Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 28.2 Excavates Include Protists with Modified Mitochondria and Protists with Unique Flagella
Excavata is one of four supergroups of eukaryotes. It includes diverse protists that are united more by molecular similarities than obvious structural traits. Many excavates live in anaerobic environments, have unusual mitochondria, or possess distinct flagella.
The Supergroup Excavata
- Originally defined by a morphological feature—an "excavated" feeding groove on the cell body.
- Divided into three monophyletic groups:
- Diplomonads
- Parabasalids
- Euglenozoans
- However, genomic data now suggest Excavata may not be monophyletic, making it the most debated supergroup.
Diplomonads
- Live in anaerobic environments.
- Have highly reduced mitochondria called mitosomes (lack electron transport chains).
- Obtain energy from anaerobic pathways.
- Typically have:
- Two equal-sized nuclei
- Multiple flagella
- Example: Giardia intestinalis, a human intestinal parasite, causes diarrhea from contaminated water.
Parabasalids
- Also anaerobic protists with reduced mitochondria, called hydrogenosomes, which produce hydrogen gas.
- Best-known species: Trichomonas vaginalis
- Sexually transmitted parasite infecting over 140 million people globally.
- Moves with flagella and undulating membranes.
- Acquired some genes through horizontal gene transfer, helping it infect host tissues.
Euglenozoans
- Include diverse heterotrophs, autotrophs, mixotrophs, and parasites.
- Unifying trait: flagella with an internal spiral or crystalline rod.
- Two key subgroups:
- 1. Kinetoplastids
- Have a single large mitochondrion containing a DNA mass called a kinetoplast.
- Some feed on bacteria; others are parasites.
- Example: Trypanosoma, causes sleeping sickness (via tsetse fly) and Chagas’ disease.
- Use antigenic variation ("bait-and-switch") to evade the immune system by changing surface proteins frequently.
- 2. Euglenids
- Possess a flagella-emitting pocket and a light-sensitive eyespot.
- Many are mixotrophs—photosynthesize when light is available, become heterotrophic otherwise.
- Others ingest prey via phagocytosis.
- Reinforced by pellicles (protein bands beneath the membrane) that offer flexibility and strength.
In a Nutshell
The Excavata supergroup contains protists with specialized traits: modified mitochondria, unique flagella, and diverse feeding modes. Though grouped by molecular data, these organisms are structurally and functionally distinct. Their diversity offers insights into early eukaryotic evolution, parasitism, and cellular complexity.