Animal Reproduction and Development Flashcards

Asexual vs. Sexual Reproduction

• Asexual Reproduction:
  • Pros:
    • Huge numerical advantage: 2 copies of genes in each offspring vs. 1 in sexual reproduction.
    • No need for a mate.
    • Avoids expensive/risky mating behaviors.
    • Avoids inbreeding depression in small populations.
  • Cons:
    • No segregation of alleles.
    • Slower natural selection.
    • Adaptation acts on the genome, not separate genes.
    • Can allow bad alleles to accrue in populations.
  • Examples:
    • Budding in hydra.
    • Fission in anemones.
    • Parthenogenesis in some lizards and insects (unfertilized eggs develop into offspring).
• Sexual Reproduction:
  • Pros:
    • Novel combinations of genes due to segregation and fertilization.
    • Allows faster adaptation.
    • Allows natural selection to act on genes as independent units.
  • Cons:
    • Need a mate.
    • Inbreeding depression in small populations.
    • Need special cells (gametes).
    • May create vulnerable life history stage.
    • Energetically expensive.
    • Only one copy of genes per offspring.
    • Mating success is typically more variable and lower on average for males.

Switching Reproductive Strategies

• Daphnia (crustaceans):
  • Reproduce both asexually and sexually.
  • Spring: Asexual reproduction by parthenogenesis (diploid female clones).
  • Fall: Sexual reproduction.
  • Switch to sexual reproduction when exposed to crowded, poor-quality water, low food availability, and short day lengths.
  • Hypothesis: Genetically variable offspring have higher fitness in degraded, changing, or uncertain environments.

Sexual Reproduction Strategies

• "Female" Strategy:
  • Make fewer, larger, and more expensive gametes/structures.
  • Invest more energy in offspring care.
  • Choosiness in mate selection.
• "Male" Strategy:
  • Make lots of small and inexpensive gametes and spread them as broadly as possible.
  • Competition with other males.
  • Has evolved multiple times independently in Eukaryotes!

Male Competition

• Competition for mating opportunities and female selection.
• Sperm competition at the cellular level.
• "Second male advantage": Adaptations to displace sperm from previous matings.
  • Occurs in sperm-storage areas where sperm from the second male displace sperm from the first male.

Animal Gametogenesis and Meiosis

• Meiosis I:
  • Crossing over between homologous chromosomes.
  • Independent assortment of chromosomes.
• Spermatogenesis:
  • All 4 spermatids mature into spermatozoa (sperm).
• Sperm Structure:
  • Acrosome: Enzyme-filled packet for digesting jelly layer or corona radiata.
  • Head: Contains nucleus.
  • Midpiece: Contains mitochondria.
  • Tail: Flagellum for movement.
  • Sperm are stripped-down, streamlined cells specialized for racing to the egg.
• Oogenesis:
  • Secondary oocyte arrested in meiosis II until fertilization, then completes meiosis.
  • Unequal allocation of cytoplasm during meiotic divisions concentrates resources into 1 ovum per oocyte.

Animal Gametes and Egg Structure

• Sea Urchin Eggs:
  • Surrounded by a jelly coat.
  • Vitelline envelope has receptors to recognize/bind to sperm.
• Human Oocytes:
  • Surrounded by a protective layer of cells called the corona radiata.
  • Zona pellucida: Term for vitelline layer of mammals.

Human Reproductive Systems

• Male:
  • Testis: Sperm production.
  • Epididymis: Sperm storage.
  • Vas deferens: Sperm delivery.
  • Seminal vesicle, prostate gland, bulbourethral gland: Produce semen.
  • Urethra: Carries semen and urine.
  • Penis: Contains erectile tissue for copulation.
    • Baculum: penis bone present in many mammals, including most primates (but not humans).
• Female:
  • Ovary: Produces eggs.
  • Oviduct (fallopian tube): Usual site of fertilization.
  • Uterus: Where the embryo develops.
  • Cervix: Opening to the uterus.
  • Vagina: Receives sperm.
• Bird Reproduction:
  • Meiosis and maturation of follicles.
  • Entry of follicle into oviduct.
  • Fertilization takes place if the hen has mated with a cock (Hens can store sperm for up to 30 days).
  • Addition of egg white and outer membranes.
  • Formation of eggshell.
  • Egg is laid.

Fertilization

• External Fertilization (spawning):
  • Mostly aquatic animals.
  • Produce large numbers of gametes.
  • Gamete release must be coordinated.
  • Synchronized release tied to environmental cues (day length, water temperatures).
  • Gametes are released in response to specific cues from individuals of the same species.
  • Pheromones often involved in synchronized gamete release.
• Internal Fertilization:
  • All terrestrial and a few aquatic animals.
  • Benefit: fewer gametes (especially costly eggs).
  • Requires either copulation or a spermatophore.
• Fertilization Challenges:
  • Sperm must find the egg.
  • Of the correct species (if external fertilization).
  • Only one sperm can fuse with the egg.
• Sea Urchin Fertilization – Cellular level
  • Chemical signals attract sperm to egg
  • Binding to jelly layer
  • Acrosome reaction
  • Flagellar hyperactivity
  • Penetration of jelly layer and vitelline envelope
  • Egg–sperm Membrane Fusion – depolarizes membrane, prevents polyspermy
  • Entry of sperm nucleus into egg
  • A wave of Ca^{2+} release starts at the point of sperm nucleus entry and spreads throughout the egg

Prevention of Polyspermy

• Cortical Granules:
  1. Release Ca^{+} ions, depolarizing the plasma membrane of egg.
  2. Release proteases that digest sperm-binding receptor proteins.
  3. Causes influx of water (osmosis).
  4. Cross-link with the vitelline envelope to form the fertilization envelope.

