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Multicellularity and the Volvocine Series
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
SA to Volume Ratio
• Inc in cell size leads to decrease in SA: V– Life processes are affected
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)
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
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
Chlamydomonas
• Unicellular flagellate
• Isogamy
Gonium
• Small colony
• 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
V. carteri
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
Advantages of Multicellularity
• Increase in size of the organism
• Permits cell specialization• Increase in surface area to
volume ratio
Disadvantages of multicellularity
• Interdependence
• Complexity