Sexual Reproduction in Flowering Plants

• Sexual reproduction involves the fusion of haploid nuclei within gametes.

The Flower

• Flowers are reproductive parts which produces fruit and seeds
• Flowers are a modified shoot bearing special modified leaves.
• The duration of growth, of a flower, is limited.
• Inflorescence is when flowers are clustered together.

Structure and Function of the Flower

• The receptacle is part of the flower stalk where the flower parts are attached.
• Nectaries produce nectar.
• The three major parts of a flower are the perianth, androecium, and gynoecium.
• The androecium and gynoecium are the flower's reproductive parts.

The Perianth

• The perianth is the flower's outer protective part and consists of sepals and petals.
• Sepals and petals attach below the reproductive parts.
• Sepals are connected below the petals and protect inner flower parts in the bud.
• Petals attract pollinators.
• Collectively the sepals form the calyx (plural calyces).
• Collectively, petals form the corolla.

The Androecium

• The androecium (pl. androecia) consists of stamens that produce pollen.
• Stamens comprise a filament connected to an anther.
• An anther has two lobes containing four pollen sacs arranged in two pairs.
• Pollen sacs are referred to as microsporangia (sing: microsporangium).
• Pollen is the microgametophyte.
• The stamens attach below the gynoecium.
• Meiosis occurs within pollen sacs, leading to pollen production.
• Pollen is shed when the anthers open.

The Gynoecium

• The gynoecium (pl. gynoecia) is the site of pollination and fertilisation.
• The gynoecium consists of all the carpels in the flower
• Carpels are composed of a stigma, style, and ovary
• Flowers may contain one or more carpels, and carpels can be separate or fused.
• Individual or fused carpels are the pistil
• The stigma collects pollen and aids in germination.
• The style is a slender stalk for pollen tube growth.
• The ovary is an enlarged carpel portion that surrounds and protects the ovules.
• The ovule is the megasporangium, containing the megagametophyte, which is the female gametophyte or embryo sac.

Types of Flowers and Plants

• A bisexual or hermaphrodite flower has both male and female reproductive parts.
• Unisexual flowers have male and female reproductive parts on different flowers.
• Staminate flowers only have an androecium.
• Carpellate flowers only have a gynoecium.
• Monoecious plants have staminate and carpellate flowers on the same plant, e.g., pumpkin (Cucurbita sp.).
• Dioecious plants have staminate and carpellate flowers on separate plants, e.g., sea grape (Coccoloba uvifera).
• Polygamous plants have both bisexual and unisexual flowers on the same plant, e.g., mango (Mangifera indica).

Anthers

• Anthers have four pollen sacs arranged in two pairs.
• The innermost layer of the anther wall is the tapetum.
• The tapetum is a layer of cells rich in food
• The formation of pollen is achieved through two separate processes
• Microsporogenesis is the formation of microspores or single-celled pollen grains within the microsporangia or pollen sacs of the anther.
• Micro gametogenesis is the development of the microgametophyte within the pollen grain.
• The anther first has a uniform mass of cells which develop into four fertile cells
• Several layers of sterile cells surround each group of fertile cells.
• The sterile cells develop into tapetum wall, which supplies food to the developing microspores.
• The fertile cells become microspore mother cells, which divide by meiosis to give rise to a tetrad of haploid microspores.
• In microgametogenesis, each microspore divides by mitosis, forming two cells within the microspore walls.
• The two cells formed are small (generative cell) and large (tube cell).
• This is the point where the pollen grain is released from the anther.
• The tube cell directs the growth of the pollen tube after the pollen comes in contact with a receptive stigma.
• The generative cell divides into two male gametes (sperm) within the pollen tube.
• Plant sperm are immotile.
• Sperm are delivered via pollen grains and pollen tube to female gametophyte
• One of the two sperm fertilises the egg cell to produce the embryo, the other fertilises the central cell to produce the endosperm.
• The anther opens to release pollen when the pollen is mature and environmental conditions are appropriate.
• Dehiscence is the shedding of pollen from an anther.
• Pollen's ability to complete fertilisation is affected by temperature and humidity.

