asexual females; this is the “twofold cost” of sexual reproduction
Despite this, almost all eukaryotic species reproduce sexually 7

8. Asexual reproduction Sexual reproduction Generation 1 Female Female
Figure
Asexual reproduction
Sexual reproduction
Female
Generation 1
Female
Generation 2
Male
Generation 3
Figure The “reproductive handicap” of sex (step 3)
Generation 4 8

9. Sexual reproduction results in genetic recombination
The resulting increased variation among offspring may enhance the reproductive success of parents in changing environments 9

10. Reproductive Cycles
Most animals exhibit reproductive cycles related to changing seasons
Reproductive cycles are controlled by hormones and environmental cues
Because seasonal temperature is often an important cue in reproduction, climate change can decrease reproductive success 10

11. Figure 36.4
Figure 36.4 Caribou (Rangifer tarandus) mother and calf 11

12. Variation in Patterns of Sexual Reproduction
For many animals, finding a partner for sexual reproduction may be challenging
One solution is hermaphroditism, in which each individual has male and female reproductive systems
Any two hermaphrodites can mate, and some hermaphrodites can self-fertilize 12

13. Figure 36.1
Figure 36.1 How can each of these sea slugs be both male and female? 13

14. Individuals of some species undergo sex reversals
For example, in a coral reef fish, the bluehead wrasse, a lone male defends a group of females
If the male dies, the largest female in the group transforms into a male
Within a few weeks, this individual can begin to produce sperm instead of eggs 14

15. External and Internal Fertilization
Sexual reproduction requires fertilization, the union of sperm and eggs
In external fertilization, eggs shed by the female are fertilized by sperm in the external environment
In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract 15

16. A moist habitat is almost always required for external fertilization to prevent gametes from drying out and to allow sperm to swim to eggs
Many aquatic invertebrates simply shed gametes into the surrounding water
In this case, timing of release of gametes is crucial to ensure that sperm and egg encounter one another 16

17. Figure 36.6
Figure 36.6 External fertilization 17

18. Ensuring the Survival of Offspring
Internal fertilization is typically associated with production of fewer gametes but the survival of a higher fraction of zygotes
Internal fertilization is also often associated with mechanisms to provide protection of embryos and parental care of young 18

19. Some other animals retain the embryo, which develops inside the female
The embryos of some terrestrial animals develop in eggs with calcium- and protein-containing shells and several internal membranes
Some other animals retain the embryo, which develops inside the female
In many animals, addition of parental care helps ensure survival of offspring 19

20. Figure 36.7
Figure 36.7 Parental care in an invertebrate 20

21. An advantage of internal fertilization over external fertilization is that
internal fertilization allows animals to reproduce sexually.
internal fertilization requires much less expenditure of resources.
internal fertilization produces more offspring, ensuring rapid population growth.
internal fertilization prevents the drying out of gametes in a dry environment.

22. Concept 36.2: Reproductive organs produce and transport gametes
Sexual reproduction in animals relies on sets of cells that are precursors for eggs and sperm 22

23. Variation in Reproductive Systems
Many (but not all) animals have gonads, organs that produce gametes
More elaborate systems include sets of accessory tubes and glands that carry, nourish, and protect gametes and sometimes developing embryos 23

24. Gametogenesis Gametogenesis is the production of gametes
Gametes are haploid (1n)
Sperm
Eggs 24

25. Oogenesis, the development of a mature egg, is a prolonged process
Spermatogenesis, the development of sperm, is continuous and prolific (millions of sperm are produced per day); each sperm takes about 7 weeks to develop
Oogenesis, the development of a mature egg, is a prolonged process
Immature eggs form in the female embryo but do not complete their development until years or decades later 25

26. Primordial germ cell in embryo
Figure 36.10ab
Primordial germ cell in embryo
Mitotic divisions
Spermatogonial
stem cell 2n
Mitotic divisions
Spermatogonium 2n
Mitotic divisions
Primary spermatocyte 2n
Meiosis I
Secondary spermatocyte
n n
Meiosis II
Figure 36.10ab Exploring human gametogenesis (part 1b: spermatogenesis, cell divisions)
Early
spermatid
n n n n
Differentiation
(Sertoli cells
provide nutrients)
Sperm cell n
n n n 26

27. (present at birth), arrested in prophase of meiosis I 2n
Figure 36.10bb
In embryo
Primordial germ cell
Mitotic divisions 2n
Oogonium
Mitotic divisions
Figure 36.10bb Exploring human gametogenesis (part 2b: oogenesis, meiosis I)
Primary oocyte
(present at birth), arrested
in prophase of meiosis I 2n 27

28. Completion of meiosis I and onset of meiosis II First polar body n n
Figure 36.10bc
Completion of meiosis I
and onset of meiosis II
First
polar
body n n
Secondary oocyte,
arrested at metaphase of
meiosis II
Ovulation, sperm entry
Figure 36.10bc Exploring human gametogenesis (part 2c: oogenesis, meiosis II)
Completion of meiosis II
Second
polar
body n
Fertilized egg n 28

29. Concept 36.4: Fertilization, cleavage, and gastrulation initiate embryonic development
Across animal species, embryonic development involves common stages occurring in a set order
First is fertilization, which forms a zygote
During the cleavage stage, a series of mitoses divide the zygote into a many-celled embryo
The resulting blastula then undergoes rearrangements into a three-layered embryo called a gastrula 29

