Animal Reproduction Modes: Asexual Reproduction

• Reproduction is often considered as mating between males and females.
• Animal reproduction takes many forms.
• Some species can reproduce without any form of sex.
• Some species individuals change sex during their lifetime.
• Some species have both male and female organs.
• Some species display a further variation with reproduction involving only a few individuals in a large population
• Reproduction is the generation of new individuals from existing ones.
• A population outlives its members only by reproduction

Forms/Mode of Reproduction

• Reproduction (or procreation or breeding) is the biological process by which new individual organisms the "offspring" are produced from their "parent" or parents.
• In the animal kingdom, reproduction can take the form/mode of asexual and sexual.

Asexual Reproduction

• An organism can reproduce without the involvement of another organism.
• New individuals are generated without the fusion of egg and sperm.
• Asexual reproduction is not limited to single-celled or unicellular organisms.
• Cloning of an organism (eukaryote or prokaryote) is a form of asexual reproduction.
• Genetically similar or identical copy of the parent is created.
• This mode of reproduction relies entirely on mitotic cell division

Mechanisms of Asexual Reproduction

• Budding
• Fission
• Fragmentation and regeneration
• Parthenogenesis

Budding

• New individuals arise from outgrowths of existing ones.
• Example is in stony corals.

Fission

• Involves splitting and separation of a parent organism into two individuals of approximately equal size.
• Examples can include:
  • Single-celled organisms like amoeba, paramecium, stentor
  • Multicellular Organisms like flatworm, hydra, corals
  • Microorganisms like Bacteria, archaea, yeast

Fragmentation and Regeneration

• Fragmentation is the breaking of the body into several pieces, followed by regeneration.
• Regeneration refers to regrowth of lost body parts or into a new organism.
• Examples can include annelid worms, corals, sponges, cnidarians, and tunicates

Parthenogenesis

• This is when an egg develops into an embryo without being fertilized.
• More common among invertebrates, but can also occur in vertebrates
• Examples include bees, wasps, and ants

Parthenogenesis Among Vertebrates

• Parthenogenesis among vertebrates is documented, but suggested to occur as a rare response to low population density.
• In Komodo dragons and hammerhead sharks, a female produced offspring when kept apart from males of its species.
• DNA analysis and evidence revealed vertebrate parthenogenesis in the wild in a group of female sawfish that were genetically completely identical to one another.

Types of Parthenogenesis

• Complete (obligate) parthenogenesis
• Incomplete (cyclic) parthenogenesis
• Paedogenetic parthenogenesis

Sexual Reproduction

• Typically requires the sexual interaction/ fusion of the gametes.
• Two specialized reproductive cells, which are haploid (contain half the number of chromosomes of normal cells (diploid)) and formed during meiosis.
• The male gamete (sperm) fertilizing a female gamete (egg) of the same species creates a fertilized zygote (diploid).
• The offspring organisms genetic characteristics are, therefore, derived from those of the two parental organisms.

The Gametes

• The female gamete, the egg, is large and non-motile.
• The male gamete, the sperm, is generally much smaller and motile.

Variations in Pattern of Sexual Reproduction

• Sexual reproduction involves the mating of a female and a male.
• Some animals develop adaptations that arose during their evolution to help them meet this challenge by blurring the distinction between male and female.
• This adaptation is common among sessile (stationary) animals like barnacles, burrowing animals like clams, and some parasites, including tapeworms.
• The evolutionary solution in this case is hermaphroditism.

Hermaphroditism

• Each individual has both male and female reproductive systems.
• Because each hermaphrodite reproduces as both a male and a female, any two individuals can mate.
• Each animal donates and receives sperm during mating.
• Examples include sea slugs.
• Hermaphrodites can also self-fertilize in some species, allowing a form of sexual reproduction that does not require any partner.
• Example includes corals.