Mechanisms of Reproduction – Comprehensive Study Notes

Foundations: Why Reproduction Matters

Ultimate biological imperative = continuity of a species
Two overarching strategies to pass on hereditary information
- Asexual reproduction → single parent, offspring are (near) clones
- Sexual reproduction → two parents, fusion of gametes, offspring genetically unique
Choice of strategy reflects ecological pressures (climate, predators, mate availability, energy budgets, disease risk)

Core features
- One parent ⇒ no gametes ⇒ no fertilisation
- Rapid population increase, low energetic cost, but minimal genetic diversity (aside from mutations)

Chromosome replicates → cell elongates → septum forms → cytokinesis
Generation times can be minutes; exponential growth Nt = N0 \times 2^{t/g} (where g = generation time)
Advantages
- Ultra-rapid colonisation, survival in stable niches
Disadvantages
- Susceptible to environmental change; wiped out en masse by antibiotics, temperature shifts etc.

Unicellular eukaryotes (e.g. Amoeba, Paramecium)
Binary fission identical to bacterial model but with mitosis of a membrane-bound nucleus
Budding (e.g. some yeast-like protists) → asymmetrical cytokinesis; smaller daughter bud eventually detaches

Budding (yeasts) = mitotic outgrowth from parent cell → genetically identical daughter
Spores (moulds, mushrooms)
- Produced asexually by mitosis within sporangia or conidiophores
- Lightweight, resistant; dispersed by air/water/animals
Switch to sexual mode when conditions deteriorate
- Two haploid (n) mycelia fuse (plasmogamy) → heterokaryon → nuclei fuse (karyogamy) → diploid zygote (2n)

Not deeply detailed in transcript, but key modes include runners (strawberry), tubers (potato), cuttings, bulbs; all mitotic growth

Definition
- Production of offspring from two parents through union of gametes: male sperm & female egg/ovum
Cellular milestones
- Meiosis: 2n \to n (halving chromosome number)
- Fertilisation: n + n \to 2n (restoring diploidy, creating zygote)
Advantages
- Genetic recombination → variation, raw material for natural selection, disease resistance
Disadvantages
- Energetically expensive (gamete production, mate search, courtship)
- Slower population growth; exposes individuals to predation during mating

All plants display alternation of generations: multicellular diploid sporophyte \leftrightarrow multicellular haploid gametophyte

Non-vascular (or minimally vascular) group
Sporophyte (diploid) produces haploid spores by meiosis
Spores → gametophyte (haploid) produces gametes by mitosis → fertilisation forms zygote → new sporophyte

Life-cycle diagram (page 6) emphasises:
- Sporangia on underside of fronds (in sori) → meiosis → spores (n)
- Spore dispersal by wind → young gametophyte → mature gametophyte with antheridia (sperm) & archegonia (egg)
- Water film required for sperm motility → fertilisation → zygote (2n) → embryo → mature sporophyte

Vascular + seeds, but no flowers; cones instead
Male & female cones on mature sporophyte; gametophyte generation occurs inside the cone
Development can span ≈ 2 years from pollination to seed release

Pollen delivered by wind/insects/birds → lands on stigma → grows pollen tube → transports TWO sperm to ovule
1. Sperm 1 + egg ⇒ zygote (2n)
2. Sperm 2 + polar nuclei ⇒ endosperm (3n) → nutritive tissue for embryo
Ovary matures into fruit (may look atypical; e.g. Banksia fruit resembles a cone but is botanically a fruit)
Seed germination cues: water, O_2, optimal temperature & photoperiod

Gametes released outside body; fertilisation in environment (usually water)
Examples
- Amphibians & bony fish: female lays eggs in water, male releases sperm over them
- Coral spawning: synchronised mass release of gametes from entire colony
Pros
- Fast, high fecundity, no gestation/parental energy cost
Cons
- Requires large gamete numbers, low success rate, no parental protection, embryos vulnerable

Male deposits sperm inside female reproductive tract (penis/copulatory organ)
Wide range of courtship rituals (links provided in transcript for enrichment)
Pros
- Can occur on dry land; fewer gametes needed; higher per-egg success; protection from predators & pathogens
Cons
- Energetically costly mating rituals & gestation; slower reproduction; risk of sexually transmitted infections; typically fewer offspring

Moisture is central: aquatic reproductive stages (sperms of ferns, external fertilisation) contrast with terrestrial adaptations (pollen grains, amniotic eggs)
Trade-offs
- Quantity vs quality (many eggs with little care vs few with intense care)
- Energy budget allocation between survival & reproduction
Evolutionary transitions show incremental solutions to challenges (e.g. seed → pollen → internal gestation in mammals)

Genetic diversity from sexual reproduction underpins crop resilience, disease resistance (e.g. rust fungi vs wheat varieties)
Asexual propagation critical in agriculture (clonal grapes, bananas); but uniformity raises vulnerability (e.g. Panama disease in bananas)
Assisted reproductive technologies in animals/humans build on understanding of internal fertilisation (IVF, artificial insemination)
Conservation: coral spawning forecasts help manage reef restoration; seed banks preserve plant diversity

Alternation of generations cycle: 2n \xrightarrow{\text{meiosis}} n \xrightarrow{\text{fertilisation}} 2n
Endosperm ploidy in angiosperms: n + n + n = 3n (triploid)
Exponential bacterial growth: Nt = N0 \times 2^{t/g}

Contrast binary fission in bacteria vs mitosis in protists & fungi.
Diagram the double fertilisation process and label ploidy levels.
Explain why external fertilisers generally release more gametes than internal fertilisers.
Describe two mechanisms angiosperms use to prevent self-pollination.
Outline advantages and disadvantages of internal fertilisation with at least one ecological example.