Angiosperm Reproduction Notes

Angiosperms reproduce sexually through mechanisms that attract animal pollinators to transfer pollen.
- Example: Flowers of the Ophrys speculum orchid resemble female Dasyscolia ciliata wasps; male wasps pollinate during mating attempts.

Pollen Transfer Process:
- Pollen grain is transferred from anther to stigma.
- Pollen tube grows down to deliver sperm to ovule:
- Egg is fertilized -> becomes a seed.
- Ovary develops into a fruit containing seeds.

Characterized by three key features: flowers, double fertilization, and fruits.
Life cycle alternates between:
- Sporophyte Generation:
- Diploid (2n) plants that produce haploid spores by meiosis.
- Larger, longer-lived than gametophytes.
- Gametophyte Generation:
- Haploid (n) plants that produce gametes by mitosis.

Flowers: Reproductive shoots of the sporophyte; consist of four floral organs:
- Carpels (pistils): Contains ovules at the base of a long style with a sticky stigma.
- Stamens: Comprises a filament topped by an anther containing microsporangia.
- Petals: Brightly colored to attract pollinators.
- Sepals: Protect unopened floral buds.
A carpel may be a simple pistil or a compound pistil (group of fused carpels).

Pollination is the transfer of pollen; can occur via:
- Wind (abiotic): 20% of angiosperms, like grasses, often produce small green flowers.
- Animals: Majority of species rely on them, with bees being the most crucial pollinators.

Bees: Most important; pollinated flowers are often brightly colored with sweet fragrances.
Moths & Butterflies: Sweet fragrance; butterflies prefer brightly colored flowers, while moths favor white/yellow.
Flies: Flowers may mimic rotten meat; flies pollinate by mistake as they lay eggs.
Bats: Light-colored, aromatic flowers; pollinated at night.
Birds: Large, bright flowers with large amounts of nectar; typically have a floral tube.

Coevolution: Occurs between flowering plants and their pollinators, influencing each other's evolution.
- Example: Darwin’s prediction of a long-tongued moth based on flower shape.

Female Gametophyte (Embryo Sac) consists of:
- Two synergids flanking the egg, guiding pollen tubes.
- Three antipodal cells at the other end.
- Two polar nuclei in the central cell.
Male Gametophyte Development:
- Inside microsporangia, undergoes meiosis to produce microspores, forming pollen grains.
- Each pollen grain consists of a tube cell and a generative cell that divides to produce two sperm.

Eventually, one sperm fertilizes the egg (zygote, 2n) and the other merges with the polar nuclei to form endosperm (3n), serving as a nutrient supply.

Seeds consist of:
- Dormant embryo and stored nutrients (endosperm).
- Protected by a hard seed coat; seeds can enter dormancy for environmental cues to germinate.
Embryonic development:
- Basal cells produce a suspensor; terminal cells form the proembryo.
- Cotyledons develop, and the seed may dehydrate to about 5–15% water.

Fruits: Mature ovaries protecting seeds, aiding in dispersal via wind or animals.
- Classified into simple, aggregate, multiple, and accessory fruits.

Asexual reproduction produces genetically identical offspring through mechanisms like fragmentation, adventitious shoots from roots (e.g., aspen trees), and apomixis.

Self-fertilization ensures ovules develop seeds but reduces genetic diversity.
Mechanisms to prevent selfing include:
- Sexual dimorphism (staminate vs. carpellate flowers)
- Spatial separation of mature stamens and carpels.

Self-incompatibility allows plants to distinguish between self and non-self pollen, preventing inbreeding:
- Gametophytic: Pollination blocked by S-alleles of the pollen.
- Sporophytic: Incompatibility between S-gene products preventing pollen from fertilizing.

Asexual reproduction is suitable for stable environments; sexual reproduction increases genetic diversity and adaptability.