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Multicellularity and the Volvocine Series

Explore the limitations of cell size, solutions to the size problem, evolution towards multicellularity, Volvocine series' modes of reproduction, and the advantages and disadvantages of multicellularity. Learn about key theories and examples from the natural world.

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Multicellularity and the Volvocine Series

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  1. Multicellularity and the Volvocine Series

  2. Limitation of Size • Size poses a lot of problems • Exchange of substances • Diffusion (surface area, concentration gradient and distance) • SA to Volume ratio • Maximum size limit of cell • Larger than size limit = multicellular

  3. SA to Volume Ratio • Inc in cell size leads to decrease in SA: V • Life processes are affected

  4. Solutions to the SA:V Problem • Avoidance • Geometric solutions • Inc SA; Dec V • Increase rate of supply • High conc. of nutrients; efficient transport • Improved efficiency to reduced demand • Specialization of cells (intercellularly and intracellularly)

  5. Movement towards Multicellularity • Occurred many times in eukaryotes • Theories: • Symbiotic Theory (e.g. endosymbiotic theory) • Syncitial Theory (slime molds and ciliates) • Colonial Theory (Haeckel, 1874) • Green algae (Chlorophyta)>7000 species • The Volvocine series

  6. Modes of reproduction • Isogamy- involves reproduction of morphologically the same gametes (+or -) • Anisogamy- (heterogamy) gametes are of different size; sperm is smaller than the egg • Oogamy-(specialized heterogamy) egg is non-motile, sperm is flagellated

  7. Chlamydomonas • Unicellular flagellate • Isogamy

  8. Gonium • Small colony • 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

  10. 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

  11. 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

  12. 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

  13. Evidence of Evolution • 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

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

  15. Disadvantages of multicellularity • Interdependence • Complexity

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