Plant Evolution and Adaptations

Plant Evolution and Adaptations

Overview

  • Plants evolved from diverse algae, leading to land plants.
  • Key features and adaptations for terrestrial life include:
    • Alternation of generations
    • Vascular tissues
    • Seeds, flowers, and fruit

Plantae and Chloroplasts

  • The Plantae kingdom consists of several clades.
  • All chloroplasts trace back to a single endosymbiotic event.
  • Diagram shows the evolutionary relationships between various groups including Ophisthokonts, Alveolates, Stramenopiles, Rhizarians, Excavates, Plantae, Amoebozoans, Fungi, Choanoflagellates, and Animals across geological time from the Precambrian to the Cenozoic era.

Chlorophytes

  • Green algae
  • Sister group to charophytes and land plants.
  • Cell walls contain cellulose (like plants).
  • Contain chlorophyll a and b, and starch for storage.
  • Can be unicellular, multicellular, or multinucleate.
  • Exhibit several symbiotic forms.

Cellular Diversity in Green Algae

  • Examples include: Chlorella, Chlamydomonas, Oedogonium, Cladophora, Volvox, and Codium.

Volvocine Evolution

  • Evolutionary progression:
    • Unicellular (e.g., Chlamydomonas)
    • Colonial (e.g., Gonium)
    • Multicellular (e.g., Volvox)
  • Illustrates the transition from unicellularity to multicellularity.
  • Highlights the evolution of division of labor.

Volvocine Evolution Details

  • Clamydomonas:
    • Unicellular
    • Isogamous (gametes are similar in size and shape)
  • Gonium:
    • Colonial
    • Isogamous
  • Volvox:
    • Multicellular
    • Division of labor: Outer sphere cells don’t divide.
    • Gonidia produce new colonies or gametes.
    • Oogamous (extreme anisogamy: large, immobile egg and small, motile sperm)

Caulerpa taxifolia

  • Invasive green algae in the Mediterranean and West Coast.
  • Composed of huge, single, multinucleated cells.
  • Rare in native tropical oceans.
  • Strong competitor in non-native habitats.

Caulerpa taxifolia - Cellular Structure

  • Features include:
    • Cell wall
    • Central vacuole
    • Cytoplasm
    • Mitochondrion
    • Nuclei
    • Chloroplast
  • Illustrates the large size and multinucleated condition of the cells.

Chlorophytes: Ulva, Sea Lettuce

  • Ulva exhibits alternation of generations.
  • Both phases are morphologically identical.

Alternation of Generations in Ulva

  • Isomorphic life cycle: Haploid and diploid phases are identical in size and shape.
  • Meiosis produces haploid spores.
  • Unfertilized haploid gamete can form another haploid individual.

Charophyta: Stoneworts

  • Most complex green alga.
  • Chara shows complex growth patterns with:
    • Nodal regions with whorls of branches
    • Hollow multinucleate internodal regions (not true leaves, seeds, roots, or branches)
  • Multicellular and complex egg-bearing structures.
  • Rounded sperm-bearing structures.
  • Secrete calcium carbonate, contributing to limestone and marl deposition.

What is a Plant?

  • Plants protect their embryos in a multicellular structure (archegonium).

Possible Algal Precursor to Land Plants

  • Coleochaetales/Coleochaetophytes are considered closest living relatives to Kingdom Plantae.
  • Some species have:
    • Parenchymatous growth
    • Nurturing of the zygotes
    • Fossil record of appropriate age
  • Examples:
    • Coleochaete (living)
    • Parka (Upper Silurian - Lower Devonian)

Comparisons to Charophycean Algae

  • Similarities:
    • Homologous chloroplasts
      • Similar structures and pigments in thylakoids/grana

Comparisons to Charophycean Algae - Pigments

  • Similar pigments:
    • Chlorophyll a and b
      • Chlorophyll \ a: C{55}H{72}O5N4Mg
      • Chlorophyll \ b: C{55}H{70}O6N4Mg

Comparisons to Charophycean Algae - Cell Walls

  • Similarities:
    • Cell walls with similar cellulose contents.
    • ‘rosettes’ of cellulose synthase.

