Sexual Reproduction in Flowering Plants

Male gametophyte development: • Takes place in the anther (specifically within the pollen sac / microsporangium) • The microspore mother cells (2n) undergo meiosis to form microspores (pollen grains) • Each pollen grain develops into the male gametophyte (microgametophyte) • A mature pollen grain contains a vegetative cell and a generative cell (which divides to form two male gametes) Female gametophyte development: • Takes place in the ovule (specifically within the nucellus of the ovary) • The megaspore mother cell (2n) undergoes meiosis to form 4 megaspores • Usually only one functional megaspore develops into the female gametophyte (embryo sac) • Mature embryo sac (7-celled, 8-nucleate): egg cell + 2 synergids + 3 antipodal cells + central cell with 2 polar nuclei

Zoospore: • An asexual reproductive structure • Produced by certain algae and fungi • Motile — has flagella for movement • Haploid (n) — formed by mitosis from haploid parent cells • Does NOT require fusion with another cell • Directly develops into a new organism • Example: Chlamydomonas, Ulothrix produce zoospores Zygote: • A sexual reproductive structure • Formed by fusion (fertilisation) of two gametes (male + female) • Non-motile in most organisms • Diploid (2n) — formed by union of two haploid gametes • Requires fertilisation to form • Develops into embryo through mitotic divisions • Example: fertilised egg in animals; oospore in plants Key difference: Zoospore is asexual, haploid, motile; Zygote is sexual, diploid, non-motile.

Geitonogamy: • Transfer of pollen grains from the anther to the stigma of another flower on the SAME plant • Genetically, it is equivalent to self-pollination (same genotype) • Functionally cross-pollination (requires a pollinating agent) • Does not produce genetic variation • Examples: Maize (different male and female flowers on same plant) Xenogamy: • Transfer of pollen grains from the anther to the stigma of a flower on a DIFFERENT plant • True cross-pollination — involves genetically different individuals • Produces genetic variation in offspring • Evolutionarily significant — promotes diversity • Requires pollinating agents (wind, insects, birds, etc.) Key difference: Geitonogamy = same plant (genetically self), Xenogamy = different plant (genetically cross).

Chasmogamous flowers: • Flowers that open (expose anthers and stigma) at the time of maturity • Normal flowers where cross-pollination can occur • Anthers and stigmas are exposed to pollinating agents • Example: Most common flowers — Rose, Hibiscus, Sunflower Cleistogamous flowers: • Flowers that do NOT open at all — remain permanently closed • Anthers and stigma remain enclosed • Only self-pollination (autogamy) can occur — never cross-pollination • Example: Viola (violet), Oxalis, Commelina — have both chasmogamous and cleistogamous flowers Can cross-pollination occur in cleistogamous flowers? No. Cross-pollination CANNOT occur in cleistogamous flowers because: 1. The flower never opens, so no pollinating agent can reach the inside 2. Pollen from another plant cannot enter the closed flower 3. Only self-fertilisation is guaranteed Note: Cleistogamy ensures seed set even in the absence of pollinators.

Flowering plants have evolved several mechanisms to prevent self-pollination (and promote cross-pollination): 1. Dichogamy (temporal separation): • Anther and stigma of the same flower mature at different times • Protandry: Anthers mature before the stigma (e.g., Sunflower, Salvia, Marigold) • Protogyny: Stigma matures before the anthers (e.g., Mirabilis, some Magnolia) • Result: When pollen is released, the stigma of the same flower is not receptive yet 2. Herkogamy (spatial separation / self-incompatibility): • Physical barriers or placement differences prevent self-pollination • Anthers and stigma are placed at different heights or positions in the flower • Example: In Primula — pin flowers (long style, short stamens) and thrum flowers (short style, long stamens) — pin style can only be pollinated by thrum pollen 3. Self-incompatibility (additional strategy — genetic): • Genetic mechanism — pollen from the same plant is rejected by the stigma • Pollen tube growth is inhibited if the same S-allele is present • Example: many plants in Solanaceae, Rosaceae 4. Unisexuality (dioecy): • Male and female flowers on different plants (dioecious) • Makes self-pollination structurally impossible • Example: Papaya, Date palm

Triple fusion is the fusion of one male gamete with the two polar nuclei (or the secondary nucleus) of the central cell of the embryo sac. It is a component of double fertilisation, unique to angiosperms. Where it takes place: • In the embryo sac (female gametophyte), in the central cell How it takes place: 1. The pollen grain lands on the stigma and germinates 2. The pollen tube grows down through the style and enters the ovule through the micropyle 3. The pollen tube releases two male gametes (n) into the embryo sac 4. One male gamete (n) fuses with the egg cell (n) → zygote (2n) — this is syngamy / true fertilisation 5. The other male gamete (n) moves to the central cell and fuses with the two polar nuclei (n + n) simultaneously → Primary endosperm nucleus (PEN) = 3n (triploid) 6. This fusion of ONE male gamete + TWO polar nuclei is called Triple Fusion Nuclei involved in triple fusion: • 1 male gamete nucleus (n) • 2 polar nuclei (n + n) → Result: Primary Endosperm Nucleus (3n, triploid) This 3n PEN develops into the endosperm, which nourishes the developing embryo.

The zygote remains dormant (does not immediately start dividing) for some time after fertilisation because: 1. Endosperm must develop first: • The endosperm (from triple fusion) develops before the zygote begins dividing • Endosperm provides nutrition for the developing embryo • Without adequate nutritional support, premature embryo development would be unsuccessful 2. Physiological conditions must be established: • The embryo sac must reach optimal hormonal and nutritional milieu • The surrounding tissues need to prepare for embryo nourishment 3. Developmental priority: • Endosperm formation takes precedence to ensure resources are available • Only after endosperm is established does the zygote begin mitotic divisions 4. Seed maturation environment: • In seeds, the zygote (now embryo) enters dormancy as the seed dries and matures • This allows seeds to survive unfavourable conditions • Dormancy is broken only when conditions (moisture, temperature, light) are favourable for germination In summary: The zygote waits for endosperm to develop first, then begins embryogenesis when nutritional and physiological support is available.

Development of male gametophyte (microgametogenesis) in angiosperms: Step 1 — Microspore mother cell: • The anther contains 4 pollen sacs (microsporangia) • Each microsporangium contains microspore mother cells (MMC) = 2n Step 2 — Meiosis: • MMC undergoes meiosis I and meiosis II • Produces a tetrad of 4 microspores (each n = haploid) • Tetrads are initially held together, then separate Step 3 — Pollen grain formation: • Each microspore develops into a pollen grain • The microspore is the first cell of the male gametophyte Step 4 — First mitotic division (within pollen grain): • The microspore nucleus divides mitotically → forms 2 cells: (a) Large vegetative cell (tube cell) — has a large, irregular nucleus (b) Small generative cell — has a dense, spindle-shaped nucleus • Pollen grain is now 2-celled Step 5 — Wall formation: • Pollen grain develops a thick wall: – Exine (outer): made of sporopollenin (extremely resistant) – Intine (inner): made of cellulose and pectin • Germ pores present where exine is thin Step 6 — Second mitotic division: • The generative cell divides to form 2 male gametes (before or after pollination) • Some plants shed pollen at 2-celled stage, some at 3-celled stage Mature pollen grain (male gametophyte) contains: • 1 vegetative cell + 2 male gametes (3-celled stage)