1. The Evolution of Multicellular Organisms
The More the Merrier?
The Evolution of Multicellular Organisms

2. The Problem of Size
All animals need to exchange substances with the environment
SURFACE AREA : VOLUME
Bacteria – /m
Whale – 0.06/m
Maximum cell size is limited
All organisms larger than size limit are MULTICELLULAR

3. Solving the SA:V problem
Geometric solutions
Increase surface area
Decrease effective volume
Increase rate of supply
High concentration of nutrients
Improve nutrient transport within
Improve efficiency to reduce demand
Division of labor within the cell
Division of labor between cells

4. Evolution of multicellularity
Evolved many times in eukaryotes
Three theories
Symbiotic Theory
Like the endosymbiotic theory
Different species are involved
Syncytial Theory
Ciliates and slime molds
Commonly occur in multinucleated cells
Colonial Theory (Haeckel, 1874)
Same species are involved
Green algae (Chlorophyta) > 7000 species
Volvox

5. Biofilms Unicellular prokaryotes Coordinate activity for own benefit
Different species, same requirements
Colony attaches to a surface
Chemical communication to others
Slime layer develops (Protein and polysaccharide)
Diffusion between cells and fluid increases
Cells regular pH and [food and wastes’ as a whole

6. Where Are Biofilms?

8. Chlamydomonas…or Volvox?
Unicellular flagellate
Individuals connected by strands of cytoplasm
Some individuals take on special roles.
Movement
Reproduction
If one cell dies, the rest survive
Volvox in Motion

9. Gonium Small colony (4, 8,16, or 32 cells) No differentiation
Intercellular communication

10. Pandorina Colony (8, 16, or 32 cells) in 1 layer Spherical
Anterior cells  larger eyespots
Coordinate flagellar movement
Colony dies when disrupted

12. Eudorina 16 or 32 cells 16 cells – no specialization
32 – 4 for motility, the rest for reproduction
Heterogamy – female gametes not released
Halves are more pronounced

13. Pleodorina
32 to 128 cells
Heterogamy – female gametes not released, in some cases becoming truly non-motile
Division of labor
Anterior vegetative cells
Larger posterior reproductive cells

14. Volvox Spherical colonies (500-50000 cells) Hollow sphere – coenobium
Cell differentiation: somatic/vegetative cells and gonidia
2-50 scattered in the posterior  reproductive
Female reproductive cells  daughter colonies
Intercellular communication possible

15. Anisogamy
Anisogamy/
Heterogamy

18. Social Amoeba: Dictyostelium discoideum
We are the scientists in the lab Looking through a microscope Those little glass slides they never lie How can this small mind cope? I've never seen anything like it before This amoeba's got a mind of its own "Amoeba" by The Adolescents

19. Amoeba Slugs Forming (About 2mm)

20. Life Cycle

21. All Cells In A Multicellular Organism Must, At Some Point…
Adhere
Communicate
Move
Differentiate
In that order? By what mechanism?

22. In case you need more videos
“Homo Amoeba” is a artistic and entertaining look at cell movement:
The BioClip “Day in the Life of Social Amoeba” deserves special mention for excellent design; you may have to contact Kota Miura (via who was kind enough to me a link to download the clip:

23. What Can We Learn From Social Amoeba?
Chemotaxis: how cells move
Cell-cell communication (chemical)
Cell connection (membrane proteins)
Behavior: Competition, altruism…and cheating
Immune systems (sentinel cells)
Molecular genetics & Development
Evo-Devo
Evolution, evolution, evolution!

24. Advantages of multicellularity
Increase in size of the organism
Permits cell specialization
Increase in surface area to volume ratio

25. Problems of multicellularity
Interdependence
Complexity

26. Images