Lecture Topic 8: Green Algae and the Origin of the Embryophytes

Background on Land Plants

• Land plants evolved approximately 450 million years ago from an aquatic green algal ancestor.
• The initial land plants likely resembled liverworts, which are a form of bryophyte.
• To comprehend the evolution of land plants, it is crucial to analyze their life cycles.

Review of Key Terms

• Mitosis: A type of cell division that results in two identical daughter cells, each with the same number of chromosomes as the parent cell.
• Meiosis: A specialized form of cell division that produces gametes with half the chromosome number of the original cell.
• Haploid: A cell or organism having a single set of unpaired chromosomes (denoted as n).
• Diploid: A cell or organism having two complete sets of chromosomes, one from each parent (denoted as 2n).
• Spores: A reproductive unit capable of developing into a new individual without fusion with another reproductive unit.
• Gametes: Sex cells (sperm and egg) that unite during fertilization to form a zygote.
• Syngamy: The fusion of two gametes to form a zygote.
• Zygote: The fertilized egg cell that results from the union of gametes.
• Embryo: The early development stage of a multicellular organism that develops from the zygote.

Life Cycles

Haplobiontic Life Cycle

• Haplobiontic: A life cycle characterized by either a haploid or diploid multicellular phase (but not both). This type lacks any alternation of generations.
  • Notation: H (haploid) or d (diploid).

Diplobiontic Life Cycle

• Diplobiontic: A life cycle that includes both haploid and diploid multicellular phases, exhibiting alternation of generations.
  • Notation: D (diploid) and h + d (haploid + diploid).

Introduction to the Viridiplantae

• Known informally as the "green plants".

Characteristics of the Viridiplantae

• The formal designation for the green algae and embryophytes.
• Common characteristics include:
  • Cell walls composed of cellulose.
  • Specialized chloroplasts featuring:
  • A double membrane.
  • Chlorophyll a and b as primary pigments.
  • Carotenoids serving as accessory photosynthetic pigments.
  • Thylakoid membranes organized into grana.
  • True starch (polysaccharide of glucose) stored within chloroplasts.
  • Comprised of two key lineages:
  • Chlorophyta (green algae)
  • Streptophyta (which includes land plants).

Overview of Chlorophyta

• Includes various classes:
  • Core chlorophytes
  • Ulvophyceae
  • Chlorophyceae
  • Trebouxiophyceae
  • Charophyceae
  • Other groups such as Picocystis, Mamiellophyceae, and others.
  • The Viridiplantae indicates both green algae and land plants, with evidence of a common ancestor approximately 1 billion years ago.

Gamete Evolution in Viridiplantae

• Isogamy: Gametes are identical in size, and both male and female gametes possess flagella.
• Anisogamy: Male and female gametes are flagellated, but differ in size, with the female being larger.
• Oogamy: In this scenario, only the male gamete has flagella, while the female gamete is significantly larger and non-motile.

Evolution of Gametangia in Viridiplantae

• Key structures include:
  • Oogonium: Structure that produces eggs.
  • Pseudoarchegonium: A type of reproductive structure that resembles an archegonium but includes jacket cells.
  • Archegonium: The female reproductive structure where eggs are produced.
  • Antheridium: The male reproductive structure that produces sperm.

Viridiplantae Cell Division

• Defined by two main mechanisms:
  • Phycoplast: Cell division where the support structure (microtubules) lies parallel to the plane of division, allowing for internal partitioning.
  • Phragmoplast: A structure that forms during cell division to aid in constructing a cell plate, dividing the cell from the outside to the inside.

Diversity in Chlorophyta

• Chlorophytes can be:
  • Unicellular
  • Colonial
  • Motile or non-motile
  • Multicellular

Examples of Multicellular Chlorophyta

• Ulothrix:
  • Filamentous structure with 1-D cell division.
  • Exhibits isogamous gametes.
• Ulva:
  • Displays 2-D cell division with a laminar body.
  • Also has isogamous gametes and pseudoparenchyma.

