Chapter 43

1. Animal Reproduction and Development: A Summary Chapter 43 AP Biology Spring 2011

2. Sexual vs. Asexual Reproduction • Asexual reproduction • Useful strategy in stable environment • Asexual reproduction by fragmentation, budding (ex. Sponge) or parthenogenesis results in offspring identical to parents

3. Costs of Sexual Reproduction • Sexual reproduction permits adaptation through variations but is biologically costly because sexes are separate • Animals must produce gametes and find each other (usually) for fertilization to occur • Through sexual reproduction, offspring are more likely to have a gene combination that is suitable to their new environment

4. Costs of Sexual Reproduction

5. Variations on Sexual Reproduction • Some animals, tapeworms and roundworms, are hermaphrodites producing both eggs and sperm at the same time • More typically, vertebrates have separate sexes that are fixed for life- either male or female

6. Variations on Sexual Reproduction • Most land animals have internal fertilization, while most marine animals use external fertilization • Offspring may develop inside mother or outside of maternal body • Most female mothers invest in yolk to help nourish developing young

7. Stages of Reproduction and Development • Gamete formation • Eggs or sperm form and mature within the parents

8. Stages of Reproduction and Development • Fertilization • Begins when a sperm penetrates an egg and is completed when the sperm nucleus fuses with the egg nucleus, resulting in formation of zygote

9. Stages of Reproduction and Development • Cleavage • Repeated mitotic divisions • Convert zygote to a blastula • Cell numbers increase but not cell size • Cleavage is over when blastula forms • Blastula’s cells is blastomeres, encloses a fluid filled cavity the blastocoel

11. Stages of Reproduction and Development • Gastrulation • Blastula enters gastrulation • Results in 3 germ layers or tissues • Ectoderm: outer layer, gives rise to nervous system and outer layers of the integument • Endoderm: inner layer, gives rise to gut and organs derived from it • Mesoderm: middle layer, muscle, organs of circulation, reproduction, excretion, and skeleton derived from it

12. Zygote  morula  blastula  gastrula

13. Stages of Reproduction and Development • Organ formation • Organ formation begins as germ layers subdivide into populations of cells destined to become unique in structure and function

14. Stages of Reproduction and Development • Growth, tissue specialization • During growth and tissue specialization, organs acquire specialized chemical and physical properties

15. Early Marching Order- Information in the Egg • Sperm contributes little more than the paternal DNA • The oocyte contains the majority of materials that will affect early development • Penetration of egg by sperm triggers structural reorganization in the egg cytoplasm

16. Early Marching Order- Information in the Egg • In a frog egg, microtubules move granules from the animal pole to form a gray crescent near the equator opposite penetration site • Near the crescent, body axis of frog embryo will become established and gastrulation will begin

17. Early Marching Order- Information in the Egg

18. Early Marching Order- Cleavage • Cleavage divides up maternal cytoplasm • After fertilization, the zygote begins a series of divisions in which each cell is pinched into two cells (blastomeres) • This process is NOT random and different blastomeres will end up with different genetic messages in a process known as cytoplasmic localization

19. Early Marching Order- Cuts • Orientation: • Radial cleavage starts with cuts perpendicular to mitotic spindle, producing cell of similar size but with different parts of cytoplasm • In other cells (frogs) cuts do not go all of the way through, so the cells produced are smaller at the animal pole

20. Early Marching Order- Cuts • Orientation • In mammals, rotational cleavage results in an inner cell mass (future embryo), which forms on the inside of a hollow sphere

21. Early Marching Order- Cuts • Complete and Incomplete: • The amount of yolk stored inside the egg also affects cleavage patterns • Incomplete: when an abundance of yolk impedes the cytoplasmic division, like in the insects, reptiles, birds, and most fishes • Complete: when little yolk is present, the first cut divides all the cytoplasm in complete cleavage (amphibians and mammals)

23. Structure of Blastula • Blastula structure varies with species cleavage patterns • In sea urchins, complete cleavage results in blastula that is a hollow ball • In highly yolky eggs (birds and some fish), a blastodisk forms on top of the blastula • In mammals, early embryo is called blastocyte

24. Zygote Blastula Gastrulation Morula

25. From Grastrula to Blastula • A hundred to thousands of cells may form at cleavage- depending on species • Starting with grastrulation, cells migrate about and rearrange themselves • In most animals, the small ball of cells formed at cleavage develops into a grastrula with three distinct germ layers; ectoderm, mesoderm, endoderm • Specific patterns of cell migration occur within the gastrulation process

26. Grastulation ectoderm mesoderm endoderm

27. From Grastrula to Blastula • Gastrulation proceeds through embryonic induction which is the process in which developmental fates of embryonic cell lineages change when exposed to signals (gene products) from adjacent tissues

28. Cell Differentiation • From grastrulation onward, cell lineages also engage in selective gene expression, which is the start of cell differentiation

29. Morphogenesis • Morphogens are signaling molecules produced by master genes • They diffuse out and form a concentration gradient in the embryo • A morphogen’s effect on target cells is proportional to its concentration

30. Morphogenesis and Pattern Formation • Morphogenesis is the progression of differentiated cells into tissues and organs; it is the result of several events • Sheets of cells expand and fold as cells change shape forming organs such as the neural tube

31. Morphogenesis and Pattern Formation • Programmed cell death helps sculpt body parts • Controlled cell death, called apoptosis, is genetically programmed elimination of tissues and cells that are used for only short periods in the embryo or adult • Pattern formation is the process by which certain body parts form in a specific place

32. Pattern Formation • Embryonic induction: developmental fates of embryonic cell lineages change when exposed to signals- gene products- from adjacent tissues • Pattern Formation: a sculpting of specialized tissues and organs from clumps of cells in the proper places in the embryo, in the proper order

33. Pattern Formation • Theory of pattern formation • The formation of tissues and organs in ordered, spatial patterns • Morphanogens and other inducer molecules diffuse through embryonic tissues, activate master genes • Products of homeotic genes and other master genes interact with control elements to map out the overall body plan

34. Pattern Formation • Evolutionary constraints on development • The basic body plans of the major animal groups have not changed due a limited number of master genes • These genes have imposed phyletic constraints in addition to the more well-known physical and architectural constraints • Physical constraint: surface area to volume ratio • Architectural constraint: imposed by body axes • Phyletic constraint: imposed by interactions among genes that regulate development in a lineage

35. Age and Death • Aging may be partly a result of time running out of internal biological clocks, which are genetically preset • Aging also may be partly an outcome of cumulative assaults on DNA and other biological molecules during the life cycle