presentation 02 - the origin of multicellularity and the volvocine series
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TRANSCRIPT
The More the
Merrier?The Evolution of
Multicellular Organisms
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
As the cell gets
larger, surface area to volume ratio gets smaller.
Solving the SA:V problem
AvoidanceGeometric solutions
Increase surface areaDecrease effective volume
Increase rate of supplyHigh concentration of nutrientsImprove nutrient transport
withinImprove efficiency to reduce
demandDivision of labor within the cellDivision of labor between cells
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
Chlamydomonas
Unicellular flagellate
Isogamy
Gonium
Small colony (4, 8,16, or 32 cells) Flat plane, mucilage No differentiation Isogamy Intercellular communication
Pandorina Colony (8, 16, or
32 cells) in 1 layer Spherical Isogamy Anterior cells
larger eyespots Coordinate
flagellar movement
Colony dies when disrupted
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
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
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
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
Advantages of multicellularity
Increase in size of the organism
Permits cell specialization
Increase in surface area to volume ratio
Problems of multicellularity
Interdependence Complexity