Comparisons to Charophycean Algae - Cell Division

  • Similarities:
    • Cell division via phragmoplasts.

Additional Similarities to Charophycean Algae

  • Homologous peroxisomes
  • Sperm
  • DNA sequences

Challenges of Living on Land

  • 1) Dessication (through evaporation and diffusion):
    • Small size and surface area:volume ratio exacerbate this.
    • Solutions:
      • Protection and nursing of smallest stages.
      • Adults have protective outer layers: cuticle and stomata.
  • 2) Water uptake and transportation:
    • Roots.
    • Vascular tissue in higher divisions.

Additional Challenges of Living on Land

  • 3) Structural support:
    • Bodies are less buoyant in air.
    • Solutions:
      • Earliest plants were small.
      • Later, specialized structural tissues (sometimes composed of dead or water-filled cells).
  • 4) Dispersal of gametes:
    • Sperm can't always swim to eggs.
    • Single-cell or early embryonic stages adapted to terrestrial dispersal.

Differences from Charophycean Algae

  • 5 differences tied to terrestrial lifestyle:
    • Apical meristems
    • Dependent embryos
    • Alternation of generations
    • Walled spores
    • Gametangia

Apical Meristems

  • All plants must live and grow in two different environments:
    • Air (stems)
    • Soil (roots)

Multicellular, Dependent Embryos

  • Embryos develop within specialized tissue (placental transfer cells).

Alternation of Generations

  • Meiosis necessitates haploid and diploid stages:
    • In animals, the haploid stage is unicellular (gametes).
    • In plants, haploid spores produce multicellular gametophytes that produce gametes.

Alternation of Generations - Diagram

  • Diploid sporophytes produce haploid spores, which produce multicellular gametophytes that produce gametes.
  • Cycle:
    • Zygote (2n) undergoes mitosis to form a sporophyte (2n).
    • Sporophyte undergoes meiosis to produce haploid spores (n).
    • Spores undergo mitosis to develop into a gametophyte (n).
    • Gametophyte produces gametes (n) through mitosis.
    • Fertilization results in a zygote (2n).

Evolutionary Novelty: Mitosis in the Diploid Zygote

  • Mitosis in the diploid zygote (sporophyte) is the evolutionary novelty.
  • Charophycean algae have a zygotic meiosis.
  • Plants have a multicellular sporophyte stage.

Walled Spores Produced in Sporangia

  • Spores lack flagella.
  • Protected by sporopollenin.
  • Adaptations to wind dispersal.

Multicellular Gametangia

  • Present in the three oldest plant divisions.
  • Archegonia (female gametangia)
  • Antheridia (male gametangia)

Gametophyte and Sporophyte

  • Zygote (2n) → Mitosis → Sporophyte (2n) → Meiosis → Spores (n) → Mitosis → Gametophyte (n) → Mitosis → Gametes (n) → Fertilization → Zygote (2n).

Hypothetical Scenario: Alternating Generations

  • Illustrative examples tracking ploidy levels through alternating generations, starting with a sporophyte (2n) leading to the formation of microspores and megaspores, then to sperm and egg production by gametophytes (1n), and finally zygote formation (2n).

Dominance Patterns of Sporophyte and Gametophyte Generations in Plants

  • Green Alga: Gametophyte dominant.
  • Bryophyte: Gametophyte dominant.
  • Fern: Sporophyte and gametophyte are independent.
  • Gymnosperm: Sporophyte dominant.
  • Angiosperm: Sporophyte dominant.

Embryophyte Phylogeny

  • All plants have protected embryos.
  • All plants have life cycles with an alternation of generations.

Embryophyte Phylogeny - Key Evolutionary Steps

  • Land plants (475 MYA):
    • Rhizoids, cuticle, and spores.
  • Origin of Stomata.
  • Vascular plants:
    • Vasculature
    • Branching sporophytes
  • Seeds (360 MYA).

