Eukaryotes: Origin and Diversification

• Chapter 26: The origin and diversification of Eukaryotes

Earth's Clock Life

• Prokaryotes were the first life forms on Earth, appearing 3.8 billion years ago.
• For over 2 billion years after their emergence, life consisted only of unicellular organisms.
• Homo sapiens appeared approximately 300,000 years ago.
• Cyanobacteria evolved the ability to photosynthesize and produce oxygen (O2).
• Eukaryotes emerged 2 billion years ago.
• Eukaryotic cells evolved from prokaryotic cells, with unicellular eukaryotes appearing first.
• Multicellularity came about later.

Eukaryotes Evolution

• The common ancestor of eukaryotes was a prokaryote archaea.
• Eukaryotes acquired organelles such as the nucleus, mitochondria, and chloroplasts.
• The evolution of the eukaryote cell happened in multiple steps.

Loss of the Cell Wall

• The loss of the cell wall allowed for a flexible cell surface, enabling the cell to grow larger and increase in volume.
• Cell volume increases proportionally with metabolic activity.

Membrane Infolding

• Nutrient and gas exchange are proportional to surface area.
• Increased cell size causes a decrease in the surface area-to-volume ratio.
• The rate of exchange can become too slow to support metabolism.
• A flexible surface folded inward, increasing the surface area for more rapid exchange.
• New enclosed environments with unique chemical reactions formed, such as the nucleus and nuclear envelope.

Eukaryote characteristics

• Cell wall lost
• Compartmentalization
• Nucleus
• Stronger cytoskeleton
• Internal membranes with ribosomes
• Lysosomes

Phagocytosis and Endosymbiosis

• New flexible cell membrane allowed phagocytosis and endosymbiosis to occur.
• Cyanobacteria were already photosynthesizing and producing oxygen (O2).

Endosymbiotic Theory

• A large archaea engulfed, but did not digest, a proteobacteria, which then evolved into the mitochondria.
  • The bacteria detoxified oxygen, reducing it to water.
• In photosynthetic eukaryotes, there were additional endosymbiotic events.
  • Primary endosymbiosis
  • Secondary endosymbiosis
  • Tertiary endosymbiosis

Primary Endosymbiosis

• It occurs in photosynthetic eukaryotes.
• Early eukaryotes (with mitochondria) engulfed and retained a cyanobacteria, allowing them to photosynthesize.
• This gave rise to chloroplasts with 2 membranes (ancestral and host plasma membranes). It involved a gram-negative cyanobacteria.
• Peptidoglycan was lost, but remnants are present in some groups.
• Chloroplasts in glaucophytes, green algae, red algae, and land plants are all similar, with 2 membranes.

Secondary and Tertiary Endosymbiosis

• Additional endosymbiotic events gave rise to chloroplasts in other eukaryotes.
• Early eukaryotes (with mitochondria) engulfed and retained a green algae, another early eukaryote which already had a 2 membrane chloroplast.
• Chloroplasts originating from secondary endosymbiosis have 3 membranes.

Domain Eukarya

• Domain Eukarya includes:
  • Bacteria
  • Archaea
  • Eukarya
  • All share a universal ancestor.

Eukaryotes

• People are familiar with multicellular eukaryotes like plants, animals, and fungi, but also unicellular eukaryotes which are common.
• Eukaryotes that are neither plants, animals, nor fungi are called protists and are in most eukaryote groups.
• Protist classification is a "garbage bin" and not scientific.
• Eukaryotes is a "monophyletic group" which include only the descendants of a common ancestor.
• Protist is a "paraphyletic group" which does not include all the descendants of a common ancestor.

Protists

1. Excavata:

• Include photosynthetic euglena.
• Unicellular
• Have a feeding groove on one side of the cell.

2. Chromalveolata:

• Alveolates: Are unicellular with sacs ("alveoli") in the cell membrane.
• Dinoflagellates - Unicellular organisms that possess two flagella. - Most are photosynthetic. - important primary producers. - form important endosymbiotic relationships. - Can cause red tides. - Coral polyps produce CO2 which zooxanthellae use for photosynthesis - Polyps then grow with the sugars, lipids and oxygen of photosynthesis.

Red Tides

• Certain dinoflagellates produce neurotoxins, and their blooms can release large amounts of toxins that pose a threat to marine life, including certain fish, marine mammals, and seabirds.
• Filter-feeding organisms like oysters, and mussels, can also accumulate these dangerous toxins. - They can be fatal if consumed by humans.

Ciliates

• Have cilia for locomotion and transporting food into their gullet.
• Have 2 types of nucleui: -Macronucleus: for the cell's functions -Micronucleus: for genetic recombination
• conjugation is when ciliates exchange genetic information without reproduction. -Provides new combinations of genes
  • Common response to unfavorable environment.

Ciliate Conjugation

• Two ciliates fuse together in the oral region.
• Micronuclei subsequently divede, cells then exchange micronuclei.
• Macronuclei disintegrate and 2 micronuclei fuse to form a new macronucleus.

Apicomplexans

• Parasites are equipped with an apical complex (organelles located in the cell's tip that enable them to infiltrate host tissue).
• Have complex life cycles that mandate two different types of organisms.
  • An example is plasmodium-malaria (human-mosquito).

Apicomplexans cycle

• In the mosquito body, human blood with plasmodium cells are transformed into infecting cells which accumulate in the salivary glands.
• As the mosquito bites a human infecting cells pass to the human blood.
• In the human body, Intecting cells enter liver cells, mature, and multiply to invade back into the bloodstream and red blood cells to form sexual cells.

Stramenopiles

• Second type of Chromalveolata is Stramenopiles: examples are diatoms and brown algae.
• This group includes unicellular and multicellular organisms with a “hairy” flagellum.

Diatoms

• Form beautiful silica frustules; look like petri dishes.
• Unicellular photosynthetic: produce 1/5 oxygen on Earth, similar to all rainforests.
• Produce oil to store energy and to float in upper photic zone.
• When die, sink to ocean floor.

Brown Algae

• Not plants!
• The brown color is due to the presence of fucoxanthin, a pigment to capture light at deeper depths.
• The largest protists there are, the giant kelps (up to 60 m long) are included.
• Resemble plants:
  • Attached to hard bottom by a holdfast
  • Have blades (leave-like structures) with gas bladders (to remain upright in the water to get more light closer to the surface)

Rhizaria

• Tiny amoeba-like organisms with calcium carbonate or inorganic tests that form sediments and sedimentary rocks.
• Their shells accumulate in sediments on the ocean floor and form sedimentary bedrock.
• Geologists use foram fossils to understand the geological history of an area, and Earth's climatic past.

Amoebozoa:

• Unicellular organisms that change their shape with pseudopods (temporary arm-like projections developed in the direction of movement).
• Some are pathogens.