The Evolution of Multicellular Organisms The More the Merrier? The Evolution of Multicellular Organisms

The Problem of Size All animals need to exchange substances with the environment SURFACE AREA : VOLUME Bacteria – 6 000 000/m Whale – 0.06/m Maximum cell size is limited All organisms larger than size limit are MULTICELLULAR

Solving the SA:V problem Geometric solutions Increase surface area Decrease effective volume Increase rate of supply High concentration of nutrients Improve nutrient transport within Improve efficiency to reduce demand Division of labor within the cell Division 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 Volvox

Biofilms Unicellular prokaryotes Coordinate activity for own benefit Different species, same requirements Colony attaches to a surface Chemical communication to others Slime layer develops (Protein and polysaccharide) Diffusion between cells and fluid increases Cells regular pH and [food and wastes’ as a whole

Where Are Biofilms?

Chlamydomonas…or Volvox? Unicellular flagellate Individuals connected by strands of cytoplasm Some individuals take on special roles. Movement Reproduction If one cell dies, the rest survive Volvox in Motion

Gonium Small colony (4, 8,16, or 32 cells) No differentiation Intercellular communication

Pandorina Colony (8, 16, or 32 cells) in 1 layer Spherical 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

Anisogamy Anisogamy/ Heterogamy

Social Amoeba: Dictyostelium discoideum We are the scientists in the lab Looking through a microscope Those little glass slides they never lie How can this small mind cope? I've never seen anything like it before This amoeba's got a mind of its own "Amoeba" by The Adolescents

Amoeba Slugs Forming (About 2mm)

Life Cycle

All Cells In A Multicellular Organism Must, At Some Point… Adhere Communicate Move Differentiate In that order? By what mechanism?

In case you need more videos http://www.youtube.com/watch?v=OX5Yiz38fgY&feature=related http://www.youtube.com/watch?v=Ql7i_TLUurM&feature=related http://www.youtube.com/watch?v=YoXWbr45rsQ http://www.youtube.com/watch?v=4VwFMqZ5KeY http://www.youtube.com/watch?v=R_nPFJlH1Vk http://www.youtube.com/watch?v=YoXWbr45rsQ “Homo Amoeba” is a artistic and entertaining look at cell movement: http://www.youtube.com/watch?v=7jd0QEe_WHE The BioClip “Day in the Life of Social Amoeba” deserves special mention for excellent design; you may have to contact Kota Miura (via www.bioclips.com) who was kind enough to email me a link to download the clip: http://www.bioclips.com/research.php3?id_article=56

What Can We Learn From Social Amoeba? Chemotaxis: how cells move Cell-cell communication (chemical) Cell connection (membrane proteins) Behavior: Competition, altruism…and cheating Immune systems (sentinel cells) Molecular genetics & Development Evo-Devo Evolution, evolution, evolution!

Advantages of multicellularity Increase in size of the organism Permits cell specialization Increase in surface area to volume ratio

Problems of multicellularity Interdependence Complexity

Images http://protist.i.hosei.ac.jp/pdb/images/Chlorophyta/Gonium/pectorale/sp_2b.jpg http://www.rbgsyd.nsw.gov.au/__data/assets/image/48212/Gonium2.gif http://www.ac-rennes.fr/pedagogie/svt/photo/microalg/pandorin.jpg http://protist.i.hosei.ac.jp/PDB/images/Chlorophyta/Eudorina/elegans/sp_5.jpg http://www.fytoplankton.cz/FytoAtlas/thm/0078.jpg