Reproduction and Development Reproduction and Development

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

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

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

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

(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

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

Asexual reproduction Sexual reproduction Generation 1 Female Female Figure 36.3-3 Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Figure 36.3-3 The “reproductive handicap” of sex (step 3) Generation 4 8

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

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

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

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

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

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

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

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

Figure 36.6 Figure 36.6 External fertilization 17

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

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

Figure 36.7 Figure 36.7 Parental care in an invertebrate 20

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.

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

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

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

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

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

(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

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

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

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

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

Sperm plasma membrane Sperm nucleus Acrosomal process Basal body Figure 35.15-5 Sperm plasma membrane Sperm nucleus Acrosomal process Basal body (centriole) Actin filament Sperm head Cortical granule Fused plasma membranes Figure 36.15-5 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

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

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 36.16 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

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

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

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

(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

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

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

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

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

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

Digestive tube (endoderm) Anus (from blastopore) Figure 36.18 Animal pole Blastocoel Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Archenteron Mesenchyme cells Blastopore Figure 36.18 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

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

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

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

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 36.19 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