Protist Evolution and Endosymbiosis

Questions and Answers

What is the current understanding of the relationships among protist groups?

They are all part of a single eukaryotic supergroup

They are divided into two distinct lineages

Our understanding of their relationships continues to change rapidly (correct)

They are all closely related and form a single clade

What is the proposed origin of mitochondria?

Endosymbiosis of a photosynthetic cyanobacterium

A fusion of multiple smaller organelles

A natural outgrowth of the host cell's membrane

Endosymbiosis of an aerobic prokaryote (correct)

Which of the following protist groups is no longer considered the oldest lineage of eukaryotes?

Parabasalids

Diplomonads

Excavates (correct)

Euglenozoans

What is a characteristic of diplomonads?

They have modified mitochondria called mitosomes Signup and view all the answers

Which of the following protist groups includes the pathogen that causes yeast infections in human females?

Parabasalids Signup and view all the answers

How are all eukaryotes, including protists, divided according to one hypothesis?

Into four supergroups Signup and view all the answers

What is the distinguishing feature of Euglenozoa?

Presence of a spiral or crystalline rod of unknown function inside their flagella Signup and view all the answers

Which of the following is a characteristic of kinetoplastids?

Presence of a single mitochondrion with an organized mass of DNA called a kinetoplast Signup and view all the answers

What is the proposed mechanism of origin for the supergroup SAR?

Secondary endosymbiosis event Signup and view all the answers

What is the function of the alveoli in alveolates?

Unknown Signup and view all the answers

Which of the following is an example of a parasitic protist?

Trypanosoma gambiense Signup and view all the answers

What is the characteristic feature of dinoflagellates?

Presence of two flagella and cellulose plates reinforcing their cell walls Signup and view all the answers

Which of the following eukaryotic supergroups includes the alveolates and the stramenopiles?

SAR Signup and view all the answers

What is the term for the organized mass of DNA found in the mitochondria of kinetoplastids?

Kinetoplast Signup and view all the answers

What is the primary function of the complex of organelles at the apex of apicomplexans?

To penetrate host cells and tissues Signup and view all the answers

Which group of protists is characterized by the presence of cilia for movement and feeding?

Ciliates Signup and view all the answers

What is the function of micronuclei in ciliates?

To facilitate conjugation Signup and view all the answers

Which of the following is a characteristic of Stramenopiles?

The presence of a 'hairy' flagellum paired with a 'smooth' flagellum Signup and view all the answers

What is the primary mode of reproduction in diatoms?

Asexual reproduction Signup and view all the answers

What is the composition of diatomaceous earth?

Fossilized diatom walls Signup and view all the answers

Which protist group is responsible for causing malaria?

Apicomplexans Signup and view all the answers

What is the current focus of research efforts related to Plasmodium?

Developing vaccines that target Plasmodium Signup and view all the answers

What is the primary characteristic that distinguishes protists from other eukaryotic organisms?

Their diversity in shape, pattern, and internal structure Signup and view all the answers

What is the metabolic process that allows some protists to produce their own food?

Photosynthesis Signup and view all the answers

What is the term for the process in which some protists can switch between autotrophic and heterotrophic metabolism?

Mixotrophy Signup and view all the answers

What is the type of reproduction that involves meiosis and fertilization in some protists?

Sexual reproduction Signup and view all the answers

How are protists classified according to their evolutionary relationships?

Into four supergroups: Excavata, SAR, Archaeplastida, and Opisthokonta Signup and view all the answers

What is the process by which one organism consumes another and forms a symbiotic relationship, leading to the formation of organelles?

Endosymbiosis Signup and view all the answers

What is the characteristic of protists that distinguishes them from other eukaryotic organisms?

Their diversity in shape, pattern, and internal structure Signup and view all the answers

What is the term for the process by which some protists exhibit alternation of generations, where two distinct generations of individuals are needed to complete a life cycle?

Alternation of generations Signup and view all the answers

What is the characteristic feature of Excavates?

Unique cytoskeleton with a groove that resembles an excavation Signup and view all the answers

Which subgroup of Excavates has a reduced number of mitochondria and is anaerobic?

Parabasalids Signup and view all the answers

What is the function of kinetic plastids in Euglenozoa?

Single large mitochondria with a large mass of DNA Signup and view all the answers

What is the characteristic of Neoglenida?

Has a pocket at one end with one or two flagella emerging Signup and view all the answers

Which subgroup of Excavates is characterized by having two nuclei and multiple flagella?

Diplomonads Signup and view all the answers

What is the example of a parasitic protist in the subgroup Euglenozoa?

Trypanosoma Signup and view all the answers

What is unique about the silica test in diatoms?

It is embedded in a matrix of organic molecules Signup and view all the answers

What is the primary function of the flagella in golden algae?

