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
Diffusion
Surface area
Difference in concentration
Distance
SURFACE AREA : VOLUME
Bacteria – /m
Whale – 0.06/m
Maximum size limit of single cell
All organisms larger than size limit are MULTICELLULAR

3. Surface area to volume ratio gets smaller as the cell gets larger!

4. Solving the SA:V problem
Avoidance
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

5. 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
Model: Volvocine series – Order Volvocales

7. Chlamydomonas
Unicellular flagellate
Isogamy

8. Gonium Small colony (4, 8,16, or 32 cells) Flat plane, mucilage
No differentiation
Isogamy
Intercellular communication

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

11. 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

12. 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

13. 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. Reproduction in the Volvox

16. Anisogamy
Anisogamy/
Heterogamy

17. Summary of Evolutionary Changes Shown
Unicellular  colonial life
Increase in # of cells in colonies
Change in shape of colony
Increase in interdependence among vegetative cells
Increase in division of labor: vegetative and reproductive cells
Isogamy  anisogamy  oogamy
Fewer female gametes are produced

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

19. Problems of multicellularity
Interdependence
Complexity

20. Images