Evolution of Angiosperms

• Angiosperms


  Flowering plants, also called Angiospermae, Magnoliophyta, or Anthophyta
• Angiosperms


  • Monophyletic group
  • Sister group to gymnosperms
  • Most numerous, diverse, and successful extant plant group
  • Contain well over 95% of all land plant species alive today
  • Grow in virtually every habitable region
  • Dominant in some aquatic and most terrestrial ecosystems
  • Exception is coniferous forests
  • Comprise the great bulk of our economically important plants, including our most valuable food crops
• Flower


  A modified, determinate shoot system bearing one or more stamens (androecium) and/or one or more carpels (gynoecium)
• Androecium


  Male parts of a flower containing the stamen and its anther and filament that function to produce pollen
• Gynoecium


  Female parts of the flower containing the carpel or pistil and its stigma, style, and ovary that function to produce ovules
• Most angiosperm flowers are bisexual (perfect), containing both stamens and carpels, but some are unisexual (imperfect), having only stamens or carpels
• Perianth


  Protects the other floral parts during floral development and functions as an attractant for pollination
• Calyx


  Generally green and photosynthetic, composed of leaf-like sepals or (if these are fused) of calyx lobes
• Corolla


  Typically colorful, showy, and odoriferous and is composed of individual petals or (if these are fused) of corolla lobes
• Tepal


  Used for similar perianth parts when the outer and inner whorl are not otherwise differentiated
• The perianth of some flowers consists of spirally arranged units that grade from sepal-like structures on the outside to petal-like structures on the inside, with no clear point of differentiation between them
• Floral development


  1. Actively dividing regions of cells grow, forming bumplike mounds of tissue (primordia)

  2. Primordia develop in whorls from outside to inside, in sequence as sepal (or outer tepal) primordia first, petal (or inner tepal) primordia second, stamen primordia third, and carpel primordia last
• ABC model of floral development

  • Gene products of the A, B, and C classes combine to produce the four major floral organs: sepals, petals, stamens, and carpels
  • Sepals are expressed by A activity alone
  • Petals by a combination of A and B activities
  • Stamens by a combination of B and C activities
  • Carpels by C activity alone
• SEPALLATA genes


  Needed in combination with A, B, and C classes to effect proper floral organ identity
• Floral organ identity genes work by producing transcription factors in the proper location of the flower, which induce the expression of other genes that bring about the development of the four floral organs
• Animal pollination


  Appears to be the primitive condition in the angiosperms, separating them from the predominantly wind-pollinated gymnosperms
• Numerous, intricate pollination mechanisms have evolved in various angiosperm lineages, largely driving the evolution of innumerable floral forms and accounting for the distinctiveness of many angiosperm families
• Animal pollinators


  • Bees
  • Butterflies and moths
  • Flies
  • Bats
  • Birds
• Flowers of many groups are quite reduced in size or structural complexity, often lacking a perianth altogether; these may be water pollinated or wind pollinated
• Stamen


  The male reproductive organ of a flower, modified microsporophylls that bears microsporangia
• Stamen


  • Some have a laminar (leaf-like) structure, to which the anther is attached or embedded
  • Most have two parts: a stalk (filament) and the pollen bearing part (anther)
  • Some lack a filament, in which case the anther is sessile (directly attached)
• Anther


  • A type of synangium, a fusion product of sporangia
  • Unique in containing two pairs of microsporangia arranged in bilateral symmetry (two mirror image halves)
  • Each pair of microsporangia is located within a discrete half of the anther called a theca
  • At maturity, the two microsporangia of a theca typically coalesce into a single, contiguous chamber called the anther locule
• Some angiosperms have anthers that are secondarily reduced to a single theca, known as monothecal or bisporangiate
• Adaptive value of angiosperm stamens over gymnosperm microsporophylls

  • Stamens are generally smaller and lighter, and occur in bisexual flowers rather than unisexual cones
  • Modifications have enabled the evolution of specialized pollination mechanisms
• Male gametophyte of angiosperms

  • Reduced, three-celled
  • No other plant group has a male gametophyte so reduced in cell number
• Development of angiosperm male gametophyte

  1. After microspore formation by meiosis, its single nucleus divides mitotically to form a tube cell and a generative cell

  2. The generative cell then divides one time, producing two sperm cells

  3. Pollen grains are shed in either a two- or three-celled condition, depending on whether the generative cell division occurs before or after pollen release
• Pollen tube


  • Angiosperms: Forms immediately after transfer of pollen to the stigma, elongates through the tissues of the stigma and style, and reaches the ovule to transport the sperm cells
  • Gymnosperms: Develops after pollen enters the ovule, functions as a haustorial device feeding from the nucellus
• The sperm cells of angiosperms lack flagella or cilia and are thus nonmotile, a derived condition among land plants
• Adaptive significance of reduced male gametophytes in angiosperms


  • Correlated with the evolution of a reduced female gametophyte and relatively rapid seed development
  • In gymnosperms, fertilization occurs long after pollination, so male gametophytes must persist and feed off the nucellus
  • In angiosperms, fertilization occurs very soon after pollination, so the male gametophyte can be lean and just function to rapidly deliver sperm cells
• Carpel


  • A modified, conduplicate megasporophyll bearing two, adaxial rows of ovules
  • The margins fuse, with certain parts differentiating into tissue for pollen reception and pollen tube growth, forming an apical stigma and style
  • At maturity, the carpel body completely encloses the ovules and seeds, accounting for the name angiosperm
• Carpel


  • May develop like a leaf, having an initially flattened, dorsiventral shape with an adaxial and abaxial surface
  • Has veins, typically one in the middle (dorsal/median vein) and two near the margins
• Pistil


  May be equivalent to one carpel (simple pistil) or composed of two or more, fused carpels (compound pistil)
• Adaptive significance of carpels

  • Function to selectively control fertilization through incompatibility reactions
  • Facilitate fruit formation and seed dispersal
• Incompatibility reactions mediated by incompatibility genes may inhibit inbreeding, allowing for reproduction only between genetically dissimilar individuals
• Fruit


  The mature ovary or ovaries (made up of one or more carpels) plus any accessory tissue
• Incompatibility reaction


  Chemical incompatibility that inhibits pollen germination or pollen tube growth, mediated by incompatibility genes
• Incompatibility reaction often occurs between

  Pollen and stigmatic regions of different species
• Incompatibility reaction may also occur between

  Individuals of the same species that are genetically similar and possess the same incompatibility alleles
• Incompatibility reactions


  Inhibit inbreeding, allowing for reproduction only between genetically dissimilar individuals of the species (i.e., promoting out-crossing)
• Carpel


  Provides some selective control as to which pollen grains contribute the sperm cells that fertilize the egg