Kingdom Protista: Eukaryotic Origins and Endosymbiosis

• Kingdom Protista

Eukaryotic Origins

• Eukaryotic cells differ from prokaryotes
• Have a complex cytoskeleton
• Show compartmentalization (nucleus and organelles)
• Have membrane bound organelles
• Eukaryotes appeared in microfossils around 1.5 BYA

Endosymbiosis

• Organelles evolved via endosymbiosis between an ancestral eukaryote and a bacterial cell
• Mitochondria: Aerobic bacteria engulfed by larger bacteria, which then became eukaryotic cells
• Chloroplasts: larger bacteria engulfed photosynthetic bacteria
• Chloroplasts derive from a single line of cyanobacteria
• Hosts are not monophyletic
• Brown algae engulfed a red algae that already had chloroplasts in secondary endosymbiosis

Evidence for Endosymbiosis

• DNA is inside mitochondria and chloroplasts with circular DNA like in bacteria
• Ribosomes inside mitochondria resemble bacterial ribosomes, are vulnerable to antibiotics
• Chloroplasts and mitochondria replicate by binary fission, not mitosis

Mitosis Evolved in Eukaryotes

• Prokaryotes carry genes on a single DNA molecule
• Eukaryotes have multiple chromosomes
• Mitosis and cytokinesis separate chromosomes and cell contents during cell division

Overview of Protists

• Most diverse of the four eukaryotic kingdoms
• Not fungi, plants, nor animals
• Unicellular, colonial, and multicellular
• Vary in size from microscopic to huge
• Vary in form and symmetry
• All types of nutrition
• Some are photosynthesizers, while others are heterotrophs

Protists and Eukaryotic Supergroups

• Protists are present in all five eukaryotic supergroups
• Protists are not monophyletic or paraphyletic
• The five eukaryotic supergroups are Excavata, SAR, Archaeplastida, Amoebozoa, and Opisthokonta

Cell Surface in Protists

• Protists are diverse at the cellular level
• They have a plasma membrane, and an extracellular matrix in some
• Diatoms and foraminifera have silica shells
• Some can form cysts that are dormant cells with resistant outer covering used for disease transmission

Locomotion in Protists

• Flagella function in movement
• Cilia function to move and eat
• Pseudopodia are used for eating, moving, locomotion, and defense, and is common in amoebas

Nutrition in Protists

• Autotrophs make their own food
• Some autotrophs are photosynthetic, while others are chemoautotrophic
• Heterotrophs eat other things or other organisms
• Phagotrophs ingest particulate food matter
• Mixotrophs are phototrophic and heterotrophic

Reproduction in Protists

• Asexual reproduction is typical
• Mitosis results in equal size daughter cells
• Budding makes one daughter cell smaller
• Schizogony results in several nuclear divisions and several individuals
• Sexual reproduction may happen regularly or under stress
• Meiosis is a major eukaryote innovation
• The union of haploid gametes is produced by meiosis and produce diploid cells allowing for genetic recombination

Protists Bridge to Multicellularity

• Can be single cells(Amoebas, euglena, Paramecium), or colonies(Volvox) or multicellular like most algae
• Multicellularity arose multiple times
• Multicellularity fosters specialization

Excavata

• Consists of diplomonads, parabasalids, and euglenozoans
• They share similarities in cytoskeletal features and DNA sequences like flagella

Diplomonads

• Unicellular
• They move with multiple flagella
• Have 2 haploid nuclei
• Giarda is a parasite that causes severe diarrhea
• They lack functional mitochondria

Parabasalids

• Have bacteria inside them
• Some live in termite guts
• Trichonympha have a symbiotic relationship with cellulose-degrading bacteria that can convert cellulose into energy
• Trichomonas vaginalis is an STD that uses flagella and has semi functional mitochondria

