What are the Differences Between Sexual and Asexual Reproduction?

• Asexual Reproduction: offspring are clones of a single parent; variation among individuals limited to mutations – Bacteria divide by binary fission – Spores in many eukaryotes (environmentally resistant

reproductive cells that can develop alone)• Sexual Reproduction: two parents donate genes to

offspring via gametes (sex cells)– Gametes are haploid, must fuse diploid zygote– Several sources of variation in addition to mutation great

physical diversity among individuals’ traits • Alternation of Generations: many organisms with both

asexual and sexual stages in life cycles; others alternate based on environmental conditions– Example: parthenogenesis in some fishes and reptiles

• Sex Determination: genetic vs. temperature-dependent

RED CLONE

YELLOW CLONE ORANGE CLONE

Fig. 7.1

Fig. 7.2

Fig. 7.4

Fig. 7.6

How are Gametes Produced? What are Some Sources of Variation Among Individuals?

• Gametogenesis: formation of gametes from somatic cells (via MEIOSIS, cell differentiation)

– Spermatogenesis: formation of sperm cells; occurs in testes• Haploid spermatids differentiate into sperm cells (with cap and

flagellum)– Oogenesis: formation of egg cells (oocytes); occurs in ovaries

• One of four grand-daughter cells absorbs cytoplasms of others (egg cells are very large cells, with RNA-rich and protein-

rich cytoplasms); polar bodies remain after formation of oocyte

• Important sources of variation for sexually reproducing organisms (other than mutation):

1. Independent assortment of chromosomes in meiosis2. Crossing over (genetic recombination) among homologous

chromosomes; more likely to occur away from centromere 3. Gene duplications, inversions, translocations, and deletions4. Non-disjunction of chromosomes / duplication of chromosomes5. Whole-genome duplications

Fig. 7.8

Figs. 7.9 and 8.1

Fig. 7.10

Figs. 7.14 and 7.15

What are the Different Forms of Early Development Among Animals?

• Forms of Cleavage (early cell divisions of zygote/embryo)– Holoblastic: cleavage through entire egg (little yolk, even distribution)

vs. Meroblastic: cleavage in cells atop a mass of yolk (telolecithal egg)– Spiral: blastomeres packed tightly, offset to layers below, “soccer ball” – Radial: blastomeres arranged in radial pattern– Discoidal: single layer of cells (blastoderm) develops atop yolk mass– Rotational: blastomeres of two-cell stage embryo divide along opposite

orientations (unique pattern of mammals) • Stages of Early Development

– Blastula: cells of embryo surround fluid-filled cavity (blastocoel)– Gastrula: invagination (blastopore) forms archenteron (gut)

• Forms of Development– Direct Development: embryo miniature adult (yolk abundant), vs.

Indirect Development: a feeding larval stage present (yolk scarce)– Deuterostome Development: blastopore anus, mouth 2°, vs.

Protostome Development: blastopore mouth; spiral cleavage– Regulative Development: fates of blastomeres determined late, vs.

Mosaic Development: fates of blastomeres determined early

Figs. 8.2 and 8.3

Fig. 8.4

Fig. 8.7

Figs. 8.8 and 8.9

Fig. 8.10

Fig. 8.14

Fig. 8.11

How do Cells Become Specialized?

• Cell Differentiation: a process where a generalized cell changes in form and function to a specialized cell (ex. neurons, RBCs)– Often triggered chemically by neighbor cells (induction)

• Cell Fate: specialized function that cell acquires • Cell Potency: range of cell types that cell could

acquire if exposed to different inductive environ-ments; potency always includes fate– Totipotent cells: unlimited potency – Pluripotent cells: high, but not unlimited potency

• Cell Determination: when potency becomes restricted to fate; timing can vary

• Heterotopic transplantation: method for testing potency and timing of cell determination

Fig. 8.16

What are the Later Stages of Development, and How are Organs

Formed?• Germ Layers and Their Derivatives

– Ectoderm: gives rise to the brain, spinal cord (via neural plate and neural tube), and epithelial tissues (earliest organs to appear)

– Endoderm: gives rise to gut lining, lungs, liver, pancreas, and pharyngeal pouches in vertebrates (derivatives of latter include gills in fishes, jaws, tonsils, and thymus in other vertebrates)

– Mesoderm: gives rise to muscles, heart (first functional organ), circulatory and urinary systems, and reproductive organs

• Organogenesis: organs differentiate from primordia following the gastrula stage (ex. limbs from limb buds, sense organs from placodes)– Vertebrate Homology: common pattern in the development of all

vertebrates apparent after neural tube forms • Pattern Formation and Limb Development

– Anterior/posterior axis: determined in oocyte by mRNA gradient– Homeotic (Hox) Genes: signal location along anteroposterior axis

(ex. cervical, thoracic, abdominal segments); mutations lead to misplaced appendages; Hox sequences conserved in animals

– Morphogens and Gradients: limb development controlled by gradient of morphogen secreted from apical ectodermal ridge

Fig. 8.26

Fig. 8.21

Figs. 8.29 and 8.30

Fig. 8.18

Fig. 8.20