Exercise 1 Examining the Origin of Multicellularity

• the first ancestral metazoan developed from a multinucleate protist that eventually formed boundaries around each nucleus leading to multicellularity


  syncytial ciliate hypothesis

  
• This suggests that animals evolved from a colony of single-celled flagellates. The cells in this colony became specialized for distinct roles and developed mutual dependence. Eventually, the individuality of cells is lost and the whole colony developed into a single multicellular organism. It is the most accepted mechanism of animal origin among biologists.


  colonial flagellate hypothesis
• What hypothesis?

  A) Colonial Flagellate Hypothesis

  
• What hypothesis?

  A) Syncytial ciliate hypothesis

  
• • definitive evidence of the origin of animals
  • single-celled protozoans that possess an apical flagellum surrounded by a collar of microvilli forming the collar complex.


  choanoflagellates
• The morphological resemblance between choanoflagellates and choanocytes of sponges inspired the hypothesis of a close relationship between choanoflagellates and animals.
• Aside from sponges, the collar complex structure is observed in the collar cells of what animals?

  some Cnidarians and some cells in animal groups such as the sensory

  epidermal cells and nephridial cells
• What resembles the single-celled flagellated protozoan?

  The sperm cells of animals, except nematodes.
• The oval and flattened body below is a multinucleated protozoan

  that parasitizes the rectum of vertebrates, especially frogs and toads.

  A) Opalina

  B) pellicle

  C) cilia

  D) ectoplasm

  E) endoplasm

  
• What origin of multicellularity can we infer from the presence of multiple nuclei evolved in Opalina?

  Syncytial Hypothesis
• What origin of multicellularity can we infer from the organization of colonies in Volvox?

  Colonial Hypothesis
• Requirements for multicellularity exhibited in Volvox:

  Cell Adhesion: extracellular matrix

  Cell-to-Cell Communication: cytoplasmic strands (exchange of information)

  Division of Labor: somatic cells (motility) and gonidia (growth and reproduction)

  Acquisition of Individuality for a Cell Aggregate: each somatic cells are equipped with 2 flagella; a cohesive unit of cells forming the colony
• Parts of Volvox

  A) somatic

  B) extracellular matrix

  C) gonidia

  D) flagella

  E) cytoplasmic strands

  
• Choanoflagellates are aquatic protists characterized by the collar of fingerlike-projections called microvilli, which surround a single flagellum. Under light microscopy, the collar appears as a single transparent unit as the microvilli are so tightly packed. The most prominent structure is the ovoid cell body.

  
• Choanocytes are cells in the sponge tissue that line the spongocoel, the main cavity of the sponge. The cells consist of an ovoid cell body and a single flagellum surrounded by microvilli.