Euglenozoa

• They can change their shape because they lack cell walls
• Among the earliest eukaryotes to possess mitochondria
• Have free-living euglenids and parasitic kinetoplastids
• They have up to 2 flagella

Euglenids

• 1/3 of euglenids have chloroplasts and are autotrophic

• The others lack chloroplasts, ingest their flood, and are heterotrophic

• Some are mixotrophic, meaning they are both Auto(like plants) and heterophic(like animals)

• Reproduce asexually via mitosis

• Contain contractile vacuoles to collect excess water

• Have stigma that detects light

• They use two anterior flagella from the reservoir, and the small chloroplasts likely evolved from a symbiotic relationship by ingesting green algae

• Euglena's photosynthetic pigments are lightsensitive

• Have parasitic kinetoplastids and a unique single mitochondrion with DNA maxicircles and minicircles responsible for raid glycolysis and unusual RNA editing

• Trypanosomes cause diseases like:

• African sleeping sickness – tsetse fly

• Leishmaniasis – sand fly

• Chagas disease – skin contact with urine or blood of infected wild animal.

• Parasitic kinetoplastids face controls due to the organisms' repetitive coat changes and mechanisms for antigen variation

• Sequencing genomes may reveal core common genes for the development of single-drug targets

SAR

• Supergroup consisting of 3 branches: Stramenopiles(brown algae), Alveolates, and Rhizaria

Stramenopiles

• Include brown algae, diatoms, and oomycetes
• Identified by very fine hairs on their flagella
• Some species have lost flagella hairs during evolution

Brown Algae

• Conspicuous seaweeds of northern regions similar to plants but are not plants
• Exhibit haplodiplontic, alternating generation lifecycles
• Sporophyte: multicellular and diploid.
• Gametophyte: multicellular and haploid.

Diatoms

• Phylum Chrysophyta
• Photosynthetic, unicellular organisms with unique double shells made of silica (glass like material/structure)
• Some move with raphes, two long grooves with vibrating fibrils

Oomycetes

• Are "water molds" like the one that caused the irish potato famine
• Can be parasites or saprobes
• Were once considered fungi
• They have motile zoospores with two unequal flagella, which are produced asexually
• They also undergo sexual reproduction
• Found in water or on land
• Phytophthora infestans caused the Irish potato famine (1845 to1849), which killed 400,000 people

SAR – Alveolata

• Are alveolates
• Have flattened vesicles called alveoli
• Include dinoflagellates, apicomplexans, and ciliates
• Share a common lineage and locomotion modes

Dinoflagellates

• Photosynthetic, unicellular with flagella in aquatic settings
• Some are luminescent
• Do not appear to be directly related to any other phylum
• Their DNA uncomplexed with histones
• Most have chlorophylls a and c and cartenoids, with some having biochemistry similar to diatoms and brown algae
• Primarily reproduce asexually but engage in sexual reproduction under starvation
• Are responsible for phenomena known as "red tide," which are population explosions of dinoflagellates
• Release toxins that may cause mortality in fish, birds, and marine mammals

Apicomplexans

• Spore-forming animal parasites
• Apical complex is a unique arrangement of organelles at one end of the cell enabling the cell to invade its host
• Plasmodium causes malaria
• Complex life cycle with sexual, asexual, and different hosts
• Eradication focused on vector elimination, drug development, vaccines and dealing with DDT-resistant mosquitoes
• Gregarines use an apical complex to attach to arthropod intestines, annelids, and mollusks
• Toxoplasma gondii leverages the apical complex to invade epithelial cells in the human gut, which causes infections in immunosuppressed and crosses the placental barrier to harm the fetus

Ciliates

• Exhibit many cilia in rows or spirals
• Beat cilia in a coordinated fashion
• The pellicle is both tough and flexible as an outer covering
• They have micronucleus for sexual reproduction, and macronucleus essential for function
• Contractile vacuoles regulate water balance, and food vacuoles digest food
• Conjugation, only different mating types can conjugate

