Angiosperms

Angiosperms

• most diverse group of land plants - over 250,000 sextant species

• phylum anthophyta

Features shared with other land plants

Alternation of generations between multicellular haploid and multicellular diploid stages

Features shared with seed plants

• Sporophyte is dominant generation/stage

• Gametophytes are microscopic and dependent on sporophyte

Features of angiosperms

Flower, double Fertilization, and Fruits

• synapomorphies of angiosperms

Flower

a specialized shoot with up to four sets of modified leaves, bearing structures that function in sexual reproduction

• main structures of flowers are attached to a

  part of the stem called receptacle

Sterile parts of flower

Sepals: A modified leaf that helps enclose and protect a flower bud

Petals: Colourful parts of a flower that attract insects and other pollinator

Flower reproductive organs for producing spores

Stamens (microsporophyll): pollen-producing organ consisting of stalk and anther, 2-4 pollen sacs

Carpels (mega sporophyll): ovule-producing organs consist of an ovary, a style and stigma

• flower may have >1 carpels, that can be fused into a pistil

• Ovary can contain 1 or more ovules within a single mother cell

• megaspore develops int female gametophyte by meiosis

Development of male gametophytes

• pollen grains consist of a two-celled male gametophyte enclosed in an outer wall rich in sporopollenin

• pollen grains are developed from microspores within the microsporangia of anthers

• Each microspore produce two-celled male gametophytes through mitosis:

  • Generative cell develops into two sperm nuclei

  • Tube cell forms pollen tube

Development of female gametophytes

• embryo sac develops within an ovule within the ovary of the flower (given ovule has two integuments surrounding megasporangium)

• four megaspores are developed from the megasporocyte by meiosis within the megasporangium

  • only one functional megaspore will form

  • megaspores produce a seven-celled female gametophyte

  • Polar nuclei: large central cel within g=female gametophyte with two nuclei

Pollination in angiosperms

A necessary reproductive process for fertilization where pollens from an anther are transferred to part of a seed plant containing the ovules

Events following successful pollination

• The tube cell produces a pollen tube

• The generative cell produces two sperm nuclei by

  mitosis

• The pollen tube grows into the ovary, to release

  sperm nuclei into an ovule, which leads to

  fertalisation

Double Fertilization

A mechanism of fertilization in which two sperm nuclei unite with two cells in the female gametophyte to form the zygote

• one sperm nucleus fertilizes the egg into a diploid zygote

• second sperm nucleus combines with two polar nuclei into a triploid (3n), which develops into endosperm and functions to nourish the embryo

Post-fertilization seed development

• Each fertilised ovule develops into a seed

• The ovary develops into a fruit enclosing the

  seed(s)

• Seed development:

  • Endosperm typically develop before embryo development

  • The triploid endosperm nucleus grows into starchy food tissues for the embryo through multiple rounds of mitosis

  • The zygote forms elongated embryo containing cotyledons, shoots, and roots

Fruit Development and function

• development takes place as sporophyte ovary thickens and mature

• dry fruit develop from drying of ovary

• fleshy fruit develop when ovary becomes thick, soft, and sweet at maturity

• fruits protect seeds, and facilitate seed dispersal

Adaptive advantages of angiosperm fertilization

• endosperm develop only after double fertilization

• fruit development is initiated by fertilization

• Minimal resources required for reproduction as size of female gametophytes is significantly reduced

• life cycle completed shorter, especially small female gametophytes (days)

Coevolution

The joint evolution of two interacting species, each in response to selection imposed by the other

Coevolution between flowers and pollinators

• corresponding shapes and size of pollen-transporting parts of pollinating animals

• mutualistic relationships: plants allocate less energy for dispersal, pollinators receive rewards for pollination

Factors contributing to adaptive radiation of angiosperms

• Presence of modified xylem with vessel tissues

• Higher rates of water transportation

• Rapid speciation from coevolution with pollinators

• Rapid reproduction

Angiosperm evolution

• divergence of angiosperms and gymnosperms took place 305 million years ago

• Amborella trichopoda most basal extant angiosperm species

Angiosperm Phylogeny

• classified based on number of cotyledons into monocots and dicots

• monocots are monophyletic, dicots are paraphyletic

  • presence of one cotyledon is synapomorphy for monocots

• Eudicot (two cotyledons) within paraphyletic dicots form monophyletic group, containing near 70% of extant angiosperm species

Monocots

• 25% of species diversity (ex. orchids, grass)

• trimerous flowers

• single cotyledon

Eudicots

• 70% of angiosperms (ex. daises, legumes)

• over 300 families

• four/five-merous flowers

• two cotyledon