Animal Reproduction

• Animals, alongside plants, are among the most successful organisms on the planet due to the diversity of their adaptations
• The alternation between the reproduction and development in animals significantly contributes to their survival for many generations
• Diverse forms of reproductive mechanisms exist in the animal kingdom, all of which allows the next generations to inherit the genetic material of the species
• Life cycle of an animal


  Begins with...
• General Animal Life Cycle


  1. Zygote

  2. Embryo

  3. Young

  4. Adult

  5. Gametes
• General Animal Life Cycle

  1. Fertilization

  2. Embryogenesis

  3. Organogenesis

  4. Gametogenesis

  5. Maturation
• Metamorphosis


  • Rapid change from an immature larvae and juvenile to a sexually mature adult
• Metamorphosis: Atlas moth (Attacus sp.)


  • Larva
  • Adult
• Metamorphosis: Sea stars


  • Juvenile
  • Adult
• Metamorphosis: Lady bugs


  • Larva
  • Adult
• Complete Metamorphosis (Holometabolous)


  • Characterized by distinct larval and pupal stages
  • Larval stage does not resemble the adult individual
  • Larval stages undergo several molting stages
  • Pupa stage is a period when drastic changes occur

  
• Complete Metamorphosis: European Stag Beetle (Lucanus cervus)


  • Larva
  • Pupa
  • Adult
• Complete Metamorphosis: Mosquito (Aedes albopictus)


  • Larva
  • Pupa
  • Adult
• Complete Metamorphosis: Monarch Butterfly (Danaus plexippus)


  • Larva
  • Early pupa
  • Late pupa
  • Adult

  
• Incomplete Metamorphosis (Hemimetabolous)


  • Characterized by an aquatic juvenile stage
  • Larval stage somehow resembles the adult stage
  • Larval stages undergo several molting stages
  • Pupa stage does not exist

  
• Incomplete Metamorphosis: Dragonflies

  • Naiad
  • Adult
• Incomplete Metamorphosis (Paurometabolous)


  • Characterized by a terrestrial juvenile stage
  • Larval stage somehow resembles the adult stage
  • Larval stages undergo several molting stages
  • Pupa stage does not exist

  
• Incomplete Metamorphosis: Grasshoppers


  • Nymph
  • Molting adult
• Asexual Reproduction

  • Fission
  • Fragmentation
  • Budding
  • Parthenogenesis
• Fission


  Splitting of the parent individual into two approximately equal halves
• Fragmentation


  Breaking body parts into fragments that will regenerate into fully functional individuals
• Budding


  Outgrowth or bud forming in the body of an adult that will detach to develop as a complete individual
• Parthenogenesis


  Development of an embryo from an unfertilized egg
• Sexual Reproduction


  • Requires two parents from each of the two sexes
  • Requires fusion of the egg cell and the sperm cell
  • Relies highly on the efficiency of gametogenesis
  • Enhances genetic variation in biological populations
• External Fertilization


  • Usually involves the release of gametes into the surroundings
  • Developing embryos are usually nourished externally or may immediately land on a substrate
  • Common in aquatic organisms such as fishes and corals
• Internal Fertilization


  • Sperm cells are usually deposited into the female reproductive tract
  • The young may be nourished in the female's body or eggs are laid after fertilization
  • Common among higher animals such as birds, reptiles and mammals
• Variations in Internal Fertilization


  • Oviparous (eggs are laid and the embryo obtains nourishment from the stored yolk nutrients)
  • Ovoviviparous (embryos are nourished in eggs, which remain in the parent's body until they hatch)
  • Viviparous (embryos are nourished by the placenta until the mother gives birth to live young)
• Mechanism of Fertilization

  1. Contact with the jelly layer or the zona pellucida

  2. Digestion by the acrosomal enzymes

  3. Species-specific fusion of gamete membranes (fast-block)

  4. Cortical reaction

  5. Formation of fertilization envelope (slow-block)
• Oviparous


  Eggs are laid and the embryo obtains its nourishment from the stored yolk nutrients
• Ovoviviparous


  Embryos are nourished in eggs, which remain in the parent's body until they hatch
• Viviparous


  Embryos are nourished by the placenta until the mother gives birth to live young
• During embryogenesis, the zygote undergoes repeated cell division and cell reorganization
• Cleavage


  The rapid cell divisions that the zygote undergoes
• Continuous division forms the solid ball of cells called morula
• The cells are then organized into a hollow ball of cells called blastula
• Gastrulation


  Involves extensive cell migration and rearrangement, which forms the three embryonic germ layers: ectoderm, mesoderm, and endoderm
• Organogenesis


  Marked by the neurulation or the formation of the neural tube through the folding of the ectodermal neural plate
• Each of the three embryonic germ layers gives rise to specific tissues and organs in the developing embryo
• Gastrulation refers to the rearrangement of the cells in the blastula
• The layers of a gastrula include the ectoderm, endoderm, and mesoderm