Diversification of Eukaryotes: Protists and the Tree of Life

Chapter 27: Diversification of Eukaryotes (Protists)

Introduction

This chapter explores the diversity, classification, and evolutionary significance of protists within the domain Eukarya. It covers the medical and ecological importance of protists, their evolutionary relationships, and the key features that distinguish major eukaryotic lineages.

Classification of Life: The Three Domains

The Three Domains of Life

• Domain Bacteria: Prokaryotic, unicellular organisms lacking a membrane-bound nucleus. Found in nearly all environments.

• Domain Archaea: Prokaryotic, unicellular organisms also lacking a membrane-bound nucleus. Many are extremophiles and may resemble the earliest forms of life on Earth.

• Domain Eukarya: Eukaryotic organisms, which may be unicellular or multicellular, and possess a membrane-bound nucleus.

Domains and Kingdoms of Life

Organisms are further classified into kingdoms within each domain. The domain Eukarya includes several kingdoms, such as Protists, Plants, Animals, and Fungi.

Domain	Kingdom/Group	Key Features	Example
Archaea	Archaea	Capable of living in extreme environments; methane-generating prokaryotes	Methanosarcina mazei
Bacteria	Bacteria	Structurally simple but metabolically diverse prokaryotes	Escherichia coli
Eukarya	Protists	Diverse group of eukaryotes, many single-celled, with both plant and animal-like characteristics	Euglena
Eukarya	Plants	Multicellular photosynthesizers	Pinus longaeva (bristlecone pine)
Eukarya	Animals	Multicellular organisms that ingest food	Homo sapiens
Eukarya	Fungi	Multicellular or unicellular organisms that absorb nutrients	Amanita muscaria

Organizing the Diversity of Life

Taxonomy and Systematics

• Taxonomy: The discipline of naming and classifying organisms according to specific rules.

• Systematics: The classification of organisms based on presumed evolutionary relationships.

Categories of classification, from most to least inclusive:

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Example: Homo sapiens (humans) are classified as follows: Domain Eukarya, Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species sapiens.

Protists: An Overview

What Are Protists?

• Protists are eukaryotic organisms that are not classified as animals, plants, or fungi.

• They can be unicellular or multicellular, and exhibit a wide range of forms and lifestyles.

• Most protists are free-living, but some are parasitic.

• Reproduction can be asexual or sexual, with some species exhibiting complex life cycles.

Importance of Protists

• Medical Importance: Some protists cause diseases in humans and crops (e.g., Plasmodium causes malaria; Phytophthora infestans caused the Irish potato famine).

• Ecological Importance: Protists are primary producers in aquatic environments, forming the base of many food chains and playing a key role in the global carbon cycle.

Key Features of Eukaryotes and Protists

Distinguishing Features of Eukaryotes

• Large cell size compared to prokaryotes

• Presence of membrane-bound organelles (e.g., nucleus, mitochondria)

• Extensive cytoskeleton

• Multicellularity evolved multiple times

• Both asexual and sexual reproduction

Protist Diversity

• Protists are a paraphyletic group: they include some, but not all, descendants of a common ancestor.

• They do not share unique derived characteristics (synapomorphies) that set them apart from all other eukaryotes.

• Protists vary greatly in size, habitat, morphology, and mode of nutrition.

Medical and Ecological Impacts of Protists

Human Health

• Malaria: Caused by Plasmodium species, transmitted by mosquitoes. Difficult to control due to rapid evolution of drug resistance and insecticide resistance.

• Crop Diseases: Phytophthora infestans (potato blight) led to the Irish potato famine.

• Harmful Algal Blooms: Caused by toxin-producing protists (e.g., dinoflagellates). Toxins can accumulate in shellfish and poison humans.

Ecological Roles

• Primary Producers: Photosynthetic protists (e.g., diatoms, algae) fix carbon dioxide and produce organic compounds, supporting aquatic food webs.

• Carbon Cycle: Protists contribute to the movement of carbon through photosynthesis, decomposition, and as carbon sinks (e.g., sedimentary rocks, petroleum).

Evolutionary Relationships and Phylogeny

Phylogenetic Analysis

• Direct sequencing of environmental DNA helps identify new protist lineages and place them on the tree of life.

• Protists are not a monophyletic group; they are paraphyletic, lacking unique synapomorphies.

Cellular and Morphological Innovations in Protists

Endosymbiosis and the Origin of Mitochondria

• Endosymbiotic Theory: Mitochondria originated when a bacterial cell was engulfed by a eukaryotic ancestor and became a symbiotic organelle.

• Evidence:

  • Mitochondria replicate by fission, have their own circular DNA, and possess double membranes.

  • Mitochondrial genes are more similar to those of alpha-proteobacteria than to nuclear genes of eukaryotes.

Structures for Support and Protection

• Protists may have cell walls, external shells, or internal rigid structures for support and protection.

• These features have evolved multiple times and contribute to the diversity of protist forms.

Multicellularity

• Multicellularity evolved independently in several eukaryotic lineages.

• Allows for cell specialization and greater organismal complexity.

Nutrition and Movement in Protists

Modes of Nutrition

• Ingestive Feeding: Engulfing food particles or other organisms (phagocytosis).

• Absorptive Feeding: Absorbing nutrients directly from the environment; includes decomposers and parasites.

• Photosynthesis: Autotrophic protists use light energy to produce organic compounds from carbon dioxide.

Movement

• Amoeboid Motion: Sliding movement using pseudopodia (requires ATP).

• Flagella and Cilia: Swimming using long flagella or short, numerous cilia. Both structures have similar internal architecture.

Reproduction in Protists

Sexual and Asexual Reproduction

• Asexual Reproduction: Based on mitosis; produces genetically identical offspring.

• Sexual Reproduction: Involves meiosis and fusion of gametes; increases genetic diversity and may help populations adapt to changing environments or resist disease.

Major Lineages of Protists

Key Lineages and Their Features

• Amoebozoa: Lack cell walls, move via amoeboid motion, produce large pseudopodia, abundant in freshwater and soil, some are parasites.

• Excavata: Some lack mitochondria (though ancestors had them), diverse feeding strategies, some are parasites.

• Plantae: Monophyletic group including red and green algae, primary endosymbiosis with cyanobacteria.

• Rhizaria: Single-celled, often with elaborate shells, move by amoeboid motion with slender pseudopodia.

• Alveolata: Unicellular, diverse morphology, some species are bioluminescent, move by cilia or flagella.

• Stramenopila: Includes both unicellular and multicellular forms, characterized by flagella with hair-like projections.

Additional info: The classification and evolutionary relationships of protists are continually revised as new molecular and morphological data become available.