Chapter 16

Explain how compartmentalization, cytoskeletal innovation, and endosymbiotic events (mitochondrial and plastid origins) gave rise to eukaryotic complexity.
Recognize "protists" as a diverse, paraphyletic group that includes all eukaryotes outside the plant, animal, and fungal kingdoms.
Identify the defining structural and functional traits that characterize Excavata and its representative lineages.
Summarize the ecological and evolutionary significance of SAR members, including the role of brown algae as foundational species.
Trace how primary endosymbiosis led to the evolution of red and green algae and ultimately to the plant lineage.
Explain how Unikonts, including choanoflagellates, illuminate the evolutionary transition to multicellularity in animals and fungi.

First eukaryotes appeared more than a billion years ago (approximately 1.8 billion years BC).

Infoldings in Plasma Membrane
- Ancestral prokaryote underwent infoldings in its plasma membrane, leading to the formation of endomembrane structures, including the nucleus and endoplasmic reticulum.
First Endosymbiotic Event
- The ancestral eukaryote consumed aerobic bacteria that evolved into mitochondria.
- Result: Modern heterotrophic eukaryote with mitochondria.
Second Endosymbiotic Event
- The early eukaryote engulfed photosynthetic bacteria, which evolved into chloroplasts.
- Result: Modern photosynthetic eukaryote.

Eukaryotic cells possess a membrane-bound nucleus and other membrane-enclosed organelles.
Organelles isolate various functions within eukaryotic cells.
The well-developed cytoskeleton of eukaryotic cells provides structural shape and anchors organelles in place.

Many genes found in eukaryotic genomes are of bacterial origin, but most cannot be traced back solely to alphaproteobacterial (mitochondrial origin) or cyanobacterial (plastid origin) ancestors.
Eukaryotes carry both bacterial and archaeal genes, having arisen from a merger of a host cell that created the Last Eukaryotic Common Ancestor (LECA).
Horizontal Gene Transfer (HGT) likely facilitated the acquisition of many bacterial genes even before the first eukaryotic common ancestor (FECA).

Evidence indicates that mitochondria evolved before plastids.
The ancestral host cell was an archaeon bearing a membrane-bound nucleus and a cytoskeleton, but details of the internalization process are still debated (e.g., symbiosis infection, slow engulfment, or phagocytosis).

Plastids evolved later when a heterotrophic eukaryote engulfed a photosynthetic cyanobacterium (a gram-negative bacterium).
This event led to the evolution of two lineages of photosynthetic protists: red algae and green algae.

Photosynthetic Presence Hypotheses:

Photosynthesis was established early in eukaryotic evolution and subsequently lost in some lineages.
Eukaryotes acquired photosynthesis multiple times through various episodes of endosymbiosis.

Excavata: Characterized by a unique cytoskeleton and an excavated feeding groove. Includes diplomonads, parabasalids, and euglenozoans.
SAR: Comprising three clades: Stramenopiles, Alveolates, Rhizarians.
Archaeplastida: Includes red algae, green algae, and plants.
Unikonta: Encompasses animals and fungi, characterized by single posterior flagellum in some cells.

Members of Excavata have distinct structural features such as an excavated feeding groove. Key groups include:
- Diplomonads
- Parabasalids
- Euglenozoans

This diverse clade includes heterotrophs, autotrophs, mixotrophs, and parasites.
The defining feature is a spiral or crystalline rod inside each flagellum.

Stramenopiles: Important for photosynthesis; features a hairy flagellum paired with a smooth flagellum. Notable groups include diatoms and brown algae.
- Diatoms: Unicellular with a glass-like wall (frustule) made of silicon dioxide; significant for their photosynthetic capabilities.
- Brown Algae: The largest and most complex multicellular algae; contain carotenoids that impart a brown color and include species commonly called kelps.

Brown Algae: Alternation of Generations

Brown algae exhibit a life cycle of alternation of generations.
- Diploid generation (sporophyte) produces spores.
- Haploid spores develop into gametophytes that produce gametes.
- Fertilization results in a diploid zygote that develops into a new sporophyte.

Brown Algae: Foundational Species

Brown algae (e.g., kelps) have crucial ecological roles.
- Ecosystem Engineers: Create underwater forests that provide habitats and nurseries.
- Community Structure Drivers: Influence trophic cascades affecting biodiversity.
- Primary Producers: Convert sunlight into biomass, forming a base for coastal food webs.
- Detritus Providers: Kelp detritus serves as food in coastal ecosystems beyond the ocean.

Dinoflagellates: Characterized by membrane-enclosed sacs (alveoli) beneath the plasma membrane. - These cells feature two flagella and can spin while moving.
Dinoflagellate Blooms: Can cause red tides due to carotenoids in their plastids, leading to massive aquatic life kills, worsened by climate change and ocean warming.

Plastids emerged when a heterotrophic protist acquired cyanobacterial endosymbionts, resulting in red and green algae.
Plants evolved from green algae, and Archaeplastida is the overarching supergroup.

Red algae possess phycoerythrin, giving them a red color, varying with depth in water (greenish-red to dark red/black).
Mostly multicellular and become the predominant large algae in warm tropical waters.

Green algae derive their color from chloroplasts.
The group is paraphyletic, consisting of chlorophytes (various habitats) and charophytes (closely related to land plants).

The supergroup Unikonta includes animals and fungi.
Some hypotheses propose Unikonts diverged first from other eukaryotic groups.
Opisthokonts: A broad group covering animals and fungi characterized by a single posterior flagellum (example: sperm cells in animals).
Choanoflagellates:
- free-living unicellular and colonial flagellate eukaryotes that are the closest living relatives to animals, showcasing similarities in cell structure and signaling pathways.

Protists exemplify the early stages of eukaryotic complexity, introducing compartmentalization, organelles, and sexual reproduction.
These organisms represent essential evolutionary leaps leading to plants, animals, and fungi.
Through endosymbiotic events, protists established the foundations of photosynthesis and cellular respiration in modern ecosystems.
Their diversity showcases the vast adaptive potential linking the microbial world to multicellular life.