To move and sense the environment Signup and view all the answers

What is the main characteristic of the life cycle of brown algae?

It is complex and consists of multiple generations Signup and view all the answers

What is the term for the process by which brown algae exhibit alternation of generations?

Heteromorphic life cycle Signup and view all the answers

What is the function of the holdfasts in brown algae?

To attach to substrates Signup and view all the answers

What is the characteristic of the gametophyte generation in brown algae?

It is haploid Signup and view all the answers

What is the primary function of the alveoli in Alveolates?

To provide an extra source of oxygen in low-oxygen conditions Signup and view all the answers

Which group of protists is characterized by the presence of a hairy flagellum?

Stramenopiles Signup and view all the answers

What is the primary mode of nutrition for some Stramenopiles?

All of the above Signup and view all the answers

Which group of protists is responsible for causing red tides?

Dinoflagellates Signup and view all the answers

What is the characteristic feature of the life cycle of Apicomplexans?

Complex life cycle that involves multiple hosts and stages Signup and view all the answers

What is the primary function of cilia in Ciliates?

To move and feed Signup and view all the answers

What is the ecological role of Diatoms?

Primary producers in aquatic ecosystems Signup and view all the answers

What is the characteristic feature of the SAR clade?

It is a monophyletic group Signup and view all the answers

What is the primary function of the stipe in brown algae?

To help hold structures upright Signup and view all the answers

What is the characteristic of the diploid generation in the life cycle of brown algae?

Produces spores through meiosis Signup and view all the answers

What is the primary characteristic of Stramenopiles?

Having two flagella, one smooth and one hairy Signup and view all the answers

What is the primary function of the blades in brown algae?

To hold photosynthetic cells Signup and view all the answers

What is the characteristic of the haploid generation in the life cycle of brown algae?

Produces gametes through mitosis Signup and view all the answers

What is the primary difference between green algae and red algae?

Their pigmentation Signup and view all the answers

What is the function of the holdfast in brown algae?

To anchor the algae to rocks or substrate Signup and view all the answers

What is the characteristic of the life cycle of brown algae?

Alternation of generations Signup and view all the answers

What is the main characteristic of the pseudopodia in Radialarians?

They are used for locomotion Signup and view all the answers

Which of the following Uniconta protists is known for causing amoebic dysentery?

Entamoeba Signup and view all the answers

What is the composition of the protective structure in Foramenifera?

Calcium carbonate Signup and view all the answers

Which of the following is a characteristic of acellular slime molds?

They have a mass of cytoplasm with many nuclei Signup and view all the answers

What is the primary function of pseudopodia in Ryze area protists?

To move around Signup and view all the answers

Which group of Uniconta protists includes animals and fungi?

Uniconta Signup and view all the answers

Study Notes

Endosymbiosis and Protist Evolution

Endosymbiosis is the process in which a unicellular organism engulfs another cell, which becomes an endosymbiont and then an organelle in the host cell
Mitochondria are believed to have evolved by endosymbiosis of an aerobic prokaryote
Plastids are believed to have evolved by endosymbiosis of a photosynthetic cyanobacterium

Diversity of Protists

Excavates are no longer thought to be the oldest lineage of eukaryotes
Many excavates have been shown to have mitochondria and have been reclassified
Our understanding of the relationships among protist groups continues to change rapidly
One hypothesis divides all eukaryotes (including protists) into four supergroups

Excavates

Characterized by a feeding groove that appears like “excavated” from the surface
Includes diplomonads, parabasalids, and euglenozoans

Diplomonads

Have modified mitochondria called mitosomes
Derive energy from anaerobic biochemical pathways
Have two equal-sized nuclei and multiple flagella
Are often parasites, such as Giardia intestinalis

Parabasalids

Have reduced mitochondria called hydrogenosomes that generate some energy anaerobically
Include Trichomonas vaginalis, the pathogen that causes yeast infections in human females

Euglenozoa

A diverse clade that includes euglenids and kinetoplastids
Some are predatory heterotrophs, photosynthetic autotrophs, and parasites
Characterized by a spiral or crystalline rod of unknown function inside their flagella

Euglenids

Have one or two flagella that emerge from a pocket at one end of the cell

Kinetoplastids

Have a single mitochondrion with an organized mass of DNA called a kinetoplast
Include free-living consumers of prokaryotes in freshwater, marine, and moist terrestrial ecosystems
Include Trypanosoma gambiense, which causes sleeping sickness in humans, and pathogenic trypanosomes that cause Chagas’ disease

SAR Supergroup

Some data suggest that the supergroup SAR is monophyletic and originated by a secondary endosymbiosis event
The proposed endosymbiont is a red alga
Includes the alveolates and the stramenopiles

