Angiosperm Sexual and Asexual Reproduction

Angiosperm Sexual Reproduction: Alternation of Generations

• Definition: The life cycle of Angiosperms involves an alternation between a diploid ($2n$) sporophyte generation and a haploid ($1n$) gametophyte generation.
• Cycle Stages and Ploidy:
  • Zygote ($2n$): Formed by fertilization.
  • Embryo ($2n$): Develops from the zygote via mitosis.
  • Mature Sporophyte ($2n$): The dominant, visible plant.
  • Sporic Meiosis: Occurs in the sporophyte to produce haploid ($1n$) spores.
  • Spores ($1n$): Undergo mitosis to develop into gametophytes.
  • Mature Gametophyte ($1n$): Produces gametes via mitosis.
  • Gametes ($1n$): Fuse during fertilization to form a zygote.

Angiosperm Microsporogenesis & Microgametogenesis

Microsporogenesis (Pollen Formation)

• Anther Structure: A cross-section of a diploid ($2n$) anther typically shows $2$ lobes, each containing $2$ pollen sacs, totaling $4$ pollen sacs, which are also called microsporangia.
• Components of an Incipient Microsporangium:
  • Sporogenous Tissue ($2n$): Located in the center, appears light, destined to form microspores.
  • Sterile Jacket ($2n$): Surrounds the sporogenous tissue, appears dark, provides protection and nourishment.
• Process: The $2n$ sporogenous tissue undergoes sporic meiosis to produce haploid ($1n$) microspores.
• Result: $1n$ microspores are found inside the mature microsporangium.

Microgametogenesis (Pollen Development)

• Starting Point: A single haploid ($1n$) microspore (still inside the anther, with its nucleus shown).
• First Mitosis: The microspore nucleus undergoes mitosis followed by cytokinesis to form two cells:
  • Generative Cell ($1n$): A smaller cell, located inside the tube cell.
  • Tube Cell ($1n$) (or vegetative cell): A larger cell that will later form the pollen tube.
• Mature Pollen (Microgametophyte): Consists of the outer tube cell and the inner generative cell.
• Second Mitosis: The generative cell typically divides again (either before or after pollination) to produce two non-motile haploid ($1n$) sperm.
• Final Structure: A mature $1n$ pollen grain (microgametophyte) contains a tube cell nucleus and two $1n$ sperm cells.
• Gametophyte Dependency: Angiosperms are characterized by a dominant sporophyte generation and a dependent gametophyte (pollen grain).

Angiosperm Megasporogenesis & Megagametogenesis

Megasporogenesis (Ovule and Megaspore Formation)

• Pistil and Ovules: The female reproductive organ is the diploid ($2n$) pistil, which contains ovules.
• Incipient Ovule: An initial $2n$ ovule develops within the pistil.
• Ovule Structure: The $2n$ ovule is comprised of:
  • Integuments ($2n$): Protective layers that surround the central tissue.
  • Nucellus ($2n$): The central tissue of the ovule, within the integuments.
  • Micropyle: An opening in the integuments, allowing entry for the pollen tube.
• Megaspore Mother Cell: A $2n$ megaspore mother cell (megasporocyte) forms within the nucellus.
• Sporic Meiosis: The $2n$ megaspore mother cell undergoes sporic meiosis.
• Megaspore Production: Four haploid ($1n$) megaspores are produced.
• Selective Survival: Typically, three of the four megaspores die and degenerate, leaving only one functional $1n$ megaspore.

Megagametogenesis (Embryo Sac Development)

• Starting Point: The single functional $1n$ megaspore within the ovule (surrounded by $2n$ integuments and nucellus).
• Mitotic Divisions: The nucleus of the functional megaspore undergoes $3$ successive rounds of mitosis without cytokinesis, resulting in $8$ haploid ($1n$) nuclei within a single cell.
• Nuclear Migration and Cellularization: These $8$ nuclei migrate to specific positions, and then cellularization occurs, forming the $7$-celled, $8$-nucleate Embryo Sac (the mature megagametophyte).
• Embryo Sac Composition:
  • Egg Cell ($E$) ($1n$): One central cell, located near the micropyle, which will be fertilized by one sperm.
  • Synergids ($s$) ($1n$): Two cells flanking the egg cell, assisting in pollen tube guidance.
  • Antipodals ($a$) ($1n$): Three cells located at the opposite end of the embryo sac.
  • Central Cell ($c$) ($1n$): A large cell containing two polar nuclei, often referred to as a binucleate central cell. This cell becomes the $3n$ Endosperm Mother Cell after fertilization.
• Gametophyte Dependency: The megagametophyte is a dependent structure, fully enclosed within the sporophyte tissues of the ovule.

