Topic 3- Eukaryotes

• compare and contrast eukaryotic cell structures

  

• sequence eukaryotic evolution

  1. Formation of First Eukaryotic Cell

  • Ancestral prokaryotic cell evolved into a eukaryotic cell through endosymbiosis.

  • Engulfed an aerobic alpha-proteobacterium, which became the mitochondrion.

  2. Primary Endosymbiosis (Origin of Mitochondria and Chloroplasts)

  • Mitochondria evolved from the engulfment of an aerobic alpha-proteobacterium.

  • Chloroplasts evolved from a cyanobacterium engulfed by a eukaryotic ancestor of plants.

  3. Diversification of Eukaryotic Supergroups

  • Four major supergroups emerged:

    • Excavata

    • SAR Clade

    • Archaeplastida (gave rise to plants)

    • Unikonta (gave rise to fungi & animals)

  4. Secondary Endosymbiosis

  • Some eukaryotic cells engulfed primary endosymbionts, leading to complex plastid evolution (e.g., red and green algae).

  5. Rise of Multicellularity

  • Multicellular organisms (e.g., plants, fungi, animals) evolved from unicellular eukaryotic ancestors.

• compare and contrast characteristics of protists

  

• compare and contrast eukaryotic supergroups

I. Introduction to Eukaryotes

A) Eukaryotic Cell Structures

• The root of the Eukarya domain is not well understood.

• The eukaryotic phylogenetic tree is a polytomy, meaning its branching order is unresolved.

• The ancestral characteristics of all eukaryotes (from Lecture 5) include:

  • DNA enclosed in a nucleus.

  • Membrane-bound organelles (such as mitochondria, chloroplasts, ER, Golgi apparatus).

  • Cytoplasm.

  • Cell sizes ranging from 10-100 micrometers.

  • Exist as single-celled organisms, colonies, or multicellular organisms.

• Endosymbiosis played a crucial role in the origin of mitochondria through the engulfment of prokaryotic micro-bacteria.

B) Endosymbiosis

• All eukaryotes have endosymbiotic relationships (Lecture Topic 6).

• Endosymbiosis: A symbiotic relationship where one cell engulfs another, and they continue living together.

• Serial endosymbiosis: Eukaryotic cells evolved through a series of endosymbiotic events.

• Primary vs. Secondary Endosymbiosis:

  • Primary endosymbiosis: A free-living bacterium is engulfed by a larger host cell, leading to the formation of an organelle (e.g., mitochondria, chloroplasts).

    • Mitochondria originated from an aerobic alpha proteobacterium engulfed by the ancestor of all eukaryotes.

    • Chloroplasts originated from a photosynthetic cyanobacterium engulfed by the ancestor of plants.

  • Secondary endosymbiosis: A eukaryotic cell that has already undergone primary endosymbiosis is engulfed by another eukaryotic cell.

    • Plastids, such as chloroplasts, originate through secondary endosymbiosis.

• The Endosymbiotic Theory states that mitochondria and chloroplasts evolved from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells.

D) Protists

• The term "protist" means “the very first” and refers to early eukaryotic organisms.

• Protists arose through endosymbiosis and are unicellular or simple multicellular organisms.

• The traditional classification of protists excludes plants, fungi, and animals.

II. Eukaryotic Supergroups

A) Archaeplastida

• Defining Trait: All members share the ancestral trait of primary endosymbiosis, where a eukaryotic ancestor engulfed a photosynthetic cyanobacterium, leading to the evolution of chloroplasts.

• Major Groups:

  • Red Algae: Contains the red pigment phycoerythrin, allowing it to absorb different wavelengths of light.

  • Green Algae: Closely related to land plants.

    • Chlorophytes: A diverse group of green algae.

    • Charophytes: The closest relatives to land plants.

• All members utilize chlorophyll a and b for photosynthesis.

B) SAR Clade

• Defining Trait: Genetic characteristics and secondary endosymbiosis of red algae.

• Monophyletic group consisting of:

  1. Stramenopiles:

     • Derived Traits: Possess two flagella—one long and hairy, the other short and smooth.

     • Many are photosynthetic algae.

     • Example: Diatoms (unicellular algae with silica cell walls).

  2. Alveolates:

     • Derived Trait: Alveoli—flattened membrane-bound sacs beneath the plasma membrane.

     • Example: Apicomplexans, a parasitic group.

       • Apical complex helps penetrate host cells (e.g., Plasmodium, which causes malaria).

  3. Rhizarians:

     • Derived Trait: Skeletal structures made of calcium carbonate or silica.

     • Example: Foraminiferans ("forams"), which have porous shells (tests) made of calcium carbonate.

C) Amoebozoa

• Defining Trait: Amoeboid movement with a blob-like shape.

• Includes slime molds and entamoebas.

D) Opisthokonta

• Defining Trait: Genetic characteristics shared among fungi and animals.

• Major Groups:

  • Nucleariids: Sister group to fungi.

  • Choanoflagellates: Sister group to animals and resemble collar cells in sponges.

E) Discoba

• Defining Trait: Crystalline rod structure in flagella.

• Also exhibits secondary endosymbiosis of green algae.

• Examples:

  • Euglenids: Photosynthetic, mixotrophic organisms.

  • Trypanosomes: Parasitic organisms.

  • Mixotrophs: Can switch between phototrophy and heterotrophy (photo-chemoautotrophs).