Evolution of Multicellularity

Question 1

Multicellularity across the tree of life

Answer 1

• Multicellularity is present in
  (nearly) all eukaryotic groups.
• Some organisms are multicellular
  only some of the time
• may be advantageous as it has emerged in multiple groups of organisms in the tree of life

Question 2

Multicellular prokaryotes - Clonal

Answer 2

All cells have same genetic ancestry

Question 3

Multicellular prokaryotes - Non- clonal

Answer 3

Have diff ancestry / genetic material

Question 4

Multicellular bacteria - Cyanobacterium

Answer 4

• Can photosynthesise + in a chain
• Some specialised for reproduction, digesting nitrogenous material etc

Question 5

Multicellular bacteria - Enterobacterium

Answer 5

many bacteria forming a film working together (colony/biofilm)

Question 6

Multicellular bacteria - Myxcobacterium

Answer 6

• Quite differentiated - on top is reproductive so only certain ones pass on their genes

Question 7

Multicellular eukaryotes forming fruiting bodies

Answer 7

Dictyostelium

• Can exist as free-living amoebe
• If they start to starve, it sends signals for other cells to congregate
• Forms a slug-like thing that can move
• Some cells die in the stalk and fruiting body/ spores reproduce=> Altruism: some cells die (stalk), others become the next generation (spores).

Question 8

Multicellularity - Adhesion

Answer 8

Cells stick together

Question 9

Multicellularity - Communication

Answer 9

Cells communicate with one another (in addition to
responding to external signals).

Question 10

Multicellularity - Differentiation

Answer 10

Cells within the organism have specialized functions

Question 11

Multicellularity - Dependency

Answer 11

Cells are dependent on one another for survival (perhaps not so much in colonial multicellular organisms/biofilms)

Question 12

How to Become Multicellular - Cell Adhesion

Answer 12

Cell adhesion molecules stick cells together
Done by proteins in membranes - may have lipid modifications

Question 13

How to Become Multicellular - Cell Signalling

Answer 13

Cell signalling proteins enable cells to talk to each other
- Also proteins
- May have conformational change as a response to chemical signals sent from other cells

Question 14

Genetic differences in uni- and multi- organisms

Answer 14

• Building blocks are present in unicellular relatives of multicellular organisms
• diffs in genomes of multicellular and its relatives that are uni-

=> found they have pretty much same cytoskeletons, receptors, adhesion molecules
=> similar genomes => change isn’t due to differing genes

• acc due to gene expression - how / when genes are turned on or off

Question 15

Choanoflagellates - unicellular relatives of animals

Answer 15

• Tiny water-dwelling organisms with flagella.
• Resemble collar cells (for feeding) in multicellular
  animals.
• Choanoflagellates an be unicellular or form
  colonies.

=> resemble sea sponges

Question 16

Choanoflagellates helping understanding multicellular animals

Answer 16

• Genome sequencing confirms that choanoflagellates are the closest living relatives of metazoans (the multicellular animals).
• Bacterial signals (sulfonolipids) promote multicellular development in choanoflagellates
• May have occurred due to outside signals

Question 17

Plant Multicellularity

Answer 17

• All land plants have a single common ancestor (we don’t know what).
• Algae (freshwater and marine) can be unicellular or multicellular (or both)
• Don’t know if common ancestor was uni- or multi-
• all similar to each other

Question 18

Experimental evolution of
multicellularity in Chlamydomonas
over many generations

Answer 18

• Over generations + grown in a lab
• Selected via settlement - the ones that sunk to bottom of test tube
• Becomes multicellular (stable population) in response to predation by Paramecium
  
  • If bigger, predation is less likely - harder to eat
• Abiotic stressors (e.g. salt stress)
  can also promote multicellularity

Question 19

Signals from bacteria control multicellular development in green seaweed

Answer 19

Ulva
- bacterial signal causes cell division (Ulva get signal)
- maibacter - signal allows Ulva to differentiate
=> a form of symbiosis

Question 20

When did multicellularity evolve?

Answer 20

• First multicellulrity - 2 billon years ago in fossil record
• Before complex life on land, it was in the sea

Question 21

Multicellularity may be later than initially thought

Answer 21

• New fossil find of complex
  calcified structures pushes back putative animal multicellularity to 890 million years ago
• May have been animal cells earlier

e.g. similar to certain sea sponges - e.g. getting o2 from bacteria that photosynthesises