Pollen Grains

• Pollen grains range in size from 10 um to 350 um.
• Pollen grains have various shapes.
• The surface of the pollen wall is the exine.
• The exine may have spines, ridges, or other surface projections
• Pollen dispersed by wind is normally smooth.
• Pollen dispersed by animals have projections that allow it to attach to the animal body.
• Copious amounts of pollen in the atmosphere can cause the allergic reaction called hay fever.

Ovules

• Megasporogenesis is the process of megaspore formation.
• Megasporangia contain megaspores that develop into megagametophytes.
• The ovule is a megaspore that develops into the female megagametophyte.
• Two protective layers called integuments surround the ovule.
• The ovule is attached to the ovary wall by a stalk called the funicle.
• The regions of the ovary wall that bears the ovule are the placentae.
• The integuments surrounding the ovule leave a small opening called the micropyle, which allows the pollen tube to grow through to reach the embryo sac.
• In the development of the ovule, a single diploid megaspore mother cell(megasporocyte) forms and meiosis occurs within the ovule.
• Four haploid megaspores are generated.
• Three of the haploid megaspores degenerate.
• One living haploid megaspore undergoes one round of mitosis and two nuclei are formed.
• Each of the nuclei from the first round of mitosis undergoes a second round of mitosis and four nuclei are formed.
• In the third round of mitosis, eight nuclei are formed.
• The eight nuclei arrange themselves into two groups of four at opposite ends of the haploid megaspore(now called the embryo sac)
• One nucleus from each end migrates to the center of the embryo sac; these two nuclei are now called the polar nuclei.
• A cell membrane forms around the polar nuclei to give the central cell.
• The three nuclei at the micropyle end become three cells: the egg cell and two cells call synergids.
• The three nuclei at the opposite end become three cells called antipodal cells.
• The mature female embryo sac comprises eight nuclei in seven cells.
• In some plants, the polar nuclei fuse to form a diploid nucleus
• When pollen grains fall on a stigma, they take up water from the surface.
• The pollen grain germinates, forming a pollen tube.
• The generative cell divides to give 2 sperm within the pollen tube.
• The pollen tube grows until it enters the ovule through the micropyle and penetrates one of the two synergids next to the egg cell.
• The synergid that was penetrated degenerates, and the two sperm are released.
• One sperm fertilises the egg cell to form the diploid zygote, and the other sperm fuses with the polar nuclei in the cell near the centre of the embryo sac to form the triploid primary endosperm nucleus.
• The union of one sperm with the egg cell and the second sperm with the polar nuclei is known as double fertilisation.
• Double fertilisation is unique to angiosperms.
• The remaining synergid and the antipodal cells degenerate after double fertilisation has occurred.
• The triploid primary endosperm nucleus divides after double fertilisation forming the endosperm, and the zygote develops into an embryo.
• Within days of fertilisation, the pistil begins to swell to accommodate the enlarging ovules.
• The flower sepals and petals wither and drop off.
• The embryos inside the ovules enlarge and differentiate
• The endosperm is also inside the ovule and this enlarges as well.
• The endosperm, rich in starch, oil, and other food reserves, remains as the food store for the developing embryo.
• The integuments surrounding the ovule develop into the seed coat, and the ovary develops into a fruit.
• Successful double fertilisation depends on the growth of the pollen tube to the unfertilised embryo sac, the release of the two sperm cells towards the egg cell and the central cell, recognition and fusion of gametes, and the growth of the zygote and endosperm.

Pollination

• Pollination is the transfer of pollen grains from the anther (androecium) to the stigma (gynoecium) of the flowering plant.
• There is wind and animal pollination

Wind Pollination

• Wind-pollinated plants have staminate and carpellate flowers on the same plant.
• Flowers generally have no nectar, no perfume, dull colours and small or no petals.
• Stamens are usually exposed.
• Anthers are suspended from long filaments hanging from the flower so they can easily lose their pollen.
• Carpellate flowers have large sticky or netlike stigmas to trap airborne pollen.
• The flowers are exposed with branches or feathery outgrowths adapted for intercepting pollen grains.
• The pollen grains are often small, light and have a smooth surface which allows them to float easily in wind currents.
• Some pollen grains are relatively large with air spaces, so they float in air easily e.g. Zea mays-corn

Animal Pollination

• Animal pollination is very efficient.
• Animal pollination promotes cross-pollination which enhances genetic variability of offspring.
• Flowers emit perfume to attract animals
• Animal pollination implements different strategies for day and night pollinators.

