Gymnosperms and Seed Evolution Quiz

Gymnosperms and Seed Evolution - Conifers

• Gymnosperms evolved from free-sporing progymnosperms about 425 million years ago.
• Seed evolution had significant ecological impacts.
• A sporophyte tissue layer protects the developing embryo.
• Nucellus is the macrosporangium of the seed plant, surrounded by integuments.
• Integuments harden into the seed coat.

Non-Seed Plant Life Cycle Disadvantages

• Water is still necessary for male gametes to reach female gametes.
• New sporophyte is temporarily dependent on small gametophyte plants (zygote and embryo).
• Many sporophytes die.

Advantages of Seed Plants

• Embryo utilizes pre-existing sporophyte photosynthetic and absorptive capabilities (leaves and roots).
• Female gametophyte (embryo sac) and embryo can remain within the maternal sporophyte, made possible by retention of the macrospore within the macrosporangium.

Transfer of Male Gametophyte

• The whole male gametophyte is now transferred to the female gametophyte (not just the male gametes).
• Vectors like wind carry mature microspore products (pollen grains) to female gametophytes.
• Pollen grain germinates on the maternal sporophyte.
• Male gametophyte develops within the microspore wall, becoming the pollen grain wall.
• Spermatophyta is the formal name for all plants that produce seeds.

Seed Plant Adaptations

• Seeds enclose the embryo in protective seed tissue, allowing for dormancy.
• Dormancy ensures the embryo can survive until favorable conditions for germination arise.
• This leads to optimal germination and seedling development.

Gymnosperm Classification

• Spermatophyta:
  • Gymnospermae: ovules are exposed to the atmosphere
  • Angiospermae (Anthophyta): ovules are enclosed within the sporophyte tissue.
• Although gymnosperm phyla exhibit morphological variation, their life cycles, gametophyte structures, and fertilization processes are relatively similar.

Early Seeds

• Early seeds of the late Paleozoic era (400-250 million years ago) typically occurred terminally on the stem axis, exhibiting bilateral or radial symmetry.
• Archaeosperma is one of the oldest plants with well-developed seed structures.
• Archaeosperma lived in the late Devonian (360 million years ago).
• Archaeosperma is interpreted as a group of loosely arranged, radially symmetric seeds.
• Every seed has a macrosporangium (nucellus) surrounded by a protective integumentary layer with finger-like outgrowths.
• Later Carboniferous fossils show typical seed structures, with progressively more deeply enclosed macrosporangia within integuments.

Pollen - Male Version of a Seed

• Pollen is the male version of a seed.
• Immature male gametophyte inside the microspore wall.
• Evolved through the progressive reduction of the male gametophyte and gradual inclusion within the spore wall.
• This evolution happened independently in Selaginella.

Selaginella and Pollen Grains

• Selaginella male gametophytes germinate from tetrad scars.
• In contrast, true pollen grains emerge from a point opposite meiotic scars.
• In seed plants, these scars are absent, likely through evolutionary time.

Extant Gymnosperm Phyla

• Four extant gymnosperm phyla: Coniferophyta, Cycadophyta, Gnetophyta, and Ginkgophyta.
• Two extinct phyla: Progymnospermatophyta and Pteridospermatophyta
• Relationships between gymnosperm phyla remain somewhat contentious but gymnosperms are generally considered to be monophyletic.

Coniferophyta (Conifers)

• Most well-known gymnosperms, including pines, cedars, cypresses, and sequoias.
• Among the oldest and tallest living things, found in cold, temperate, and arid habitats worldwide.
• Commercially valuable resources like wood, paper, resin, turpentine, and taxol.

Pinus (Pine Tree)

• More than 100 species, confined to the Northern Hemisphere.
• Needle-shaped leaves in groups of 2-5 on short branches.
• Leaves adapted to minimize water loss (thick cuticle, sunken stomata).
• Contain resin ducts for protection against freezing and insects/fungi.
• Commercially important sources of turpentine and resin.

Pinus Life Cycle - Male

• Heterosporous, developing spores into two types of gametophytes.
• Pollen grains form in microsporangia of male cones (microstrobili).
• Male cones borne in groups on lower branches.
• Papery bracts (microsporophylls) spirally arranged.
• 2 microsporangia below the bract.

Pinus Life Cycle - Female

• Female cones (macrostrobili) larger than male cones, with bracts that become woody.
• 2 ovules develop on the top of each macrosporophyll.
• Each ovule has a macrosporangium (nucellus).
• Nucellus surrounded by integumentary layer.
• Tiny opening in integument (micropyle).
• Integument hardens into seed coat.

Pinus Life Cycle - Fertilization

• Pollen grain carried by wind lands in sticky pollination droplet on micropyle.
• Pollen tube germinates and grows to the archegonium.
• Zygote develops within the seed, into an embryo.
• Seeds released after dormancy.
• Seeds germinate to form new sporophyte generation.
• One sperm fuses with the egg to form a zygote, creating the embryo.
• The other parts of the male gametophyte disintegrate.

Test your knowledge on the evolution of gymnosperms and seed plants. This quiz covers key concepts including the advantages of seed plants, the life cycle of non-seed plants, and the transfer of male gametophytes. Explore the ecological impacts of these evolutionary advancements!