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 problemAvoidanceGeometric 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