Lecture Notes on Eukaryotic Evolution and Protist Diversity

Lecture Overview

  • Lecture presentations by Nicole Tunbridge and Kathleen Fitzpatrick
  • Topic: Chapter 28 - The Origin and Evolution of Eukaryotes
  • © 2021 Pearson Education Ltd.

Introduction to Eukaryotes

  • Focus of the lecture: Eukaryotes

Eukaryotic Diversity and Protists

General Concepts

  • Eukaryotes include diverse, mostly unicellular groups informally known as protists.
  • Protist Definition: An informal term for all eukaryotes that are not plants, animals, or fungi.
      - This classification is no longer considered a kingdom due to closer relationships between some protists with plants, fungi, or animals than with other protists.

Cell Structure

  • Eukaryotic cells possess a nucleus and other membrane-enclosed organelles, which isolate functions within the cell, making them more complex than prokaryotic cells.
  • The cytoskeleton in eukaryotic cells allows for asymmetric shapes and adaptability.
  • Most eukaryotic lineages consist primarily of protists, and most protists are unicellular organisms.

Structural and Functional Diversity in Protists

  • Protists represent the greatest structural and functional diversity among eukaryotes.
  • While most protists are unicellular, some exist as colonies or multicellular forms.
  • Unicellular protists are complex as each cell must perform all life functions independently.
  • Certain unicellular protists possess unique organelles, such as dinoflagellates having an eye-like organelle called an ocelloid.

Nutritional Diversity

  • Protists exhibit nutritional variety:
      - Photoautotrophs: Contain chloroplasts.
      - Heterotrophs: Absorb organic molecules or ingest larger food particles.
      - Mixotrophs: Combine photosynthesis and heterotrophic nutrition.

Reproductive Diversity

  • Reproduction can be asexual or sexual, with some protists having both phases in their life cycle.
  • All three basic sexual life cycle types (animal, plant, and fungal) are represented in protists.

Endosymbiotic Theory

Concept of Endosymbiosis

  • Endosymbiosis: A relationship where one organism lives inside the cells of another (the host).
  • Evidence indicates much of protistan diversity stems from endosymbiosis, where mitochondria and plastids arise from engulfed bacteria.
  • Mitochondria evolved before plastids from an anaerobic bacterium, specifically alpha proteobacteria, suggesting each evolved only once in life's history.

Early Eukaryotic Features

  • The ancestral host cell was relatively complex with eukaryotic features, potentially linked to lokiarchaeotes as candidate ancestors.

Plastid Evolution

  • Plastids evolved later when a heterotrophic eukaryote engulfed a photosynthetic cyanobacterium.
  • Key lineages of photosynthetic protists, like red and green algae, arose from this plastid-bearing ancestor.
  • Plastids of red algae and green algae exhibit two membranes, with transport proteins homologous to those found in cyanobacteria.

Supergroups of Eukaryotes

  • Current understanding divides all eukaryotes into four supergroups: Excavata, SAR, Archaeplastida, Unikonta.

Excavata

  • Includes three clades: parabasalids, diplomonads, and euglenozoans.
  • Example: Giardia intestinalis, a diplomonad parasite causing intestinal infections in mammals.

SAR Supergroup

  • Comprises three large clades: Stramenopila, Alveolata, and Rhizaria.
  • Examples: Diatoms (photosynthetic stramenopiles), rhizarians (amoebas like Globigerina).

Archaeplastida

  • This supergroup encompasses red and green algae and plants.
  • Features unicellular, colonial, and multicellular species like Volvox (multicellular green alga).

Unikonta

  • Comprises amoebas with lobe- or tube-shaped pseudopodia, animals, fungi, and non-amoeba protists closely related to animals or fungi.
  • Example: Amoeba proteus, a tubulinid amoeba.

Excavates and Their Characteristics

General Features

  • Characterized by their cytoskeleton and the presence of an excavated feeding groove.
  • Three monophyletic groups: diplomonads, parabasalids, and euglenozoans.

Diplomonads

  • Lack plastids; possess reduced mitochondria known as mitosomes, which do not conduct electron transport.
  • Obtain energy through anaerobic pathways, possess two equal-sized nuclei and multiple flagella.
  • Many are parasites (e.g., Giardia intestinalis).