SAR - Rhizaria

• Use pseudopods for locomotion
• Three distinct monophyletic groups: Radiolaria, Foraminifera, and Cercozoa

Radiolarians

• Phylum Actinopoda
• Glassy exoskeletons made of silica
• Have needlelike pseudopods for feeding, moving, and defense

Foraminifera 1

• Heterotrophic marine protists
• Have pore-studded shells(like snail shells) comprised of tests, through which thin podia emerge
• They use podia for swimming and feeding
• Experience complex life cycles with both haploid and diploid generations
• Limestones are rich in forams and comprise the White Cliffs of Dover

Cercozoa

• Morphologically are diverse and primarily soil protists
• They use flagella or pseudopods for locomotion
• Some have silica-based shells made of scales or plates

Archaeplastida

• Includes Rhodophyta, Chlorophyta, Charophytes, and land plants
• These photosynthetic organisms get their chloroplast through primary endosymbiosis

Rhodophyta

• Red algae range from microscopic to very large
• They don't have flagella or centrioles
• They have red accessory photosynthetic pigments
• Their cells have both haploid and diploid phases

Green Algae

• Consist of 2 lineages called Chlorophyta
• Unusual diversity and lines of specialization, and the Charophytes gave rise to the land plants

Unicellular Chlorophytes

• Early green algae probably resembled Chlamydomonas reinhardtii
• Diverged from land plants over 1 B YA
• Several lines of evolutionary specialization derived from chlorophytes
• Genome of Chlamydomonas reinhardtii has been sequenced; organism is workhorse for comparative genetic studies; widely used host for recombinant protein expression

Cell Specialization in Colonial Chlorophytes

• Multicellularity arose many times in the eukaryotes
• Colonial chlorophytes are examples of cellular specialization, which are a hollow sphere of 500 to 60,000 cells each having 2 flagella

Haplodiplontic Life Cycles In Multicellular Chlorophytes

• Haplodiplontic life cycles found in some chlorophytes and the streptophytes
• Ulva is a multicellular chlorophyte where both gametophyte and sporophyte generations
• Ulva consists of flattened sheets two cells thick

Charophytes

• Green algae distinguished from chlorophytes by phylogenetic relationship to land plants via molecular evidence from rRNA and DNA sequences

• They show haplontic life cycles

• The 2 candidate charophyte clades thought to be most closely related to land plants are Charales and Coleochaetales

• Both clades create green mats around freshwater edges

• One species successfully transitioned to land by adapting

Amoebozoa

• Amoebas move via pseudopods, flowing projections of cytoplasm.
• Pseudopods extend and pull the amoeba forward to engulf food.
• An amoeba puts a pseudopod forward and then flows into it
• Microfilaments of actin and myosin assist

Amoeba

• Most amoebas are free living
• Found in soil and freshwater
• Some are parasitic
• Acanthamoeba enters the body and passes the blood-brain barrier to cause inflammation and death

Plasmodial Slime Molds

• Stream along as a plasmodium
• Nonwalled, multinucleate mass of cytoplasm.
• Form feeding phase which ingests bacteria/organic material
• When food/moisture are scarce, the organism forms sporangia, where spores are produced

Cellular Slime Molds

• Important for studying cell differentiation for simple systems
• Individual organisms behave as separate amoebas
• Move through soil ingest bacteria
• When food is scarce, organisms aggregate to form a slug
• The slug may make spore cells

Opisthokonta

• Choanoflagellates
• Unicellular organisms/fungi and animal common ancestor
• They have a single emergent flagellum surrounded by a contractual collar; structure in sponges which are animals
• Colonial choanoflagellates resemble close sponge relatives

Protists are eukaryotic organisms with complex cytoskeletons and compartmentalization. Organelles evolved through endosymbiosis, where bacteria were engulfed by ancestral eukaryotes. Mitochondria and chloroplasts have bacterial-like DNA and replicate by binary fission.