Alveolata

Characterized by membrane-bounded sacs (alveoli) just under the plasma membrane
Function of the alveoli is unknown
Includes dinoflagellates, apicomplexans, and ciliates

Dinoflagellates

Have two flagella and each cell is reinforced by cellulose plates
Are abundant components of both marine and freshwater phytoplankton
Are a diverse group of aquatic phototrophs, mixotrophs, and heterotrophs
Toxic “red tides” are caused by dinoflagellate blooms

Apicomplexans

Are parasites of animals, and some cause serious human diseases
Spread through their host as infectious cells called sporozoites
Have a complex of organelles specialized for penetrating host cells and tissues
Most have sexual and asexual stages that require two or more different host species for completion
Include Plasmodium, which causes malaria

Ciliates

A large and varied group of protists, named for their use of cilia to move and feed
Have large macronuclei and small micronuclei
Genetic variation results from conjugation, in which two individuals exchange haploid micronuclei

Stramenopila

Includes important phototrophs as well as several clades of heterotrophs
Most have a “hairy” flagellum paired with a “smooth” flagellum
Includes diatoms, golden algae, brown algae, and oomycetes

Diatoms

Unicellular algae with a unique two-part, glass-like wall of hydrated silica
Are a major component of phytoplankton and are highly diverse
Fossilized diatom walls compose much of the sediments known as diatomaceous earth
Reproduction is mainly asexual

Introduction to Protists

Protists are a diverse group of eukaryotic organisms exhibiting a wide range of shapes, patterns, and internal structures.

Characteristics of Protists

Protists are eukaryotic, meaning their cells contain a nucleus and numerous membrane-bound organelles.
Most protists are single-celled (unicellular) organisms, but some live in groups (colonies) and a few are multicellular.
Protists have diverse subcellular structures, including organelles and other internal features.

Nutrition and Metabolism

Photoautotrophic protists produce their own food through photosynthesis using chloroplasts.
Heterotrophic protists obtain nutrients by ingesting or absorbing them from their surroundings.
Some protists can switch between autotrophic and heterotrophic metabolism, a process known as mixotrophy.

Reproduction and Life Cycles

Some protists reproduce asexually, while others are capable of sexual reproduction through meiosis and fertilization.
Some protists, like kelp, exhibit alternation of generations, where two distinct generations of individuals are needed to complete a life cycle.

Classification of Protists

There are four supergroups of protists: Excavata, SAR, Archaeplastida, and Opisthokonta.
Each supergroup has a hypothesized common ancestor, and the protists are considered polyphyletic, meaning they have multiple ancestors.

Endosymbiosis

Endosymbiosis is a process where one organism consumes another and forms a symbiotic relationship, leading to the formation of organelles like mitochondria and chloroplasts.
Scientists hypothesize that endosymbiosis played a role in the origin of certain organelles in eukaryotic cells.

Protist Super Groups

Protist classification is based on morphological and DNA data, and they have unique characteristics such as nutritional styles and replication methods that distinguish them from other organisms.

Super Group: Excavates

Excavates have a unique cytoskeleton with a groove-like structure resembling an excavation.
They have modified mitochondria.
They possess two major groups with flagella that are distinct from other organisms.

Subgroup: Diplomonads

Diplomonads have modified mitochondria called microsomes.
They lack an electron transport chain and use anaerobic pathways for energy production.
They have two nuclei and multiple flagella.
Many diplomonads are parasites, such as Giardia lamblia, which causes diarrhea and vomiting in humans.

Subgroup: Parabasalids

Parabasalids have a reduced number of mitochondria.
They are anaerobic and produce hydrogen gas as a byproduct.
Example: Trichomonas vaginalis, a sexually transmitted disease causing a burning, itching sensation in women.

Subgroup: Euglenozoa

Euglenozoa have a rod-shaped structure inside their flagella.
They have kinetic plastids, which are single large mitochondria with a large mass of DNA.
They are free-living parasites, such as Trypanosoma, which causes African sleeping sickness and Chagas disease.

Subgroup: Neoglenida

Neoglenida have a pocket at one end with one or two flagella emerging.
They are mixotrophic, able to undergo photosynthesis, ingest other organisms, or synthesize their own energy.
Example: Euglena, a common microscopic organism found in pond water.

SAR Clade

The SAR clade is a supergroup of protists that includes Stramenopiles, Alveolates, and Rhizarians, and is thought to be a monophyletic group.

Alveolates

Alveolates are characterized by the presence of membrane-bound pockets called alveoli, which may serve as an extra source of oxygen in low-oxygen conditions.
Alveolates include dinoflagellates, apicomplexans, and ciliates.

