THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
17. Protists: Advent of the Eukaryotes
17.8. The Road to Animals
Just as plants (and undoubtedly fungi) had protist ancestors, so too did animals, in this case an unusual single-celled het- erotroph called a choanoflagellate.
Choanoflagellida Are the Direct Ancestors of Animals
Choanoflagellates are unicellular, heterotrophic protists with a single, long flagellum. The flagellum is surrounded by a funnel-shaped contractible collar of closely placed filaments, not unlike the woven strands of a basket. The whipping motion of the flagellum draws water into the funnel, forcing it past the filaments, which strain bacteria from the water. The choanoflagellate protist feeds on these bacteria. Precisely this same type of filtering cell is found in sponges, the most primitive of animals (discussed in chapter 19).
While no choanoflagellate is multicellular, some are colonial, forming spherical assemblies (figure 17.19) that look very much like freshwater sponges. This close relationship between choanoflagellates and animals is also seen when molecular comparisons are made. A cell surface receptor used to initiate an intercellular signal pathway is the same in both choanoflagellates and sponges. Detailed genomic comparisons are not yet available, but can be expected to further confirm this relationship.
Figure 17.19. Colonial choanoflagellates.
Colonial choanoflagellates resemble their close animal relatives, the sponges.
Key Learning Outcome 17.8. Choanoflagellates have a unique cell structure also found in sponges, and are believed to be the direct ancestors of animals.