Endosymbiosis and the origins and diversification of complex multicellular life

Learning Objectives:

•  Describe the basic evolutionary relationships between Bacteria, Archaea, Eukaryotes

• Describe how prokaryotic cells exchange genetic information

• Explain the theory of endosymbiosis and the evidence that supports it

• Identify the key adaptations of complex multicellular life forms (cell adhesion, cell communication, bulk transport systems)

Part I: Protists and the Origins of Mitochondria

Recall: All life involves two categories of cells

• Single celled

• Cell size: small (1-10 microns)

•  No nucleus

•  Usually no organelles

•  Diverse metabolism

•  Single strand of circular DNA

•  Cell division by binary fission

•  Single or multicellular

•  Cell size: large (10-100 microns)

•  Envelope bound nucleus

•  Membrane-bound organelles

• Metabolism usually aerobic

•  DNA as chromosome pairs

• Cell division by mitosis and meiosis

Eukaryotes arose fromwithin Archaea

It may seem strange, but scientists now believe that eukaryotes are a specialized

group of Archaea which developed new features (a nucleus, mitochondria, chloroplasts) that allowed them to thrive

Bacteria arose ~3.5 billion years ago, and eukaryotes ~2.1 billion years ago.

Why did prokaryotes remain single-celled organisms, while eukaryotes diversified in size and complexity?

Recall: Eukaryotic cells have internal membrane-bound organelles

Specialization is the main advantage of having subcellular compartments.

• Different compartments with different conditions

• The presence of a nucleus allowstranscription and translation to be separated

Eukaryotes:

Synapomorphies shared by all eukaryotes:

• Nucleus and endomembrane system

• Mitochondria (or mitochondrial genes)

Eukaryotes include the lineages of fungi, animals, and land plants.

All other eukaryotes are collectively referred to as Protists.

Protists are a paraphyletic group

•  Broad size range

• Amazing diversity

• An incredible innovation arose in protists that led to the evolution of species that live in a wide array of habitats

The origins of eukaryotic cells

• Most subgroups of the major lineages of eukaryotes are unicellular, as

are all bacteria and archaea

•  With this in mind, biologists believe the first eukaryote was also a single-

celled organism that had:

•  Mitochondria

•  A nucleus and endomembrane system

• A cytoskeleton

• So where did the complexity of eukaryotes originate?

Mitochondria!

•  There are many features that distinguish prokaryotic cells from eukaryotic cells, the mitochondria being one of the most significant

•  Recall: mitochondria are membrane-bound organelles where aerobic

• respiration occurs Phospholipid bilayer double membrane

•  They have their own (circular) DNA (mtDNA) and are self-replicating

So how did early, simple life forms acquire mitochondria in the first place?

The theory of endosymbiosis

•  Larger cells (archaea) engulfed bacteria (phagocytosis)

•  Host cell established symbiotic relationship with the bacterium

•  The bacterium that was ingested was capable of aerobic respiration

Evidence for endosymbiosis and the origin of the mitochondrion

• Similar size to α-proteobacteria

•  Replicate by fission

•  Have their own ribosomes and manufacture there own proteins

• Have double membranes

•  Circular genomes, organized much like a bacterial chromosome

•  Mitochondrial gene sequences are more closely related to sequences from α-proteobacteria than to sequences from nuclear DNA of eukaryotes

Structural evidence: size, surrounded by two membranes, circular DNA genome, ribosomes

Reproduction: binary fission

Mitochondria contain proteins encoded by both the nuclear and mitochondrial genomes

•  Mitochondria contain about 1500 proteins, BUT <50 of these proteins are

encoded by genes found on the mitochondrial chromosome.

• The rest are encoded by nuclear genes

Horizontal gene transfer is relatively common in prokaryotes

HGT Can Occur in three ways:

•  Transformation – uptake of DNA from the environment

•  Transduction – transfer of DNA from one cell to another by bacteriophage (viruses)

•  Conjugation – transfer of DNA directly from one bacterial cell to another cell

Genetic diversity is promoted by horizontal gene transfer

→ Cell to cell transfer of genetic information within and between species

In this way you can have transfer of genetic information in the absence of sexual reproduction

Part II: The Origins of Chloroplasts and the Nuclear Envelope

Where did chloroplasts come from?

