Gymnosperms

Origin of seed plants

• about 360 million years ago

Seed

Adaption of some terrestrial plants, consisting of embryo packaged along with a store of food within a protective coat

• domestication of seed plants began around 8,000 years ago, enabling permanent human settlements

Seed plants have reduced gametophytes

• Gametophytes develop within spore walls that are retained within the parent sporophyte tissues

• From basal to derived, the gametophyte is reduced in size and becomes dependent on sporophyte (sporophyte has more roles):

  • Non-vascular bryophytes possess independent, dominant gametophytes; a gametophyte-dependent sporophyte

  • Seedless vascular plants have dominant sporophyte; an independent gametophyte

  • Seed plants have dominant, independent sporophyte; a microscopic, dependent gametophyte

Heterospory

• All seed plants are heterosporous

• Megasporangia produce megaspores, which give rise to female gametophytes

• Microsporangia produce microspores, which give rise to male gametophytes

• In seed plants, spores are retained within the sporophytes without being dispersed

Seed plants have ovules

Ovule contains megasporangium (2n) , megaspore (n, produced by meiosis), and one or more protective integuments (2n)

• Gymnosperm ovules have one integumentary

• Angiosperm ovules usually have two integuments, female gametophytes are reduced to embryo sacs with few nuclei and cells

• Ovules develop into seeds after fertilization

Seed plants have pollen

• Microspores aren’t dispersed, instead develop into male gametophytes (n) within pollen grains

• cell walls of pollen grains coated with sporopollenin, protecting pollen

  • Pollen grains are male gametophtrs enclosed within pollen wall

Pollen Grain vs Male Gametophyte

Pollen grain:

• outer layer rich in sporopollenin produced by sporophyte,

• contains a male gametophyte with at least one generative cell and tube cell developed into nuclei and pollen tube after pollination

Male gametophytes:

• transported to ovule within pollen grain

• sperms not released in the external environment for fertilization

Seed Development and Function

• Develop from fertilized ovule, composed of embryo, nutrient supply, and outer seed coat

• Seed contains next generation sporophyte surrounded by tissues of parent gemtophyte

• Size varies, primarily influenced by nutrient storage derived from gametophyte

Pollination

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

• In seed-less plants, single-celled flagellated sperms require water to swim to the archegonium of the female gametophyte

• In seed plants, the entire pollen grain along with the male gametophyte is dispersed. If the pollen germinates, a pollen tube is developed to discharge sperm nuclei into the female gametophytes

Benefits:

• Sporopollenin can protect male gametophyte

• Pollens can survive long-distance transportation by biotic or abiotic agents

Seed dispersal vs Spores

Advantages of seed dispersal over spores:

• protective coats protect embryo

• energy supply of stored food and can germinate in dark

Trade-off:

• larger seeds with more stored energy are produced in lower quantity

• smaller seeds with less stored energy are produced in larger quantity

Origin and Diversification of Seed Plants

• first documented about 360 million years ago

• 250,000< angiosperm extant species

• 900 gymnosperms extant species

• angiosperms began to replace gymno sperms in ecological systems in cretaceous period

Gymnosperms

Group of vascular plants with seeds exposed on sporophytes that form strobili

• sporophyte (2n) as mature tree is dominant generation

• typical gymnosperm has both pollen and ovulate cones

• gymnosperms develop seeds from fertilized ovules

• pollens containing male gametophytes are transferred to ovules before sperm are released

Life Cycle of Conifer

1. Tree has both ovulate and pollen cones

2. Microsporocytes divide by meiosis, producing haploid microspores that develop into a pollen grain

3. Pollination occurs when pollen grain reaches ovule, pollen grain germinates forming pollen tube digesting its way to megasporangium

4. Megasporocyte undergoes meiosis, producing 4 haploid megaspores

5. Megaspores develop into female gametophyte with 2-3 archegonia

6. Sperm cells developed in pollen tube extend to female gametophyte, causing fertilization

7. One zygotę develops into embryo, ovule becomes seed consisting of embryo, food supply, and seed coat

Diversity of gymnosperms

Four phyla, monophyletic group:

• Cycadophya - about 130 species

• Ginkgophya - singular species

• Ginetophyta - about 75 species

• Coniferophyta - about 600 species

Phylum Ginkgophyta

A division with Ginkgo biloba as the only extant species originated from China

• G. bilboa associated with human habitats, no wild population

• have fan-shaped leaves and flagellated sperms

• G. bilboa have separate gametes in separate individuals

• G. bilboa are deciduous

Phylum Gnetophyta

Small group of gymnosperms recovered from molecular evidence

• contains about 75 species in three genera: Ephedra, Gnetum, Welwitschia

• can be found in tropical and desert environment

• Xylem contains vessel elements