Angiosperm Double Fertilization

• Pollen Tube Arrival: The mature pollen grain germinates on the stigma, and the pollen tube grows through the style towards the ovule, guided by signals from the synergids.
• Synergid Interaction: The pollen tube contacts a receptive synergid. The receptive synergid will typically undergo Programmed Cell Death (PCD).
• Double Fertilization Event: Two distinct fertilization events occur simultaneously:
  • First Fertilization: One $1n$ sperm cell fuses with the $1n$ egg cell ($E$), resulting in a diploid ($2n$) Zygote.
  • Second Fertilization: The other $1n$ sperm cell fuses with the $1n$ binucleate central cell (containing two polar nuclei), forming a triploid ($3n$) Endosperm Mother Cell.
• Significance: Both fusion events are essential for seed development and are collectively known as Double Fertilization.

Angiosperm Seed Development

• Components Post-Fertilization:
  • Zygote ($2n$): Formed by the fertilization of the egg.
  • Endosperm Mother Cell ($3n$): Formed by the fertilization of the central cell.
  • Integuments ($2n$): The outer protective layers of the ovule.
• Conversions During Seed Development:
  • The $2n$ Zygote develops into the $2n$ Embryo (the young sporophyte) through mitotic divisions.
  • The $3n$ Endosperm Mother Cell develops into the $3n$ Endosperm, which serves as the primary food storage tissue providing nutrients for the developing embryo and germinating seedling.
  • The $2n$ Integuments develop into the $2n$ Seed Coat, which provides protection for the embryo and endosperm, and can also aid in seed dispersal.
• Mature Angiosperm Seed: Contains the $2n$ embryo, $3n$ endosperm, and $2n$ seed coat.
• Examples of Seed Structure (e.g., Castor Bean Seed):
  • Longitudinal View: Reveals the seed coat, endosperm, cotyledon, and embryo axis.
  • Cross-sectional View: Shows the protective seed coat surrounding the embryo and the nourishing endosperm, with typical dimensions like $2$ mm.

Asexual (Clonal) Reproduction in Angiosperms

• Definition: A form of reproduction that does not involve gametes or fertilization, resulting in offspring that are genetically identical to the parent plant.

Rhizomes

• Description: Horizontal, underground stems that grow new shoots from nodes.
• Structure: Features include adventitious roots and scale leaves at nodes.
• Example: American Beachgrass.

Stolons (Runners)

• Description: Horizontal, above-ground stems that produce new plants at their nodes.
• Example: Strawberry plants typically propagate via stolons.

Roots

• Description: Adventitious buds can form on roots, leading to new shoots that emerge from the ground, creating clonal colonies.
• Example: The "Pando" colony of Populus tremuloides (Quaking Aspen) in Bryce Canyon, Utah, is a single clonal organism estimated to be $14,000$ years old.

Tubers

• Description: Swollen, underground stem portions that store food.
• Structure: They possess 'eyes' which are axillary buds capable of sprouting new plants. A rhizome often terminates in a swollen tuber.
• Example: The potato is a classic example of a tuber.

Corms

• Description: Short, vertical, swollen underground plant stems that serve as storage organs for food.
• Structure: Differentiated into nodes and internodes, covered by scale leaves. Daughter corms can form from axillary buds.
• Examples: Crocus and Nut Sedge.

Bulbs

• Description: Underground storage organs consisting of a short stem enclosed by thickened, fleshy leaves that store food.
• Structure: Contain a flower bud and lateral buds that can develop into new plants, as seen at the end of a growing season.
• Example: Ramps (wild leeks) reproduce using bulbs.

Plantlets

• Description: Miniature plants that form on the margins or surfaces of leaves, often in specialized structures.
• Structure: These plantlets can detach from the parent leaf and grow into new, independent plants.
• Example: Kalanchoe sp. is known for forming plantlets on its thickened, fleshy leaves.