•  An extension of endosymbiosis theory contends that the eukaryotic chloroplast originated when a protist engulfed a cyanobacterium

•  Cyanobacteria changed the world! They can photosynthesize: harvest energy from sunlight to produce glucose

•  Aquatic

•  Among the oldest fossils (3.5 bya)

• One of the largest groups of bacteria

Cyanobacteria evolved specialized photosynthetic structures called thylakoids

6CO 2 + 6H 2O + light energy → C 6H12O 6 + 6O 2 

In this way, they filled the world with oxygen!

Photosynthetic cyanobacteria changed Earth’s atmosphere

Photosynthesis and respiration lie at the heart of eukaryotic life

Evidence for endosymbiosis and the origin of chloroplasts

•  Cyanobacteria-like characteristics

•  Circular DNA with genes similar to those in cyanobacteria

• In mosses, chloroplasts are surrounded by a layer of peptidoglycan (which also makes up the cell wall of bacteria).

•  There are examples of extant (currently existing) endosymbiotic cyanobacteria living in cells of certain protists and animals.

Hypothetical Timeline:

•  Aerobic respiration is a very efficient pathway for producing lots of ATP

(energy) in the presence of O 2

•  Aerobic respiration conferred a big selective advantage – this resulted in selection for aerobic organisms

Origin of the nuclear envelope

•  Leading hypothesis: the nuclear envelope arose due to infoldings of the plasma

membrane

•  Why was this development significant? It helped separate transcription and translation.

Part III: The Origins of Multicellularity

The origins of multicellularity

•  Mutations that led to multicellularity probably first caused cells to simply stick together after cell division

•  Selection pressures

•  Eventually cells became specialized for different functions

•  KEY POINT OF MULTICELLULARITY: not all cells express the same genes

Protists are unicellular or multicellular

•  Most currently described groups of protists contain only unicellular organisms

•  Multiple groups of protists exhibit at least some cases of simple multicellularity. Two groups (red algae and brown algae) exhibit complex multicellularity.

What does “simple multicellularity” mean?

In organisms that have a simple multicellular structure:

•  Usually cells come together by aggregation, mitosis

•  Cells are joined together minimally by adhesion molecules

•  Limited communication or transfer of resources between cells

•  Little or no differentiation of specialized cell types; most of the cells retain a full range of functions (including reproduction)

•  Every cell is in contact with the external environment

→ Overall, cells show little differentiation and remain in close contact with their environment

Why would single cells aggregate?

Cellular slime molds are single-celled protists (Amoebozoans) that can form multicellular aggregates.

Complex multicellularity

Complex multicellularity is only found in six clades of eukaryotes: animals,

land plants, red algae, brown algae, and two groups of fungi (Ascomycetes

and Basidiomycetes).

 →Overall, cells form unique tissues with unique functions

 →Complex multicellularity is exclusive to eukaryotes, perhaps thanks to cytoskeleton

Common characteristics of most complex multicellular organisms

•  Clonal development (mitosis)

•  Sophisticated mechanisms for cell adhesion

•  Specialized pathways for cell communication

•  Tissue and organ differentiation

• Only a small subset of cells contribute to reproduction

•  Cell or tissue loss can be lethal for the organism

•  Only some cells directly contact the environment

The evolution of complex multicellularity

Complex multicellularity evolved several times!

Part IV: General Requirements for Multicellular Life

General requirements for multicellular life

Cells must:

1. Stick together

2. Communicate/interact with one another (and by extension, share

molecules/nutrients with one another)

3. Have a genetic program to guide growth and development

#1: How do eukaryotic cells stick together?

Choanoflagellates have genes for adhesion proteins and can be stimulated

to form multicellular structures

In higher animals, cell-cell adhesion becomes even more complex...

Humans have >1000 genes that code for cell adhesion proteins

#2: Multicellularity depended upon the evolution of bulk transport mechanisms

• Diffusion: Movement of molecules from areas of high to low concentration acting over

small distances

• Bulk transport: The means by which molecules move through organisms at rates beyond those possible by diffusion across a concentration gradient

Bulk transportation in complex multicellular organisms circumvents the constraints of size imposed by diffusion 

Cell communication occurs via molecular signals between cells

In plant: plasmodesmata connect cells

In animals: gap junctions connect cells

What can these molecular signals do in us?

Oxygen was necessary for complex multicellularity and larger body size to evolve

#3: A genetic program guides development

Hox genes & Hox proteins