Embryo Development Strategies

• Oviparity: Egg laying; embryo develops in the external environment; all food from yolk (e.g., snake).
• Viviparity: Live birth; embryos develop in the mother, rely on the mother for food; requires connection to the reproductive tract for nutrient and gas exchange (e.g., fish).
• Ovoviviparity: Embryos develop in the mother’s body, rely on yolk for food; no placenta or nutrient and gas exchange (e.g., shark).
• Evolution of Viviparity:
  1. Egg laying is probably the ancestral condition.
  2. Ovoviviparity evolved independently in two groups of Sceloporus lizards.
  3. Ovoviviparity correlates with living in cold climates.
     • Lineages that vary in some interesting trait (e.g., viviparity) provide opportunities to study trait evolution.

Mammalian Placenta and Viviparity

• Reptiles, birds, and egg-laying mammals lay amniotic eggs.
• Why did placenta and viviparity (live bearing) evolve?
  1. Offspring develop at a more constant, favorable temperature.
  2. Offspring are protected.
  3. Offspring are portable—mothers are not tied to a nest.

Animal Development

• Stages:
  • Fertilized egg → Cleavage → Blastula (blastocyst in mammals) → Gastrula → Newborn

Cleavage and Blastocyst Formation

• Cleavage: Produces a blastocyst.
• Blastocyst Structure:
  • Inner cell mass: Gives rise to the embryo.
  • Trophoblast: Gives rise to the placenta (in part).
  • Blastocoel: Fluid-filled chamber.
• Implantation: Occurs in the uterine wall.

Gastrulation

• Gastrulation Sets the Stage for Body Development in Deuterostome Animals
• Gastrulation produces 3 germ layers, creates Head-to-Tail and Back-to-Belly axes that determine body form.

Germ Layers and Organogenesis

• Ectoderm:
  • Nervous system
  • Cornea and lens of the eye
  • Epidermis of skin
  • Epithelial lining of mouth and rectum
• Mesoderm:
  • Skeletal system
  • Circulatory system
  • Lymphatic system
  • Muscular system
  • Excretory system
  • Reproductive system
  • Dermis of skin
  • Lining of body cavity
• Endoderm:
  • Epithelial lining of the digestive tract, respiratory tract, reproductive tract, and urinary tract
  • Liver
  • Pancreas
  • Thyroid gland
  • Parathyroid glands
  • Thymus

Endocrine Control of Reproduction

• Puberty: Transition to sexual maturity.
  • Average age of puberty in girls has decreased ~5 years in the last 200 years.
  • Does body fat trigger puberty? Maybe. Leptin produced by fat may trigger GnRH.

Menstrual Cycle

• Female reproductive cycle of humans.
• Menstruation = shedding of uterine lining.
• Most mammals have an estrous cycle that does not include menstruation.
• Hormones Involved:
  • Ovarian: Estradiol, progesterone (both estrogens).
  • Pituitary: Follicle-stimulating hormone (FSH), luteinizing hormone (LH).
• Hormonal Changes:
  • Estradiol surges in the follicular phase, progesterone in the luteal phase.
  • LH surge induces ovulation.
  • "The pill" contains progesterone that suppresses LH, FSH, and GnRH, so no ovulation.

Birth Control Methods

• Barrier Methods:
  • Condom: 85% effectiveness.
  • Female condom: 79% effectiveness.
  • Diaphragm: 88% effectiveness.
  • Sponge: 84% effectiveness.
  • Spermicide: 71% effectiveness.
• Behavioral Methods:
  • Rhythm method: 80% effectiveness.
  • Withdrawal: 80% effectiveness.
• Hormone-Based Methods:
  • The pill, the patch, the ring, the shot, the implant: 92-99.9% effectiveness (depends on delivery system used).
• Emergency Contraception:
  • 92% effectiveness.
• Pregnancy Termination:
  • Mifepristone: Blocks progesterone receptors so menstruation occurs even after fertilization and implantation (92% effectiveness).
• Other:
  • Intrauterine device (IUD): Small, T-shaped structure inserted into the uterus; induces the uterus to produce substances hostile to sperm and eggs; some IUDs also secrete hormones (99% effectiveness).

Male Birth Control

• Hormonal male BC: Challenge to suppress production of millions of sperm (vs. 1-2 eggs) without causing low testosterone side-effects.
• Vasalgel: Fill vas deferens with biodegradable gel, reversed with injection that breaks down gel.
  • In use in India: 100% effective & reversible, lasts >15 years.
  • May be available in the US by 2026.

Learning Objectives

• Understand pros and cons of:
  • Sexual vs. asexual reproduction
  • Internal vs. external fertilization
  • Viviparous, oviparous, ovoviviparous
• Understand the importance of meiosis in gametogenesis and how it generates new combinations of genes.
• Know the basic structure and function of bird and human reproductive tracts.
• Hormones: LH, FSH, Estrogen & Progesterone effects on ovulation and pregnancy, menstrual cycle of humans.
• Understand the process of fertilization and prevention of polyspermy.
• Know: cleavage, blastula, gastrulation/gastrula, 3 germ layers, basic points of organogenesis.
• Understand hormonal control of puberty and menstrual cycle, kinds of birth control.