30. EMBRYONIC DEVELOPMENT Sperm Zygote
Figure 36.14
EMBRYONIC DEVELOPMENT
Sperm
Zygote
Adult
frog
Egg
FERTILIZATION
CLEAVAGE
Metamorphosis
Blastula
GASTRULATION
Figure Developmental events in the life cycle of a frog
ORGANO- GENESIS
Larval
stages
Gastrula
Tail-bud
embryo 30

31. Fertilization
Molecules and events at the egg surface play a crucial role in each step of fertilization
Sperm penetrate the protective layer around the egg
Receptors on the egg surface bind to molecules on the sperm surface
Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg 31

32. Sperm plasma membrane Sperm nucleus Acrosomal process Basal body
Figure
Sperm plasma
membrane
Sperm
nucleus
Acrosomal
process
Basal body
(centriole)
Actin
filament
Sperm
head
Cortical
granule
Fused
plasma
membranes
Figure The acrosomal and cortical reactions during sea urchin fertilization (step 5)
Acrosome
Perivitelline
space
Hydrolytic enzymes
Jelly coat
Fertilization
envelope
Vitelline layer
Sperm-
binding
receptors
Egg plasma membrane 32

33. The events of fertilization also initiate metabolic reactions that trigger the onset of embryonic development, thus “activating” the egg
Activation leads to events such as increased protein synthesis that precede the formation of a diploid nucleus 33

34. Acrosomal reaction: plasma membrane
Figure 36.16 1 2 3 4 6 8 10 20 30 40 50 5 60 90
Binding of sperm to egg
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
Seconds
Increased intracellular calcium level
Cortical reaction (slow block to polyspermy)
Formation of fertilization envelope complete
Figure Timeline for the fertilization of sea urchin eggs
Increased protein synthesis
Minutes
Fusion of egg and sperm nuclei complete
Onset of DNA synthesis
First cell division 34

35. Diploidy is first reestablished following
fertilization.
gastrulation.
parthenogenesis.
organogenesis.
ovulation.

36. Cleavage and Gastrulation
Fertilization is followed by cleavage, a period of rapid cell division without growth
Cleavage partitions the cytoplasm of one large cell into many smaller cells
The blastula is a ball of cells with a fluid-filled cavity called a blastocoel
The blastula is produced after about five to seven cleavage divisions 36

37. (a) Fertilized egg 50 m Figure 36.17a
Figure 36.17a Cleavage in an echinoderm embryo (part 1: fertilized egg)
(a) Fertilized egg 37

38. (b) Four-cell stage 50 m Figure 36.17b
Figure 36.17b Cleavage in an echinoderm embryo (part 2: four-cell stage)
(b) Four-cell stage 38

39. (c) Early blastula 50 m Figure 36.17c
Figure 36.17c Cleavage in an echinoderm embryo (part 3: early blastula)
(c) Early blastula 39

40. (d) Later blastula 50 m Figure 36.17d
Figure 36.17d Cleavage in an echinoderm embryo (part 4: later blastula)
(d) Later blastula 40

41. After cleavage, the rate of cell division slows
The remaining stages of embryonic development are responsible for morphogenesis, the cellular and tissue-based processes by which the animal body takes shape 41

42. During gastrulation, a set of cells at or near the surface of the blastula moves to an interior location, cell layers are established, and a primitive digestive tube forms
The hollow blastula is reorganized into a two- or three-layered embryo called a gastrula 42

43. The cell layers produced by gastrulation are called germ layers
The ectoderm forms the outer layer and the endoderm the inner layer
In vertebrates and other animals with bilateral symmetry, a third germ layer, the mesoderm, forms between the endoderm and ectoderm 43

44. Digestive tube (endoderm) Anus (from blastopore)
Figure 36.18
Animal
pole
Blastocoel
Mesenchyme cells
Vegetal plate
Vegetal
pole
Blastocoel
Archenteron
Mesenchyme cells
Blastopore
Figure Gastrulation in a sea urchin embryo
50 m
Key
Blastocoel
Future ectoderm
Ectoderm
Future mesoderm
Archenteron
Future endoderm
Mouth
Blastopore
Mesenchyme
(mesoderm forms
future skeleton)
Digestive tube (endoderm)
Anus (from blastopore) 44

45. Development must occur in which of the following sequences?
cleavage  blastula  gastrula  morula  
cleavage  gastrula  morula  blastula
cleavage  morula  blastula  gastrula
gastrula  morula  blastula  cleavage
morula  cleavage  gastrula  blastula

46. Sea urchins and other echinoderms are deuterostomes, as are chordates
Cell movements and interactions that form the germ layers vary among species
One distinction is whether the mouth develops at the first opening that forms in the embryo (protostomes) or the second (deuterostomes)
Sea urchins and other echinoderms are deuterostomes, as are chordates 46

47. Each germ layer contributes to a distinct set of structures in the adult animal
Some organs and many organ systems derive from more than one germ layer 47

48. ECTODERM (outer layer of embryo)
Figure 36.19
ECTODERM (outer layer of embryo)
Epidermis of skin and its derivatives (including sweat glands,
hair follicles)
Nervous and sensory systems
Pituitary gland, adrenal medulla
Jaws and teeth
Germ cells
MESODERM (middle layer of embryo)
Skeletal and muscular systems
Circulatory and lymphatic systems
Excretory and reproductive systems (except germ cells)
Dermis of skin
Adrenal cortex
Figure Major derivatives of the three embryonic germ layers in vertebrates
ENDODERM (inner layer of embryo)
Epithelial lining of digestive tract and associated organs
(liver, pancreas)
Epithelial lining of respiratory, excretory, and reproductive tracts
and ducts
Thymus, thyroid, and parathyroid glands 48