Multicellularity and Choanoflagellates

Multicellularity

• Multicellularity is defined as an organism consisting of more than one cell type.
• True multicellularity evolved independently at least ten times: in animals (metazoa), six times in plants, and three times in fungi.
• All multicellular organisms evolved from unicellular ancestors.

Choanoflagellates

• Choanoflagellates are single-celled organisms closely related to animals (metazoa).
• They feature a collar region and a flagellum.
• Choanoflagellates are heterotrophic, consuming bacteria.
• The collar traps bacteria, and the flagellum propels the organism.

Choanoflagellates vs. Sponges

• Both choanoflagellates and sponges have choanocytes.
• Choanoflagellates are single-celled, while sponges are multicellular.
• Choanoflagellates are the sister lineage to all animals.
• Choanocytes in sponges are specialized cells that resemble choanoflagellates.

Cell to Cell Communication

• Signal transduction cascades involve: transcriptional changes, cell shape alterations, and phosphorylation of proteins.
• Phosphorylation is common in signal transduction, switching proteins on or off.
• Key amino acids involved in phosphorylation are serine, threonine, and tyrosine.

Cell-to-Cell Adhesion

• Cell-to-cell adhesion involves homotypic molecules that connect to actin filaments.
• Alpha and beta catenin mediate the connection between adhesion molecules and actin.
• Evolutionarily, cadherin genes exhibit unique features in species like M. brevicollis compared to most animals. Specifically, cadherin genes lack certain domains present in animal cadherins.

Body Plan Axes

• Animals can have one or more body axes.
• Radiata (e.g., jellyfish) have a singular anterior-posterior axis.
• Bilateral animals (e.g., mammals) have multiple axes, including dorsal-ventral and left-right axes.

Wnt Signaling

• WNT is a secreted signaling ligand interacting with Frizzled receptors.
• Wnt signaling cascade is complex and impacts cell adhesion, movement, and cytoskeleton rearrangement.
• In the absence of WNT, GSK3β phosphorylates and degrades β-catenin, preventing it from entering the nucleus and regulating Wnt-responsive genes (which are "off").
• Conversely, WNT binding to the receptors leads to the stabilization of β-catenin, which translocates into the nucleus and activates downstream Wnt-responsive genes.

Radial Symmetry

• WNT signaling plays a crucial role in establishing the primary body axes in radiata.
• Wnts influence the development of anterior-posterior body axes.
• High WNT gradients are implicated in tail development of vertebrates.

Nerves

• Sponges lack nerves.
• All animals with bilateral symmetry possess nervous systems.

Sensory Cells

• Flask cells in sponges potentially evolved into neurons in other animals.
• Flask cells in sponges exhibit functionalities akin to synapses.
• The evolution of flask cells is speculated to have facilitated sensory diversification.
• This evolution potentially laid the groundwork for more intricate environmental interpretation in nerve systems.