Transition in Viridiplantae

• Observes a shift from filamentous growth to parenchymatous growth.
• Illustrates various modes of cell division:
  • Primarily in one plane; occasionally in others leading to varied growth forms.

Charophycean Algae and Embryophytes

• Charophyceae and Coleochaetophyceae are the closest relatives to embryophytes.
• These groups exhibit several advanced characteristics:
  • Phragmoplastic cell division
  • Presence of plasmodesmata
  • Formation of a single apical cell
  • Structure resembling pseudo-parenchyma
  • Oogamous gamete formation with a pseudo-archegonium
  • Presence of sporopollenin in cell walls.
• Interestingly, Zygnematophyceae features a primitive appearance yet is closely related to embryophytes.

Major Evolutionary Trends in Viridiplantae

• Transition from unicellular to multicellular forms.
• Shift from multicellular-filamentous to multicellular-parenchymatous structures.
• Evolution from isogamous to oogamous gametes.
• Transition from non-archegoniate to archegoniate reproductive structures.
• Adoption of phycoplast to phramoplast mechanisms in cell division.

The Transition to Land: Embryophytes

Challenges in Transition

• Adaptation from an aquatic to terrestrial environment involved overcoming various stresses:
  • Salt water and rising irradiance levels.
  • Encountering drought as a terrestrial stressor.
  • Interaction potential with substrate microbiota.

Shared Characteristics of Embryophytes

• Formation of true archegonia rather than pseudo-archegonia.
• Exhibit a diplobiontic life cycle with two multicellular generations.
• Development of a cuticle for moisture retention.
• Production of multicellular sporangia and gametangia.
• Embryogenesis occurring within female sex organs and resulting in true parenchyma derived from an apical meristem.

Additional Embryophyte Adaptations

• Possess vascular tissues enabling upright growth and support of weight.
• Synthesize chemical compounds that provide protection against UV damage.
• Develop complex polymers to prevent water loss:
  • Waterproof cuticle composed of cutin and wax.
  • Drought-resistant spores that contain sporopollenin.
  • Incorporation of lignin into cell walls for structural support.

Gamete Production in Embryophytes

• In many organisms, including animals and algae, gametes arise directly from the multicellular life stages.
• In embryophytes, gametes can be produced by either mitosis or meiosis, depending on the organism's ploidy level.
• Notably, embryophytes exhibit two distinct multicellular life stages (haploid and diploid).
  • Haploid stage produces gametes via mitosis.
  • Diploid stage yields spores through meiosis.

Alternation of Generations in Embryophytes

• Embryophytes alternate between a multicellular diploid sporophyte and a multicellular haploid gametophyte, designated as generations.
• This life cycle is termed diplobiontic.
• The cycle involves:
  • Formation of zygote via fertilization.
  • Development of sporophyte (2n) and gametophyte (n) generations.
  • Requirement of either meiosis or fertilization to progress between generations.

Major Groups of Embryophytes

Domain: Eukarya

Kingdom: (Viridi)Plantae

• Nonvascular, Seedless Vascular:
  • Liverworts (Marchantiophyta)
  • Hornworts (Anthocerophyta)
  • Mosses (Bryophyta)
• Characteristics:
  • Lack vascular tissues, true leaves, stems, or roots.
  • Dominant gametophyte stage; sporophyte is dependent.
  • Requires water for sexual reproduction; utilizes spores for dispersal.
• Seed Vascular:
  • Gymnosperms:
  • Cycads (Cycadophyta)
  • Ginkgo (Ginkgophyta)
  • Gnetophytes (Gnetophyta)
  • Conifers (Coniferophyta)
  • Angiosperms (Magnoliophyta):
  • Utilize enclosed seeds for dispersal and possess flowers.
• Both vascular groups:
  • Have vascular tissues and true leaves, stems, and roots.
  • Dominant sporophyte with dependent gametophyte.
  • Do not require water for sexual reproduction.