Parabasalids

  • Possess reduced mitochondria called hydrogenosomes that produce energy anaerobically, releasing hydrogen gas as a byproduct.
  • Best-known species: Trichomonas vaginalis, a sexually transmitted parasite infecting millions annually.

Euglenozoans

  • Diverse clade comprising heterotrophs, autotrophs, mixotrophs, and parasites characterized by a spiral or crystalline rod in their flagella.
      - Includes kinetoplastids and euglenids.
Kinetoplastids
  • Have a single mitochondrion containing organized DNA (kinetoplast).
  • Free-living species consume prokaryotes, while others like Trypanosoma parasitize hosts, causing sleeping sickness.
Euglenids
  • Feature one or two flagella emerging from a cell pocket.
  • Many are mixotrophs that alternate between autotrophy and heterotrophy based on conditions.

SAR - A Diverse Group

Overview

  • Defined by DNA similarities, includes clades: Stramenopila, Alveolata, Rhizaria.

Stramenopiles

  • Include significant photosynthetic organisms; often possess hairy and smooth flagella.
  • Key groups: Diatoms, oomycetes, and brown algae.
Diatoms
  • Unicellular algae characterized by a two-part, glass-like wall made of silica, providing protection.
  • Comprising roughly 100,000 species, they significantly contribute to oceanic phytoplankton and affect global CO2 levels.
Brown Algae
  • The largest and most complex algal forms.
      - Color due to carotenoids; marine environment common.
      - Structure includes holdfasts, stipes, and blades.
  • Economically important; for example, Laminaria is used in cosmetics and food.
  • Exhibit alternation of generations in their life cycles, with structurally different components.
Oomycetes
  • Include water molds mistaken for fungi due to filamentous structures but are distinguished by cell walls composed of cellulose.
  • Obtain nutrients via parasitism or decomposition (e.g., Phytophthora infestans causing potato late blight).

Alveolates

  • Defined by membrane-enclosed sacs (alveoli) beneath the plasma membrane.
  • Includes three major clades: Dinoflagellates, apicomplexans, ciliates.
Dinoflagellates
  • Found in marine and freshwater phytoplankton, noted for two flagella and armor-like cellulose plates.
  • Responsible for red tides, which can be toxic to marine life, exacerbated by climate change.
Apicomplexans
  • Almost entirely parasitic, with complex life cycles involving multiple hosts (e.g., Plasmodium, the malaria-causing parasite).
  • Evade host immune systems by changing surface proteins; significant global health impact.
Ciliates
  • Named for their use of cilia for movement and feeding.
  • Characterized by having micronuclei and macronuclei, featuring mechanisms for genetic variation through conjugation and asexual reproduction via binary fission.

Rhizarians

  • Many are amoebas utilizing pseudopodia to feed and move.
  • Defined by threadlike pseudopodia in their structures, unified into three clades: radiolarians, forams, cercozoans.

Archaeplastida

Overview

  • Comprises red algae, green algae, and plants.

Red Algae

  • Contain phycoerythrin, affecting their coloration; primarily multicellular.
  • Reproduction often features alternation of generations; significant in ecosystems and human activity (e.g., nori for sushi).

Green Algae

  • Characterized by chloroplasts similar to plants; includes charophytes (closest relatives to plants).
  • Exhibit complex life cycles involving both sexual and asexual reproduction.

Unikonta

Overview

  • Encompasses amoebozoans and opisthokonts (animals and fungi).

Amoebozoans

  • Defined by lobe- or tube-shaped pseudopodia.
  • Includes tubulinids and slime molds, the latter resembling fungi due to spore-producing fruiting bodies:
      - Plasmodial Slime Molds: Form multinucleated plasmodium.
      - Cellular Slime Molds: Form aggregates of separate cells for migration and fruiting body formation.

Ecological Roles of Protists

Symbiotic Relationships

  • Many protists play roles as symbionts, such as dinoflagellates within coral polyps or protists aiding termites with wood digestion.
  • Some protists act as parasites (e.g., Plasmodium, Pfiesteria, Phytophthora).

Producers in Ecosystems

  • Photosynthetic protists are vital producers, forming the basis of food webs in aquatic systems.
  • Their populations fluctuate with nutrient availability, impacting ecosystems significantly.

Climate Change Impact

  • Increasing sea temperatures hinder nutrient upwelling, affecting phytoplankton health and, subsequently, marine ecosystems and fisheries.