SUMMARY: The closest relative to multicellular animals are single-celled organisms called choanoflagellates. These protists look like collar cells. These are cells that have a microvillar ring (the collar) around a flagellum. An example of these cells in animals are sensory epidermal cells. To get a better look at differences in single versus multicellular choanocytes, scientists reconstructed the cell shape and cell organelle distribution in single-celled choanoflagellates and multicellular sponge choanocytes (these are basically the collar cells that make up the inner lining of a sponge). The results showed that multicellular choanocytes had less mitochondria, increased amounts of the endoplasmic reticulum, different types of Golgi-associated vesicles, and a more amoeba-like shape. Scientists hypothesized that these differences could be due to the fact that the multicellular choanocytes don’t swim, but tumble, thus needing less energy. The increase in ER size and the amoeba shape could be due to communication needs within the multicellular organisms, as well as a way to sense chemicals in the surrounding environment.
LESSON COMMENTS: This article can be introduced in a biology or advanced biology class during a unit on cell organelles and cellular respiration. The introduction and results contain pictures, diagrams, and graphs that show the differences between single-celled and multicellular choanocytes. Teachers can ask guiding questions to allow students to make the connection between mitochondria and the ER (the close relationship between these organelles is discussed in the discussion section). This article can also be used in middle school to introduce students to the difference between single and multicellular organisms. Once again, the graphs and diagrams are an easy way to present this study to younger students. Other topics that this article covers include: evolution, adaptation, taxonomy, classification, protein synthesis, cell transport, and cell communication/chemotaxis.
Laundon, D., Larson, B. T., McDonald, K., King, N., & Burkhardt, P. (2019). The architecture of cell differentiation in choanoflagellates and sponge choanocytes. PLoS biology, 17(4), e3000226. doi:10.1371/journal.pbio.3000226