Lecture Notes on Multicellularity

Multicellularity

• Multicellularity: Organisms containing more than one cell.
• Unicellular organisms: Composed of a single cell that performs all functions.
• Multicellular organisms: Composed of many specialized cells working together.
• Challenge: Coordinating cells to communicate and function together for survival.
• Multicellularity arose relatively recently in evolution.

Evolutionary Timeline

• Earth formed: Unicellular prokaryotes (bacteria and archaea) originated.
• Bacteria and Archaea: Prokaryotes without a nucleus or membrane-bound organelles.
• Archaea: Ancient organisms, some are extremophiles (surviving in extreme conditions).
• Eukaryotic cell: Developed from bacteria and archaea, with a nucleus and organelles, initially unicellular.
• Multicellular organisms: Arose much later, initially simple, then complex animals.
• Phylogenetic Tree: Bacteria and archaea (prokaryotes) gave rise to eukaryotes, with multicellularity arising later in specific lineages (animals, fungi, plants).

Advantages of Multicellularity

• Multicellularity arose independently multiple times.
• Animals are more closely related to choanoflagellates (single-celled) than to plants or fungi.
• Independent evolution suggests a selective advantage.

Unicellular vs. Multicellular Organisms:

• Unicellular:
  • Simple body organization.
  • Single cell performs all life processes.
  • Entire surface exposed to the environment.
  • Limited capacity for repair; short lifespan.
• Multicellular:
  • Complex organization.
  • Division of labor: cells specialized for specific functions.
  • Some cells provide protection.
  • Capacity to replace damaged cells; longer lifespan.
  • Potential for more complex behaviors.

Simple vs. Complex Multicellularity

Simple Multicellularity:

• Cells come together to form a colony.
• Limited coordination and differentiation.
• Individual cells may have different genomes.
• Reversible: cells can disperse and survive independently.
  *Example: Choanoflagellates

Complex (Obligate) Multicellularity:

• Distinct, differentiated cell types.
• Regulated developmental process from a fertilized egg (zygote).
• Multicellularity required for survival; irreversible.
  *Example: Animals

Benefits of Simple Multicellularity

• Enhanced prey capture.
• Protection from predation (colony formation).
• Survival strategy under limited nutrients (cannibalism).
• Limitation: Non-identical cells can limit communication and lead to "cheater" cells.
• Dictyostelium example: Fruiting body formation with stalk cells dying while spore sac cells benefit, thus the cells that end up in the spore sac are taking advantage of the altruism of the stalk cells.

Complex Multicellularity

• Found in animals, land plants, brown algae, fungi, and red algae.
• Develops from a single cell (clonal multicellularity).
• Genetically identical cells are crucial for complex body patterns.
• Cell-cell communication depends on identical signaling components.
• Complex multicellularity has led to species diversification. Many species are complex multicellular animals
  *Simple multicellular organisms have never evolved complex forms (e.g., different body structures and behaviors of different species)
• LEGO Analogy:
  • One LEGO brick: Unicellular organism (limited possibilities).
  • Box of identical LEGO bricks: Simple multicellularity (clusters, limited shapes).
  • Box of diverse LEGO bricks: Complex multicellularity (various cell types, many species).

Mechanisms Leading to Multicellularity

Aggregate Multicellularity:

• Separate cells come together to form a cluster.
• Evolved multiple times in eukaryotes.
• Occurs in response to adverse conditions.
• Cells are quiescent with limited motility.
• Reversible; cells can disperse and divide independently.

Clonal Multicellularity:

• Serial cell division without dispersion of sister cells.
• Starts with a zygote (fertilized egg).
• Developmental mechanisms lead to a fully formed organism.
• Changes in the cell cycle and extracellular matrix are involved.
• Phylogenetically more widespread and morphologically diverse.
• Underlies complex multicellularity.

Multicellularity in Animals:

• Starts with a fertilized egg (zygote).
• Zygote divides to form clones of cells (blastocyst).
• Cells undergo differentiation to form various cell types.
• Efficient communication leads to complex morphologies and behaviors.

Origin of Obligate Multicellular Life

• Unicellular life transitioned to simple multicellular organisms.
• Changes in the cell cycle & extracellular matrix caused clustering.
• Cells became specialized, leading to division of labor.
• Specialized cells became dependent on each other (mutual dependence).
• Loss of ability to switch back to being unicellular.
• Further specialization led to organs and more complex functions.

Requirements for Embryonic Development

• Occurs through clonal expansion.
• Regulated cell cycle.
• Regulated programs of selectivity.
• Cell differentiation.
• Signal transduction.
• Cell motility and adhesion.
• Ancestral single-celled organisms had the basics.
• Clonal multicellularity in animals was marked by diversification of transcription factor families and signaling molecules.

Arising of Multicellularity in Animals

• Phylogenetic tree shows relationships through evolution.
• Choanoflagellates: Unicellular organisms, closest living relatives.
• Sponges: Animals with spicules (support) and choanocytes (similar to choanoflagellates).
• Choanocytes capture nutrients; division of labor.
• Supports the idea that animals evolved from choanoflagellate-like organisms.

Model Study System: Volvocales

• Line of aquatic green algae with varying complexity.
• Can exist as single-celled or multicellular forms.
• Volvox has division of labor with cells specialized for reproduction.
• Changes in cytokinesis & extracellular matrix resulted in cell attachment changes and drove multicellularity.

Multicellularity and Cancer

• Multicellular organisms require cooperation and altruism among cells.
• Cancer: Loss of cooperation; cells act independently, like unicellular organisms.
• Hallmarks of cancer relate to traits of multicellularity like proliferation control, cell recognition, and cell adhesion.
• Cancer cells revert to a unicellularity phenotype, reactivating ancestral traits for selfish behaviors.