Dinoflagellates

Dinoflagellates have two flagella, one at the terminal end and one around the central groove.
They can be photoautotrophic, heterotrophic, or mixotrophic.
Some dinoflagellates are responsible for red tides, which can cause gastrointestinal poisoning in humans.

Apicomplexans

Apicomplexans have a pointy structure called an apex, which is used to penetrate host cells.
They have a complex life cycle that involves multiple hosts and stages.
Apicomplexans are responsible for diseases such as malaria, which kills approximately 900,000 people per year.

Ciliates

Ciliates have organized rows of cilia, which are used for locomotion.
They have a complex life cycle that involves sexual and asexual stages.
Ciliates can reproduce through binary fission, but may not produce genetically identical clones.

Stramenopiles

Stramenopiles are characterized by the presence of a hairy flagellum.
They include diatoms, golden algae, brown algae, and oomycetes.
Stramenopiles have a diverse range of strategies for obtaining energy and nutrition, including photoautotrophy and heterotrophy.

Diatoms

Diatoms are single-celled or colonial organisms that can be found in freshwater and seawater.
They are abundant in aquatic ecosystems and play a key role in the food chain.
Diatoms have a silica test, which is made of hydrated silicon material, and is embedded in a matrix of organic molecules.

Golden Algae

Golden algae have two flagella, one hairy and one smooth.
They can be photoautotrophic, heterotrophic, or mixotrophic.
Golden algae are sometimes colonial and have a yellowish color due to the presence of a specific type of photosynthetic pigment.

Brown Algae

Brown algae have a complex life cycle and a range of morphologies.
They are often found in the ocean and provide food and shelter for young fish.
Brown algae can grow up to 100 meters in length and have a complex structure, including holdfasts, stipes, and blades.

Alternation of Generations

Brown algae exhibit an alternation of generations, where a diploid sporophyte generation alternates with a haploid gametophyte generation.
The sporophyte generation produces spores, while the gametophyte generation produces gametes.
This alternation of generations results in a heteromorphic life cycle, where the different generations have different forms.

Brown Algae

Brown algae are multicellular protists that resemble plants, but have distinct structures with different functions.
Holdfast anchors the algae to rocks or substrate, but does not absorb nutrients.
Stipe helps hold structures upright, similar to a stem or trunk.
Blades hold photosynthetic cells, similar to leaves.
Some brown algae have bladders that help them float upright and grow towards the surface of the water.
Brown algae have an alternation of generations life cycle, with two generations having different numbers of chromosomes.

Life Cycle of Brown Algae

Diploid generation produces haploid spores through meiosis.
Haploid generation produces gametes (eggs and sperm) through mitosis.
Fertilization of gametes produces a new diploid sporophyte, which grows into a mature sporophyte.
Sporangia are sacs containing spores that develop into male and female gametophytes.

Characteristics of Brown Algae

Belong to the stramenopiles group.
Have two flagella, one smooth and one hairy.
Sporophytes are responsible for producing spores, which develop into gametophytes.
Gametophytes are responsible for producing gametes, which fertilize to produce a new sporophyte.

Red Algae

Red algae have an abundance of the accessory pigment ficocyanin, which gives them a red color.
Some red algae are edible and used in sushi rolls.
Most red algae are multicellular and live in the ocean.

Green Algae

Most green algae are multicellular and live in freshwater.
They are filamentous, with cells arranged in a long chain.
Some green algae, like sea lettuce and Caulerpa, live in the ocean.
Green algae are important contributors to the food chain, producing energy for other organisms.

Amoeboid Protists

Move using pseudopodia (false feet) or extensions of their cell membrane.
Classified into two groups: Ryze area (AR) and Uniconta.

Ryze Area (AR)

Have thin, slender extensions called pseudopodia used for locomotion and capturing prey.
Examples include:
- Foramenifera: have a protective structure called a test made of calcium carbonate, with tiny holes (perforations) for pseudopodia to emerge.
- Radialarians: have tests made of silica, with intricate patterns and shapes.

Uniconta

Include animals, fungi, and protists.
Examples include:
- Slime molds:
  - Acellular slime molds: many cells with no plasma membrane dividing them, forming a mass of cytoplasm with many nuclei.
  - Cellular slime molds: individual amoeba cells with their own plasma membrane, forming aggregates.
- Tubulinea:
  - Have amoeba-like movement, but with lobe-like pseudopodia.
  - Feed on other protists, algae, and bacteria.
- Entamoeba:
  - Can be parasites of the gastrointestinal system, causing amoebic dysentery.
  - Entamoeba histolytica is notorious for causing severe gastrointestinal disease.

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Description

Learn about the process of endosymbiosis, mitochondria and plastid evolution, and protist diversity, including the debunking of Excavates as the oldest eukaryote lineage.