Presentation 02 - The Origin Of Multicellularity And The Volvocine Series

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Presentation 02 - The Origin Of Multicellularity And The Volvocine Series

  1. 1. The More the Merrier?<br />The Evolution of Multicellular Organisms<br />
  2. 2. The problem of size<br />All animals need to exchange substances with the environment<br />Diffusion<br />Surface area<br />Difference in concentration<br />Distance<br />SURFACE AREA : VOLUME<br />Bacteria – 6 000 000/m<br />Whale – 0.06/m<br />Maximum size limit of single cell<br />All organisms larger than size limit are MULTICELLULAR<br />
  3. 3. As the cell gets larger, surface area to volume ratio gets smaller.<br />
  4. 4. Solving the SA:V problem<br /><ul><li>Avoidance
  5. 5. Geometric solutions
  6. 6. Increase surface area
  7. 7. Decrease effective volume
  8. 8. Increase rate of supply
  9. 9. High concentration of nutrients
  10. 10. Improve nutrient transport within
  11. 11. Improve efficiency to reduce demand
  12. 12. Division of labor within the cell
  13. 13. Division of labor between cells</li></li></ul><li>Evolution of multicellularity<br />Evolved many times in eukaryotes<br />Three theories<br />Symbiotic Theory<br />Like the endosymbiotic theory<br />Different species are involved<br />Syncytial Theory<br />Ciliates and slime molds<br />Commonly occur in multinucleated cells<br />Colonial Theory (Haeckel, 1874)<br />Same species are involved<br />Green algae (Chlorophyta) > 7000 species<br />Model: Volvocine series – Order Volvocales<br />
  14. 14.
  15. 15. Chlamydomonas<br />Unicellular flagellate<br />Isogamy<br />
  16. 16. Gonium<br /><ul><li>Small colony (4, 8,16, or 32 cells)
  17. 17. Flat plane, mucilage
  18. 18. No differentiation
  19. 19. Isogamy
  20. 20. Intercellular communication</li></li></ul><li>Pandorina<br />Colony (8, 16, or 32 cells) in 1 layer<br />Spherical<br />Isogamy<br />Anterior cells  larger eyespots<br />Coordinate flagellar movement<br />Colony dies when disrupted<br />
  21. 21.
  22. 22. Eudorina<br /><ul><li>16 or 32 cells
  23. 23. 16 cells – no specialization
  24. 24. 32 – 4 for motility, the rest for reproduction
  25. 25. Heterogamy – female gametes not released
  26. 26. Halves are more pronounced</li></li></ul><li>Pleodorina<br />32 to 128 cells<br />Heterogamy – female gametes not released, in some cases becoming truly non-motile<br />Division of labor<br />Anterior vegetative cells<br />Larger posterior reproductive cells<br />
  27. 27. Volvox<br />Spherical colonies (500-50000 cells)<br />Hollow sphere – coenobium<br />Cell differentiation: somatic/vegetative cells and gonidia<br />2-50 scattered in the posterior  reproductive<br />Female reproductive cells  daughter colonies<br />Intercellular communication possible<br />
  28. 28.
  29. 29. Summary of Evolutionary Changes Shown<br /><ul><li>Unicellular  colonial life
  30. 30. Increase in # of cells in colonies
  31. 31. Change in shape of colony
  32. 32. Increase in interdependence among vegetative cells
  33. 33. Increase in division of labor: vegetative and reproductive cells
  34. 34. Isogamy  anisogamy  oogamy
  35. 35. Fewer female gametes are produced</li></li></ul><li>Advantages of multicellularity<br />Increase in size of the organism<br />Permits cell specialization<br />Increase in surface area to volume ratio<br />
  36. 36. Problems of multicellularity<br />Interdependence<br />Complexity<br />

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