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