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The More the Merrier?

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  1. 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 – 6 000 000/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

  6. Chlamydomonas • Unicellular flagellate • Isogamy

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

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

  9. 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

  10. 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

  11. 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

  12. Reproduction in the Volvox

  13. Anisogamy Anisogamy/ Heterogamy

  14. 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

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

  16. Problems of multicellularity • Interdependence • Complexity

  17. Images • http://protist.i.hosei.ac.jp/pdb/images/Chlorophyta/Gonium/pectorale/sp_2b.jpg • http://www.rbgsyd.nsw.gov.au/__data/assets/image/48212/Gonium2.gif • http://www.ac-rennes.fr/pedagogie/svt/photo/microalg/pandorin.jpg • http://protist.i.hosei.ac.jp/PDB/images/Chlorophyta/Eudorina/elegans/sp_5.jpg • http://www.fytoplankton.cz/FytoAtlas/thm/0078.jpg