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, insome 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