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The Evolution of Multicellularity: From Unicellular to Complex Organisms

This comprehensive exploration delves into the evolution of multicellular organisms, highlighting the challenges of size and the necessity of substance exchange with the environment. It discusses diffusion, surface area-to-volume ratios, and the limits of single-cell size. The text examines three main theories of multicellularity evolution: Symbiotic, Syncytial, and Colonial Theories, featuring examples from green algae and other eukaryotes. The advantages, such as increased organismal size and cell specialization, are contrasted with the challenges of interdependence and complexity.

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The Evolution of Multicellularity: From Unicellular to Complex Organisms

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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

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