Plant Classification

  • Nonvascular Land Plants:
    • Hepatophyta (Liverworts): No stomata; gametophyte flat or leafy.
    • Bryophyta (Mosses): Filamentous stage; gametophyte leafy; sporophyte grows apically.
    • Anthocerophyta (Hornworts): Embedded archegonia; sporophyte grows basally.
  • Vascular Plants:
    • Lycopodiophyta (Lycophytes): Microphylls in spirals; sporangia in leaf axils.
    • Monilophyta (Horsetails, Ferns): Simple leaves in whorls or frondlike compound leaves.
  • Seed Plants:
    • Gymnosperms: Cycadophyta (Cycads), Ginkgophyta (Ginkgo), Gnetophyta (Gnetophytes), Coniferophyta (Conifers).
    • Angiosperms: Flowering plants.

Nonvascular Plants

  • New plant features:
    • Apical growth
    • Embryophytes
  • Restricted to moist habitats
  • Gametophyte stage is most prominent.

Examples of Nonvascular Land Plants

  • Liverworts
  • Hornworts
  • Mosses

Habitat Restrictions for Nonvascular Plants

  • Gametophytes release flagellated sperm.

Reasons for Moist Habitat Restriction

  • Lack of vascular tissues to take up and transport water (but rhizoid bundles can wick and retain water).
  • Grow primarily horizontally rather than vertically.

Homosporous Life Cycle

  • Haploid gametophyte produces both “male” antheridium and “female” archegonium gamete-producing structures.
  • These produce haploid gametes by mitosis.
  • Syngamy results in diploidy.
  • Alternation of generations.

Gametophyte Prominence

  • Gametophyte is the prominent stage in nonvascular plants.

Liverworts

  • Gametophytic thallus with sexual structures: archegonia and antheridia.
  • Gemmae cups for asexual reproduction.

Cross Section of Marchantia Thallus

  • Specialized air-flow regulating cells surrounding the opening (analogous, but probably not homologous, to stomata).
  • Chlorenchyma (photosynthesis).
  • Parenchyma (storage).

Life Cycle of Marchantia

  • Diagram showing the life cycle, including:
    • Mature sporophyte with seta and sporangium.
    • Spores and elaters for dispersal.
    • Gametophytes with archegoniophores and antheridiophores.
    • Fertilization and embryo development.

Leafy Liverwort

  • Lophocolea bidentata:
    • Overlapping leaves.
    • Creeping growth form.

Sporophytes of Leafy Liverwort

  • Young sporophytes topped with capsules of Lophocolea bidentata.

Hornworts

  • Thallus resembles Coleochaetales.
  • Spores often occur in tetrads.
  • Stomata present on sporophytes.

Hornworts - Additional Details

  • Diploid sporophyte grows imbedded into the haploid gametophyte thallus.
  • Sporophytes have elater-like structures (resembling those in liverworts).
  • Meristematic region of sporophyte.
  • Sporophyte is green and has stomata.

Mosses in a Wet World

  • Moist habitat is ideal for bryophytes.
  • Require liquid water for fertilization.
  • Flagellated sperm require water to swim to the egg in the archegonium.

Gametophyte and Sporophyte Structure in Mosses

  • Protonema (early gametophyte stage).
  • Leafy gametophyte.
  • Sporophyte with capsule and peristome.

Stomata in Mosses

  • Mostly two cells, but in some species, it's a single cell.
  • Found only in sporangia of the sporophyte.

Rhizoids vs. Roots

  • Bryophytes:
    • Rhizoids: Filaments used for attachment and rudimentary absorption.
    • Adaptations like bundles of rhizoids can wick and retain water.
  • True roots:
    • Have vascular tissue.
    • Store photosynthetic products.

Conducting Vessels in Mosses

  • As mosses grew taller, conducting vessels (hydroids) evolved in the sporophyte filament.
  • Continuous space in the hydroid cells.

