Diversity of Eukaryotes: Protists and the Domain Eukarya

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Chapter 27: Diversity of Eukaryotes – The Protist

Chapter Opening

This chapter explores the distinguishing features of eukaryotic organisms, focusing on protists, their evolutionary origins, and their classification within the Domain Eukarya.

Key features of eukaryotes: Eukaryotes possess membrane-bound organelles, including a nucleus, and are generally larger and more complex than prokaryotes.
Synapomorphy for Domain Eukarya: The presence of a nuclear envelope, mitochondria, and other membrane-bound organelles are shared derived traits (see Figure 27.1).
Definition of protists: Protists are a diverse group of mostly unicellular eukaryotic organisms that do not fit into the traditional categories of plants, animals, or fungi.

Section 27.1: Why Do Biologists Study Protists?

Introduction

Protists are studied for their ecological importance, evolutionary significance, and their role in human health and disease. They represent a major branch of eukaryotic diversity.

Historical context: Organisms called "protists" were once grouped together due to their unicellular nature, but modern classification recognizes their diversity.
Ecological roles: Protists include algae (photosynthetic), water molds, and amoeboids, which play key roles in aquatic food chains and nutrient cycling.
Kingdom Protista: The term is now less commonly used as protists are distributed among several eukaryotic lineages.
Food chains: Protists are primary producers, consumers, and decomposers in aquatic ecosystems.

Terms to define:

Carbon sink: A reservoir that absorbs and stores carbon from the atmosphere.
Global carbon cycle: The movement of carbon among Earth's atmosphere, oceans, and living organisms.
Sedimentary rock: Rock formed from the accumulation of sediment, often containing fossilized protists.
Malaria: Disease caused by Plasmodium (a protist) and transmitted by mosquitoes.
Plankton: Microscopic organisms drifting in aquatic environments, including phytoplankton (photosynthetic) and zooplankton (heterotrophic).

Section 27.2: How Do Biologists Study Protists?

Introduction

Biologists use molecular phylogeny and morphological traits to classify protists and understand their evolutionary relationships within Eukarya.

Molecular phylogeny: DNA sequence analysis and morphology are used to divide Domain Eukarya into major lineages (see Figure 27.7 and Table 27.2).
Shared traits: The nuclear envelope, mitochondria, and cytoskeleton are hypothesized to be derived from a common ancestor of Eukarya.
Comparisons: Eukaryotes are distinguished from prokaryotes (Bacteria and Archaea) by their complex cell structure.

Terms to define:

Bilateria: Animals with bilateral symmetry.
Cilium (plural: cilia): Short, hair-like structures for movement.
Flagellum (plural: flagella): Long, whip-like structures for movement.
Light microscopy: Technique for viewing cells using visible light.
Transmission electron microscopy (TEM): Technique for viewing ultrastructure of cells.
Unikonta: A major eukaryotic lineage including animals, fungi, and some protists.

Section 27.3: What Themes Occur in the Diversification of Protists?

Introduction

The diversification of protists is driven by evolutionary innovations such as the origin of the nuclear envelope, endosymbiosis, and multicellularity.

Origin of nuclear envelope: Hypothesized to provide adaptive advantages by separating transcription and translation.
Endosymbiosis theory: Proposed by Lynn Margulis, suggests mitochondria and chloroplasts originated from engulfed prokaryotes.
Multicellularity: Evolved independently in several protist lineages, allowing for specialization and increased size.
Sexual and asexual reproduction: Protists exhibit both, contributing to genetic diversity.
Life cycle patterns: Protists show diverse life cycles, some similar to those in plants, animals, and fungi.

Terms to define:

Absorptive feeder: Organism that absorbs nutrients from its environment.
Amoeboid movement: Movement by cytoplasmic streaming and pseudopodia.
Detritus: Dead organic matter.
Endosymbiosis theory: Theory explaining the origin of mitochondria and chloroplasts.
Primary endosymbiosis: Initial engulfment of a prokaryote by a eukaryote.
Secondary endosymbiosis: Engulfment of a eukaryotic cell that already contains an endosymbiont.
Multicellularity: Existence of organisms composed of multiple cells.
Sexual and asexual reproduction: Modes of reproduction involving or not involving gametes.

Scientist to know: Lynn Margulis (proposed the endosymbiosis theory).

Section 27.4: Key Lineages

Introduction

There are six major lineages in the Domain Eukarya, each defined by unique morphological characteristics (synapomorphies). Understanding these lineages is essential for classifying eukaryotic diversity.

Major lineages: Amoebozoa, Opisthokonta, Excavata, Plantae, Rhizaria, Alveolata, Stramenopila.
Synapomorphies: Each lineage has distinct traits, such as the loss of mitochondria in Excavata or the presence of alveoli in Alveolata.
Phylogenetic tree: Figure 27.7 and Table 27.2 illustrate the evolutionary relationships and shared derived traits (labeled 2-8).
Example: Excavata includes lineages that lack mitochondria, which is a derived trait.

Terms to define:

Amoeba: Unicellular organism with flexible shape and pseudopodia.
Bioluminescence: Production of light by living organisms.
Foraminifera: Marine protists with calcium carbonate shells.
Harmful algal bloom: Rapid growth of algae that can produce toxins.

Taxonomic Table: Major Eukaryotic Lineages

Purpose

This table summarizes the major lineages, representative groups, and ecological or human impacts of eukaryotes. It is used for classification and comparison.

Domain	Kingdom or Lineage	Phylum or Group	Genus or Species	Short Description	Ecological or Human Impact
Eukarya	"Protists" (multiple lineages)	Amoebozoa	Lobose amoebas	Amoeboid movement	Model organism
Eukarya	Alveolata	Ciliates	Paramecium	Surface alveoli	Primary producer
Eukarya	Alveolata	Apicomplexans	Plasmodium	Parasitic	Causes malaria
Eukarya	Opisthokonta	Animals and fungi		Multicellular	Decomposers, consumers
Eukarya	Excavata	Euglenids	Trypanosoma	Flagellated	Causes sleeping sickness
Eukarya	Plantae	Green algae		Photosynthetic	Primary producer
Eukarya	Rhizaria	Foraminifera		Calcium carbonate shell	Marine sediment formation
Eukarya	Stramenopila	Water molds	Phytophthora infestans	Filamentous	Plant pathogen
Eukarya	Stramenopila	Brown algae		Large multicellular	Habitat formation
Eukarya	Stramenopila	Diatoms		Silica shell	Primary producer

Phylogenetic Tree: Shared Traits in Domain Eukarya

Explanation

The phylogenetic tree (see Figure 27.7) shows the evolutionary relationships among major eukaryotic lineages. Shared ancestral traits (at node 1) and derived traits (synapomorphies, labeled 2-8) define each group.

Shared ancestral traits: Nuclear envelope, mitochondria, cytoskeleton.
Synapomorphies: Unique features for each lineage, such as loss of mitochondria in Excavata or presence of alveoli in Alveolata.

Key Concepts and Formulas

Endosymbiosis theory equation:

Carbon cycle equation:

Additional info:

Protists are no longer classified as a single kingdom; instead, they are distributed among several eukaryotic supergroups.
Some protists cause diseases (e.g., malaria, sleeping sickness), while others are essential for ecosystem functioning (e.g., phytoplankton).
Multicellularity and sexual reproduction evolved multiple times independently in eukaryotes.