The document discusses the evolution of multicellular organisms from single-celled ancestors. As single cells increase in size, their surface area to volume ratio decreases, limiting nutrient exchange. Multicellularity solves this issue by allowing division of labor between specialized cells and improved transport within the organism. The evolution of multicellularity is seen in the Volvocine algae series, from unicellular Chlamydomonas to colonial Volvox, with increasing cell number, shape complexity, specialization, and reproductive strategies over generations. Multicellularity provides advantages like increased size and specialization but also new challenges of interdependence and complexity.

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

2. The problem of sizeAll animals need to exchange substances with the environmentDiffusionSurface areaDifference in concentrationDistanceSURFACE AREA : VOLUMEBacteria – 6 000 000/mWhale – 0.06/mMaximum size limit of single cellAll organisms larger than size limit are MULTICELLULAR

3. As the cell gets larger, surface area to volume ratio gets smaller.

4. Solving the SA:V problemAvoidance

5. Geometric solutions

6. Increase surface area

7. Decrease effective volume

8. Increase rate of supply

9. High concentration of nutrients

10. Improve nutrient transport within

11. Improve efficiency to reduce demand

12. Division of labor within the cell

13. Division of labor between cellsEvolution of multicellularityEvolved many times in eukaryotesThree theoriesSymbiotic TheoryLike the endosymbiotic theoryDifferent species are involvedSyncytial TheoryCiliates and slime moldsCommonly occur in multinucleated cellsColonial Theory (Haeckel, 1874)Same species are involvedGreen algae (Chlorophyta) > 7000 speciesModel: Volvocine series – Order Volvocales

15. ChlamydomonasUnicellular flagellateIsogamy

16. GoniumSmall colony (4, 8,16, or 32 cells)

17. Flat plane, mucilage

18. No differentiation

19. Isogamy

20. Intercellular communicationPandorinaColony (8, 16, or 32 cells) in 1 layerSphericalIsogamyAnterior cells  larger eyespotsCoordinate flagellar movementColony dies when disrupted

22. Eudorina16 or 32 cells

23. 16 cells – no specialization

24. 32 – 4 for motility, the rest for reproduction

25. Heterogamy – female gametes not released

26. Halves are more pronouncedPleodorina32 to 128 cellsHeterogamy – female gametes not released, in some cases becoming truly non-motileDivision of laborAnterior vegetative cellsLarger posterior reproductive cells

27. VolvoxSpherical colonies (500-50000 cells)Hollow sphere – coenobiumCell differentiation: somatic/vegetative cells and gonidia2-50 scattered in the posterior  reproductiveFemale reproductive cells  daughter coloniesIntercellular communication possible

29. Summary of Evolutionary Changes ShownUnicellular  colonial life

30. Increase in # of cells in colonies

31. Change in shape of colony

32. Increase in interdependence among vegetative cells

33. Increase in division of labor: vegetative and reproductive cells

34. Isogamy  anisogamy  oogamy

35. Fewer female gametes are producedAdvantages of multicellularityIncrease in size of the organismPermits cell specializationIncrease in surface area to volume ratio

36. Problems of multicellularityInterdependenceComplexity