Theme 5 Flashcards
In animal reproduction, new individuals are produced by:
Existing individuals
Animal Reproduction
- Generally involves dominant diploid stage producing haploid gametes, which fuse to produce a new diploid zygote
- Zygote must undergo growth and differentiation
- Cells that give rise to gametes are generally isolated early in development
Asexual Reproduction
- Genes from one parent
- Fission
- Budding
- Fragmentation
- Parthenogenesis – develop from an unfertilized egg
- Based upon mitosis
Asexual Reproduction Pros
- Don’t need males
- Rapid reproduction
- Saves energy
Asexual Reproduction Con
Lack of genetic variability
Sexual Reproduction – genes from two parents
- Eggs (oogenesis) and sperm (spermatogenesis) are the gametes
- Dioecious (gonochoristic)
- Monoecious/hermaphrodite (simultaneous vs sequential hermaphrodites)
- Fertilization and development
- Based upon meiosis
Sexual Reproduction Pro
Generates genetic variation
Sexual Reproduction Cons
- Fertilization necessary - complicated
- Development must proceed from zygote
- Costs of eggs and sperm
- Males don’t produce offspring
Spermatogenesis
- Sperm cells are highly modified and simplified
- Mitochondria provide ATP for the flagellum
- Head consists of acrosome, nucleus, centrioles
- Male strategy is to produce abundant small, motile, cheap gametes
Oogenesis
- Egg contains a large amount of cytoplasm, organelles, stored nutrients (yolk)
- Contained within the vitelline membrane (zona pellucida) – carries receptors for sperm
- May be wrapped within a jelly layer, hard shell
- Female strategy is to produce a few large, well-provisioned gametes
- Forced by male strategy
Hermaphroditism
- The adult organism possesses functional reproductive systems of both sexes, either simultaneously or sequentially
- Common in some taxa (annelids, gastropods)
Fertilization and Early Development
- Bringing sperm and egg together
- The one, right sperm, for the egg
- Protection of the developing embryo
- Events in early development
- Cleavage, blastula, gastrula (tissue layers)
- Chordate (vertebrate) development
Fertilization 1bringing sperm and egg together
- Diploid is the dominant life cycle stage (usually)
Fertilization:
- External fertilization (e.g. broadcast spawning)
- Internal fertilization
- Aquatic vs terrestrial environments
External Fertilization
- Only in aquatic animals
- Gametes small, high mortality
- Release of gametes by both sexes into the environment requires coordination of gametogenesis in response to seasonal environmental cues
- Coordinated gamete release may be due to stimulus through courtship movements, calls, or pheromone release
Internal Fertilization
- Large differences in gamete size
- Copulation: generally requires copulatory organ
- Spermatophore: package of sperm and mucus must be introduced into female’s reproductive tract
- Stronger sexual dimorphism
- Terrestrial and aquatic organisms
Fertilization 2one sperm and the right sperm for the egg
Challenges for internal vs external fertilization – making sure that the right sperm encounters the egg, and that only one sperm fertilizes it
- External fertilization: possibility of interspecific sperm-egg encounters (timing of breeding, molecular mechanisms of identifying appropriate sperm)
- Internal fertilization: pre-and post-zygotic isolating mechanisms (behavioural, physical constraints, genetic incompatibility)
Acrosomal Reaction (fast block)
Sperm cell membrane fuses with egg cell membrane, initiates wave of depolarization – subsequent sperm can’t fuse with egg = external fertilization
Cortical Reaction (slow block)
Cortical granules fuse with egg membrane, vitelline coat becomes impenetrable to sperm = external and internal fertilization
