Module 1&2 | Week 1&2

• Plants have evolved different reproductive strategies for the continuation of species. Some plants reproduce sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual reproduction.

  
• Pollinators


  • Butterflies
  • Birds
  • Bees
• Sexual Reproduction in plants

  Plants have two distinct stages in their life cycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and female gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte.

  
• Alternation of generations


  The cycle of the haploid gametophyte alternating with the diploid sporophyte during the sexual reproduction process of angiosperms.

  
• Stages of alternation of generations

  • The diploid sporophyte has a structure called sporangium
  • The sporangium undergoes meiosis and forms haploid spores
  • The spore develops into a gametophyte which is haploid in nature
  • The gametophyte has the reproductive organs which undergo mitosis to form haploid gametes
  • The gametes fertilize to form a haploid zygote which matures into a mature sporophyte

  
• The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.
• Flowers contain the plant's reproductive structures. A typical flower has 4 main parts-or whorls-known as the calyx, corolla and androecium and gynoecium.

  
• Gymnosperm reproduction


  The female gametophyte is present on exposed bracts of the female cone. Double fertilization is absent. The male and female gametophyte structures are present on separate male and female cones. Wind plays an important role in pollination.
• Asexual reproduction in plants

  Produces plants that are genetically identical to the parent plant because no mixing of male and female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when compared with plants and produced from sexual reproduction because they carry genes identical to those of their parents.
• Types of asexual reproduction in plants

  • Corm
  • Bulbs
  • Stem tubers
  • Rhizomes
  • Adventitious roots
  • Stolons

  
• Advantages of asexual reproduction

  The resulting plant will reach maturity faster and be sturdier than a seedling.
• Methods of asexual reproduction

  • Natural (e.g. growth from buds, adventitious roots, leaf buds)
  • Artificial (e.g. grafting, cutting, layering, micropropagation)

  
• Life cycle of an angiosperm

  The adult, or sporophyte, phase is the main phase. Angiosperms are heterosporous, producing microspores that develop into pollen grains (the male gametophytes), and megaspores that form an ovule containing the female gametophytes. Double fertilization occurs, with one sperm fusing with the egg to form the zygote, and the other sperm fusing with the central cell to form the endosperm.

  
• Pollination


  The transfer of male gametes (pollen grains) from the anther to the stigma. It is a prerequisite for fertilization and can be carried out by biotic (insects, animals) or abiotic (wind, water) agents.
• Fertilization


  The fusion of haploid male and female gametes to form a diploid zygote, initiating the development of a new individual. In plants, it follows pollination and involves the hydration and activation of pollen grains, formation of the pollen tube, and release of the sperm into the ovule.

  
• Most flowers carry both stamens and carpels, but some species self-pollinate. Self-pollination is a severe form of inbreeding and can increase the risk of genetic disorders.
• Pollen tube


  Filaments that eventually lead to the formation of the pollen tube
• Male gamete in plants

  Non-motile, depends on the pollen tube for transfer from the stigma to the ovary
• Pollen tube formation


  1. Penetrates the stigma

  2. Elongates along the extracellular matrix of style

  3. Reaches the ovary

  4. Penetrates the ovule through the micropyle

  5. Bursts in the embryo sac, releasing the sperm
• In gymnosperms, the formation of the pollen tube doesn't occur as the ovules are not enclosed with the ovary
• Angiosperms undergo double fertilization where two sperms from each pollen tube fertilize two cells present in the ovary

  
• Perfect flowers


  Flowers that contain both stamens and carpels
• Self-pollination


  A severe form of inbreeding that can increase the number of genetic defects in offspring
• Types of plants based on flower gender

  • Monoecious (one house - perfect flowers or imperfect flowers on one plant)
  • Dioecious (two houses - male and female flowers on separate plants)
• Sexual reproduction produces genetically diverse offspring that differ from their parents
• Sexual reproduction requires maintaining two different types of individuals, males and females, which can limit the ability to colonize new habitats
• Hermaphroditism


  Animals where one individual has both male and female reproductive parts
• Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring</b>
• Asexual reproduction


  Produces offspring that are genetically identical to the parent
• In a stable or predictable environment, asexually reproducing species may be at an advantage, but in an unstable or unpredictable environment they may be at a disadvantage
• Fission


  An organism splits into two separate organisms
• Budding


  Outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals
• Fragmentation


  Breaking of the body into two parts with subsequent regeneration
• Parthenogenesis


  A form of asexual reproduction where an egg develops into a complete individual without being fertilized
• Developmental stages in animals

  • Gametogenesis
  • Fertilization
  • Cleavage
  • Gastrulation
  • Organogenesis
  • Growth
• Gametogenesis


  The stage of animal development where the end-product is the haploid gamete
• Meiosis in gametogenesis


  1. Meiosis I (chromosome number reduced to half)

  2. Meiosis II (each cyte divides into two gametes)
• Heterogamy


  Male and female gametes are drastically different in size and shape
• Spermatogenesis


  1. Spermatogonia divide to produce more spermatogonia or become primary spermatocytes

  2. Primary spermatocyte divides into two secondary spermatocytes in Meiosis I

  3. Each secondary spermatocyte divides into two spermatids in Meiosis II

  4. Spermatids differentiate into spermatozoa

  
• Oogenesis


  1. Oogonia divide to produce more oogonia or become primary oocytes

  2. Diploid primary oocyte divides unequally to produce one secondary oocyte and a polar body in Meiosis I

  3. Secondary oocyte divides unequally during Meiosis II to produce an ootid and another polar body

  4. Ootid matures into a functional ovum