[bio] 02 - Origin Of Multicellularity (calsado)

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

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