Lecture 1.4 - Eukaryotes and Protists

Introduction to Eukaryotes

• Discussion of eukaryotes' features: nucleus, mitochondria
• Eukaryotes evolved from prokaryotic cells
• Key question: How did prokaryotic cells evolve into eukaryotic cells?

Origins of Eukaryotic Organelles

Formation of the Nucleus

• Thought to have originated from infolding of the plasma membrane
• Creation of a membrane-bound organelle through this process

Endosymbiotic Theory

• Mitochondria:
  • Originated via endosymbiosis
  • Primal eukaryotic cell engulfed a free-living bacterium
  • Relationship evolved into co-dependency
  • Mitochondria resemble alpha proteobacteria
  • Have their own DNA similar to bacteria
  • Reproduce independently, similar to bacteria
• Plastids (including Chloroplasts):
  • Origin similar to mitochondria
  • Eukaryotic cells engulfed a photosynthetic bacterium
  • Became co-dependent over time
  • Have their own DNA and reproduce independently

Order of Endosymbiotic Events

• Mitochondria endosymbiosis occurred before plastids
• All cells with plastids also have mitochondria

Introduction to Protists

Protists Overview

• Kingdom Protista is paraphyletic
• Protists include all eukaryotes not classified as plants, fungi, or animals
• Mostly unicellular, diverse in form and function
• Can be autotrophs, heterotrophs, or mixotrophs
• Capable of phagocytosis
• Most are motile using flagella, cilia, or pseudopodia
• Complex life cycles

Phylogenetic Tree of Eukaryotes

• Six major supergroups (clades) exist within Eukarya
• Protists distributed across these supergroups

Supergroups

1. Archaeplastida

• Includes photosynthetic organisms
• Red Algae: Distinct from plants, use different pigments

2. Amoebozoa

• Gymnamiba and Entamoebas:
  • Naked amoebas and amoebas with tests (shells)
  • Includes some pathogenic species
• Slime Molds:
  • Produce spores, similar to fungi
  • Two types: Plasmodial (multi-nucleated mass) and Cellular (form aggregates)

3. Opisthokonta

• Includes animals and fungi
• Choanoflagellates:
  • Colonial cells with flagella
  • Closest living ancestors to animals
• Nucleariids:
  • Amoeba with thin pseudopodia

4. Rhizaria

• No shared synapomorphies across all members
• Forams:
  • Amoeba with thin pseudopodia, form complex tests
  • Significant carbon sinks
• Radiolarians:
  • Amoeba with silica shells, important for fossil records
• Cercozoans:
  • Diverse, some photosynthetic, some parasitic to fungi

Conclusion

• Endosymbiosis of plastids occurred independently multiple times
• This lecture covers the basis of eukaryotic cell evolution and introduces the diversity of protists. Further exploration will continue in the next lecture.