Seedless Vascular Plants: Pteridophytes

  • Vascular structures:
    • Leaves, roots, xylem, and phloem.
  • Cell walls reinforced with lignin.
    • Tracheids vs. Hydroids
  • Still need water for fertilization.
  • Sporophyte is prominent stage.

Vascular Structures

  • Presence of well-developed vascular tissues in seedless vascular plants.

Xylem and Phloem

  • Xylem: transports water and minerals up.
  • Phloem: transports sugars down.

Early Evolution of Land Plants: Devonian Period

  • Examples:
    • Cooksonia (with round sporangia).
    • Zosterophyllum (with clustered sporangia).
    • Agalophyton (with elongate, solitary sporangia).
    • Psilophyton (with sterile and fertile branchlets).

Early Tracheophytes - Rhynia

  • Lack leaves and roots, but had tracheids and an underground horizontal stem (rhizome).
  • Rhizome may have led to the forerunner of roots with the development of minute rhizoids.
  • Height of these plants was about 0.5 m.

Phylogeny of Tracheophytes

  • Key evolutionary features:
    • Tracheids (vascular cells); branching independent sporophyte; rooting structures
    • Microphylls (Lycophytes)
    • Megaphylls; overtopping growth form (Euphyllophytes)
    • Seeds (Seed plants)
    • Flowers; double fertilization (Angiosperms)

Phylum Lycophyta: Club Mosses

  • Both homosporous and heterosporous life cycles are found in this group.
  • Lycophytes share a similar chloroplast DNA sequence with the bryophytes and green algae.
  • Differs from the inverted sequence of the ferns and seed plants.

Homosporous Lycopyte Life Cycle

  • Lycopodium has underground gametophytes that are normally closely associated with mycorrhizal fungi from which they derive their nutrition.

Heterosporous Life Cycle

  • Two distinct gametophytes: mega- and micro- (female and male, respectively).
  • These develop from two different kinds of spores produced by the sporophyte.

Heterosporous Life Cycle in Lycophytes - Selaginella

  • Selaginella has two kinds of spores produced in the micro- and megasporangium.

Ferns and Their Allies

  • Ferns and seed plants are more closely related than either is to the lycophytes.
  • Based on:
    • Megaphyll leaf structure
    • Base pair sequence of six genes
    • 30,000 base pair inversion in the chloroplast DNA

Phylum Pterophyta: Ferns

  • Examples of ferns from various regions:
    • Lindsaea (Costa Rica)
    • Tree fern, Cyathea (Costa Rica)
    • Elaphoglossum (Peru)
    • Pleopeltis polypodiodes (on tree, Arkansas)

Major Features of Ferns

  • Fern prothallus (gametophyte) with rhizoids, archegonia, and antheridia.
  • Sori (sorus, sing) on the undersides of pinnae.

Sporophyte Dominance in Pteridophytes

  • Sporophytes are larger and more complex than gametophytes.

Pteridophyte Life Cycle

  • Spore → Young gametophyte → Mature gametophyte (with archegonium and antheridium) → Egg and Sperm → Fertilization → Zygote → New sporophyte → Mature sporophyte (with sporangium).

Gametophytes Produce Male and Female Gametes

  • Antheridia (male).
  • Archegonium (female).

Lower Plants - Not Primitive or Uncompetitive

  • Example:
    • Lygodium microphyllum (climbing fern)
      • Invasive species.
      • Biocontrol candidate Cataclysta camptozonale

Horsetails

  • The “scouring rushes” extend back to the Devonian and were maximally abundant about 300 MYA.
  • In the Carboniferous, they reached heights of 38+ m and 45 cm diameter.
  • Today, there is one genus, Equisetum.
    • Homosporous, fertile shoots lacking chlorophyll.
    • Vegetative shoots.

Whisk Ferns

  • Psilotum, a seedless vascular species with two living genera, has no roots or leaves.
  • They form subterranean gametophytes with antheridia and archegonia in which fertilization occurs and the sporophyte initially develops.