The More the Merrier?

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The More the Merrier?

• The Evolution of Multicellular Organisms

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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

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Surface area to volume ratio gets smaller as the
cell gets larger!
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Solving the SAV 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

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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) gt 7000 species
• Model Volvocine series Order Volvocales

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Chlamydomonas

• Unicellular flagellate
• Isogamy

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Gonium

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

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Pandorina

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

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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

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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

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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

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Reproduction in the Volvox
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Anisogamy
Anisogamy/ Heterogamy
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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

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Advantages of multicellularity

• Increase in size of the organism
• Permits cell specialization
• Increase in surface area to volume ratio

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Problems of multicellularity

• Interdependence
• Complexity

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Images

• http//protist.i.hosei.ac.jp/pdb/images/Chlorophyt
  a/Gonium/pectorale/sp_2b.jpg
• http//www.rbgsyd.nsw.gov.au/__data/assets/image/4
  8212/Gonium2.gif
• http//www.ac-rennes.fr/pedagogie/svt/photo/microa
  lg/pandorin.jpg
• http//protist.i.hosei.ac.jp/PDB/images/Chlorophyt
  a/Eudorina/elegans/sp_5.jpg
• http//www.fytoplankton.cz/FytoAtlas/thm/0078.jpg