Self-Fertilisation

• Self-fertilisation occurs when fertilisation happens in one individual and it fertilises itself.
• Self-fertilisation can lead to inbreeding.
• Inbreeding increases the homozygote frequency and decreases heterozygote frequency.
• Vigour and fertility decrease as inbreeding increases – inbreeding depression.
• Harmful recessive alleles tend to become homozygous through inbreeding.
• Self-pollinating plants often have small, inconspicuous flowers compared to cross-pollinating plants.
• Inbreeding affects basic Mendelian traits and quantitative traits such as body size, vigour, fertility, and yield in agricultural crops.
• Vigour and fertility decrease as inbreeding increases; this is known as inbreeding depression.

Cross-Fertilisation

• Cross fertilisation occurs between two different individuals.
• The separation of staminate and carpellate flowers on monoecious plants can reduce the incidence of self-pollination.
• The maturation of the staminate and carpellate flowers at different times enhances the chance that cross fertilisation will occur.
• In protogyny, the stigmas of a single bisexual flower are receptive before the anthers in the same bisexual flower shed pollen (e.g., Soursop, sugar apple, water lilies).
• Protandry is when the anthers shed pollen before the stigmas are receptive (e.g., The daisy family of plants).
• Other mechanisms to limit self-fertilization include the arrangement of stamens and stigmas within the flower.
• For example, the stigma can be positioned higher than the anthers within the flower receptacle and pollinators, guided on a path so that they do not touch the stamen and stigma.

Self-Incompatibility

• Self-incompatibility is a mechanism that eliminates self-fertilisation.
• It is the inability of a bisexual plant to produce zygotes with its own pollen
• Promotes genetic variation and limits inbreeding
• It is genetically controlled by multiple alleles of a self-incompatibility gene S.
• A diploid bisexual plant has two of these alleles.
• Fertilisation will not be successful if a pollen grain has the same S allele as the stigma of the plants on which it falls.
• Selfing between plants with the same S alleles is prevented regardless of how genetically different they are.
• The most common type of self-incompatibility is called gametophytic incompatibility.
• The genotype (presence of S alleles) of the pollen determines if fertilisation will be successful.
• Pollen cannot fertilise an ovule if the pollen grain carries an S allele that is present in the maternal reproductive tissue.
• Fertilisation between two plants with the same two self-incompatibility alleles will not be successful.
• Fertilisation can occur and zygotes can be produced if self-incompatibility alleles are different.

Fruit and Seed Development

• Fruit is a matured ovary.
• The ovary wall (pericarp) and related structures develop into a fruit.
• As the ovule develops into a seed, the ovary thickens and develops into a fruit.
• The seed is a mature ovule that contains an embryo and nutritive tissues (endosperm).
• Nutritive tissues consist of two cotyledons or one cotyledon and an endosperm.
• The endosperm is a specialised tissue that contains starch, oils, and proteins, which are used as food for the germinating embryo and the young seedling.
• The seed coat (testa) surrounds the outside of a seed and develops from the integuments.
• The embryo consists of an epicotyl, radicle, plumule, hypocotyls, and cotyledons.
• The plumule develops into the shoot, and the radicle develops into the root.
• The hypocotyl connects the epicotyl and the radicle
• The cotyledons are the seed leaves.
• The cotyledons may become green and photosynthetic when the seed germinates and the embryo begins to grow into a seedling.
• The radicle emerges before shoot so that the seedling can obtain water for growth.
• Dicotyledons are flowering plants with 2 cotyledons.
• Monocotyledons have one cotyledon.

Fruit and Seed Dispersal

• Dispersal can be dispersed via wind water, or by animals.
• Wind dispersal can be assisted by hairs on seeds (e.g., Trinidad vine), wings on seeds, and other adaptations such as small dust-like seeds
• The Mahogany tree produces light, papery brown seeds shaped like propellers that twist in the wind.
• Seeds dispersed due to water have a tough outer covering and are able to float (e.g., coconut).

Genetic Consequences of Sexual Reproduction

• Sexual reproduction increases genetic variation.
• Plant populations that practice self-fertilisation has a high proportion of genetically similar individuals.
• There is a greater level of genetic diversity in plant populations that practice cross-fertilisation.