Gymnosperm Evolution, Pollen and Ovules

Gymnosperm Evolution

• In the late Devonian period, there were also ancient gymosperms called seed ferns, or Pteridospermales
• Fossil evidence indicates that by the late Devonian period (370 mya), some plants, called progymnosperms, had begun to acquire adaptations characteristic of seed plants
• Some progymnosperm species were homosporous, and some were heterosporous
• Archaeopteris is a progymnosperm

Pollen and Production of Sperm

• Microspores develop into pollen grains which contain the male gametophytes
• Pollination is the transfer of pollen to the part of the seed plant containing the ovules

Ovules

• An ovule consists of a megasporangium, megaspore, and one or more protective integuments
• Gymnosperm megaspores have one integument
• Angiosperm megaspores have two integuments

Evolutionary Advantage of Seeds

• A seed develops from the whole ovule
• A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat
• Seeds have evolutionary advantages over spores:
  • Seeds may remain dormant for days to years, until conditions are favorable for germination
  • Seeds may be transported long distances by wind, animals, pr water

Gymnosperms and Angiosperms

• Living seed plants are divided into two clades: gymnosperms and angiosperms
• Gymnosperms appear early in the fossil record and dominated the Mesozoic terrestrial ecosystems
• Gymnosperms were better suited than nonvascular plants to drier conditions
• Angiosperms evolved from the gymnosperms

Phylum Cycadophyta

• Individuals have large cones and palm-like leaves, tracheids, ovulate and microsporangia cones on separate plants
• Cycads thrived during the Mesozoic era, but few species exist today
• Cycad sperm is multiflagellated, and some species rely on insects like beetles for pollination, similar to angiosperms

Phylum Ginkgophyta

• This phylum consists of a single living species: Ginkgo biloba
• Ginkgo biloba has a high tolerance to air pollution and is cultivated as a popular ornamental tree, with tracheids, ovules, and microsporangia on separate plants
• Ginkgo biloba sperm is also multiflagellated
• The fleshy seed coat of Ginkgo biloba has a vile odor due to butanoic and hexanoic acids, similar to fatty acids found in rancid butter and Romano cheese

Heterospory

• The ancestors of seed plants were likely homosporous, while seed plants are heterosporous
• Megasporangia produce megaspores that give rise to female gametophytes (seeds)
• Microsporangia produce microspores that give rise to male gametophytes (pollen)

Phylum Gnetophyta

• This phylum comprises three genera
• Gnetophyte species vary in appearance and habitat; some are tropical, while others live in deserts
• Gnetophytes have tracheids and vessel elements, non-motile sperm, and ovulate and microsporangiate cones on separate plants
• Ephedra exhibits double fertilization like angiosperms, but it produces extra embryos instead of a 3n endosperm

Cones

• Male cones release pollen grains and rely on wind to reach female cones
• Female cones have ovules on protruding scales and product seeds when fertilized
• Wind dispersal of pollen is highly inefficient and pines have been using it to pollinate successfully for 200 million years

Pine Life Cycle

• The pine tree is the sporophyte and produces sporangia in male and female cones
• Small cones produce microspores called pollen grains that each contain a male gametophyte and are dispersed by wind

Gymnosperm Characteristics

• Gymnosperms bear "naked" seeds, typically on cones, that are not enclosed by ovaries
• Gymnosperms consist of four phyla: Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta

Phylum Coniferophyta

• Coniferophyta is the largest of the gymnosperm phyla
• Most conifers are evergreens, carrying out photosynthesis year round, and possess tracheids, non-motile sperm, and ovulate and microsporangiate cones on the same plant
• Pine trees usually disperse their offspring via windblown seeds
• Cones are modified leaves

Asexual Plant Reproduction

• A species of sea grass Posidonia australis, in Shark Bay, Australia is believed to be the largest plant on Earth
• Samples spanning 111 miles were proven to belong to a single plant by genetic testing
• The existing meadows are a single plant that originated from a seed 4500 years ago, and its underground root system is home to marine animals

Flower Structure

• A flower is a specilized chute with up to tour types of modified leaves:
  • Sepals enclose the flower
  • Petals attract pollinators with bright colors
  • Stamens produce pollen on terminal anthers
  • Carpels produce ovules
• A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received

Angiosperm Adaptations

• Angiosperms are seed plants with reproductive structures called flowers and fruits, making up the clade of plants known as vessel seeds
• Angiosperms are the most widespread and diverse of all plants

Flower Diagram

• Flowers have a stigma, carpel, style, ovary, stamen, anther, filament, petal, sepal, and ovule

Fruits

• A fruit (the ovary) typically consists of a mature ovary but can also include other flower parts
• Fruits protect seeds and aid in their dispersal
• Seeds can be carried by wind, water, or animals to new locations

Pollination Coevolution

• Nectar and brightly colored reproductive parts will attract animal dispersers
• Animals have adapted to plant phenolic secretions, and many plants have coevolved adaptations to facilitate pollination including:
  • Colors and patterns
  • Flower structure
  • Odors
  • Nectar

Angiosperm Characteristics

• All angiosperms are classified in a single phylum, Anthophyta (Magnoliophyta)
• The name angiosperm comes from the Greek word Anthos meaning flower
• Flowers are angiosperm structures specialized for sexual reproduction
  • Many species of angiosperms are pollinated by insects or animals
  • While others are wind pollinated, and in some cases pollination occurs via water transport

Frugivorous Seed Adaptations

• Frugivorous animals have coevolved unique adaptations with fruit-bearing plants
• Seeds coats need to be protected going through the digestive tract
  • Some seeds are resistant to digestive enzymes and can withstand low pHs
  • Some seeds actually need disruption of the seed coat to germinate, accomplished by either acid/enzyme action, or scarification to start the seed formation

This lesson explores gymnosperm evolution, focusing on seed ferns and progymnosperms like Archaeopteris. It covers the development of microspores into pollen grains, pollination, and the structure of ovules in gymnosperms versus angiosperms. The evolutionary advantages of seeds are also discussed.