Endosymbiotic Hypothesis and Eukaryotic Cells

Origin of Eukaryotic Cells

• The first cells that appeared were prokaryotes (before the nucleus).
• Originated 1500 to 2000 million years ago.
• Evolutionary transition involved morphological complexity, incorporating genomes, discovering sexual reproduction, and leading to multicellular organisms.

Endosymbiotic Hypothesis

• The eukaryotic nucleus contains genes derived from bacteria.
• Ribosomes, organelles and their phylogeny suggest a shared origin with prokaryotic ribosomes.
• Antibiotic specificity supports the theory.

Endosymbiotic Hypothesis: Antibiotic Specificity

• Tigecycline is an antibiotic used to treat bacterial infections.
• It inhibits bacterial protein translation by blocking the ribosomal A-site.
• While cytotoxic to human cells, its mechanism of inhibition is unclear.

Endosymbiotic Hypothesis: Hidrogenosomas

• Phylogenetic analyses associate them with prokaryotes.

Primary and Secondary Endosymbiosis

• Primary Endosymbiosis: A cell of Eukarya acquires a mitochondrion and a nucleus.
• Secondary Endosymbiosis: A cell from a cyanobacterial lineage of Bacteria acquires a chloroplast from red algae, leading to:
  • Dinoflagellates
  • Apicomplexans
  • Stramenopiles.
• A cell from a green algae lineage acquires a chloroplast from green algae leading to:
  • Green Algae
  • Euglenida
  • Chlorarachniophytes.

Eucariogenesis

• Eukaryotes originated from a prokaryotic ancestor.
• LECA (Last Eukaryotic Common Ancestor) is a key point in eukaryotic evolution.
• Debates on the origin of LECA from Bacteria or Archea.
• Monophyletic origin of Eukaryotes.

Eucariogenesis: Phylogeny

• A comparison of 3D and 2D phylogenetic trees shows the relationship between Bacteria, Archaea, and Eukarya.

Diplomonads and Parabasalids

• Diplomonads:
  • Two nuclei of the same size.
  • Mitosomes.
  • Compact genome with few introns.
  • Giardia intestinalis causes diarrheal disease transmitted through water.
• Parabasalids:
  • Parabasal body supporting the Golgi complex.
  • Hidrogenosomas.
  • Found in the gastrointestinal and urogenital tracts.
  • Trichomonas vaginalis is a sexually transmitted disease.

Trichomonas

• Trichomonas is a flagellated protozoan with a parabasal body and hidrogenosomas.
• It's a human pathogen that causes sexually transmitted infections.

Euglenozoa

• A diverse group of unicellular flagellated eukaryotes.
• They can be free-living or parasitic.
• Some are pathogenic and have a kinetoplast (DNA in the unique mitochondrion).
• Euglenids are mobile, non-pathogenic, and phototrophic or chemotrophic.

Euglenozoa: Kinetoplastids and Euglenids

• Kinetoplastids:
  • Kinetoplast.
  • Unique mitochondrion.
  • Aquatic environments. Some are pathogenic (e.g., Trypanosoma brucei, T. cruzi, Leishmania).
• Euglenids:
  • Non-pathogenic.
  • Chemo- or phototrophic.
  • Found in freshwater or saltwater environments.
  • Phagotrophic (feeding on bacteria by phagocytosis).

Alveolates

• Characterized by alveoli, cytoplasmic sacs below the plasma membrane.
• Alveoli may help maintain osmotic balance.
• Three main groups:
  • Ciliates (e.g., Paramecium)
  • Dinoflagellates (photosynthetic, some toxic; e.g., Gonyaulax - red tide)
  • Apicomplexans (e.g., Toxoplasma, Plasmodium, Eimeria).

Alveolates: Ciliates, Dinoflagellates and Apicomplexans

• Ciliates:
  • Have cilia at some point.
  • Use them for movement and feeding.
  • Have a food vacuole.
  • Micronucleus for genetic recombination.
  • Macronucleus for basic cell functions.
  • Examples: Balantidium coli, Paramecium
• Dinoflagellates:
  • Phototrophic freshwater organisms.
  • Secondary endosymbiosis.
  • Flagella with spinning motion.
  • Free-living or symbiotic.
  • Some produce neurotoxins (e.g., Gonyaulax, Pfiesteria).
• Apicomplexans:
  • Non-phototrophic.
  • Obligate parasites of animals.
  • Produce sporozoites, the transmission stage of the parasite.
  • Apicomplexan refers to their specialized organelles at the sporozoite's end.
  • Degenerate chloroplasts (fatty acids, isoprenoids, heme group).
  • Examples: Plasmodium, Toxoplasma, Eimeria (coccidia).

Paramecium

• Micronuclei function in genetic recombination.
• The macronucleus controls cellular activities.
• Specialized cellular structures: Contractile vacuole, Anal pore, Pellicle, Oral groove, Digestive vacuole, Trichocyst, Fibrils, Alveolus, Cilium, Cilia, Alveoli.

Stamenopiles

• 'Stramen' refers to the straw-like hairs (pili) on their flagella.
• Includes chemotrophs (heterotrophs) and phototrophs (autotrophs), from micro- to macro- organisms.

Stamenopiles: Diatoms, Oomycetes, Crisophytes and Brown Algae

• Diatoms:
  • Unicellular, photosynthetic.
• Oomycetes:
  • Aquatic fungi.
  • Filamentous and coenocytic (multinucleate) growth.
• Crisophytes:
  • Chrysophytes or golden-brown algae.
• Brown Algae:
  • Large and multicellular.

Amoebozoa

• Gymnamoebas:
  • Free-living protozoa.
  • Use pseudopods (false feet) for movement (e.g., Amoeba)
• Entamoebas:
  • Parasites of vertebrates and invertebrates.
  • Found in the oral cavity or gastrointestinal tract.
  • Entamoeba histolytica is a human parasite.
• Slime Molds:
  • Form fruiting bodies with spores.
  • Mobile, moving over solid surfaces.
  • Two types:
    • Plasmodial (multinucleate)
    • Cellular (individual cells)

Amoebozoa: Amoeba, Slime Molds and Entamoeba

• Amoeba
• Slime Molds
• Entamoeba

Red and Green Algae: General Properties

• Photosynthetic.
• Contain chlorophyll a.
• Red algae:
  • Do not have chlorophyll b.
  • Contain phycobiliproteins, giving them a reddish color due to phycoerythrin.
  • Lack flagella.
  • Used in agar and sushi production.
  • Some live acid hot springs.
• Green algae:
  • Have chlorophyll a and b, giving their characteristic green color.
  • Closely related to plants.
  • Unicellular (e.g., Chlamydomonas, Dunaliella, Ostrococcus tauri) or macroscopic (e.g., Chara, Volvox).

Red and Green Algae: Organisms

• Red algae:
  • Polysiphonia
  • Cyanidium
  • Galdiera
• Green algae:
  • Chlamydomona
  • Dunaliella
  • Ostrococcus tauri
  • Chara
  • Volvox

Explore the origin of eukaryotic cells and the endosymbiotic hypothesis in this quiz. Learn about the evolutionary transition from prokaryotes to complex multicellular organisms and how antibiotic specificity supports this theory. Delve into the significance of ribosomes and organelles in understanding cellular evolution.