Multicellularity Study Guide PDF

**[Multicellularity ]** What is it? ----------- - An organism that consists of more than one cell type - True multicellularity evolved independently at least 10 times (one in animals metazoa, 6 times in plants and 3 times in fungi - All precursers were unicellular ### Choanoflagellate...

**[Multicellularity ]** What is it? ----------- - An organism that consists of more than one cell type - True multicellularity evolved independently at least 10 times (one in animals metazoa, 6 times in plants and 3 times in fungi - All precursers were unicellular ### Choanoflagellates - Closest living single -celled relative to metazoa. - Have a collar region as well as a flagella. - They are bacteria eating eukaryotes - The collar traps bacteria and the flagellum is used for movement. - Actin:Tubulin double antibody immunofluorescence - Collar : Actin - Flagellum: tubulin - Some choanoflagellates form colonies - Choanoflagellates comprise the sister lineage to all animals - Choanoflagellates\> Porifera \> Sponges - Choanoflagellates are very similar to choanocytes found in sponges Basic sponge anatomy: A diagram of a human body Description automatically generated ### Choanoflagellates vs Sponges 1. Cell to cell communication: - Signal transduction cascade- transcriptional , cell shape change - Phosphorylation is common end result of this cascade and it can switch a protein on/off - Serine, threonine and tyrosine can be phosphorylated - Receptor tyrosine kinase- span the membrane and cause cross phosphorylation of each other as well as downstream enzymes - Cytoplasmic tyrosine kinase - Blue box: common ancestor to choanoflagellates + animals - Animals + choanoflagellates significantly increased the number of receptor tyrosine kinases in their genomes - Csk is a negative regulator of Src - CsK does phosphorylate Src at Y530 in M.brevicollis - Increased complexity of kinase regulation in animals - Phosphorylation of tyrosine residue can either activate or inhibit the function of an enzyme  'Active' Src \> (Csk) 'inactive' Src 2. Cell to cell adhesion: - In animals.... Homotypic cell cell adhesion molecules. Connects to actin filaments via alpha and beta catenin. - Unusual cadherin genes in the M.brevicollis genome - SH2, IgG and Hh domains are NOT ASSOCIATED with cadherins in most animals - Conserved Beta-catenin binding domain is missing in choanoflagellate cadherins. - Most of the cadherin is found in the collar of microvilli. -- E Cadherin can act as a bacterial receptor. - HYPOTHESIS- Cadherins first evolved to bind bacteria before being co-opted for cell-cell adhesion RECAP: - Choanoflagellates significantly increase the number of receptor tyrosine kinases in their genomes before metazoan evolution - Regulation of kinases increased in complexity - Cadherins likely evolved for 'bacterial capture' before being co-opted for cell-cell adhesion. ### Body plan axis - Animals have one or more body plan axes - Sponge larva and other radiata (e.g. jellyfish) have a single anterior-posterior axis - Bilateral animals have 2 or 3 axes. Dorsal-ventral axis and left-right axis ### Wnt - WNT is a secreted ligand and binds to Frizzled receptors - **In absence of WNT:** Frizzled is unbound, GSK3beta phosphorylates Beta-catenin. Beta-catenin is degraded. TCF is inactive. Wnt-responsive genes are OFF. GSK3 is a serine/threonine protein kinase. TCF is a TF that binds DNA in sequence-specific manner - **In presence of WNT:** Beta-catenin is stabilised. Beta-catenin translocate to the nucleus. Binds TCF TF Wnt-responsive genes are on. - WNT signalling response is complex -- cell adhesion/movement, cytoskeleton rearrangement - Inhibition of the kinase GSK3beta will result in activation of the Wnt signalling pathway. - Wnt pathway only in animals. ### Radial symmetry - Wnt establish the primary axis of radiata - Wnts establish the primary anterior-posterior axis of all animals - High gradient of inhibition from anterior to posterior - Posterior defects from Wnt3a inhibition. - High Wnt signalling is important for tail formation in most vertebrate embryos ### Nerves - Sponges do not have any nerves - All bilaterally symmetrical animals have nerves - Sensory input \> ciliary locomotion - Sensory input \> muscle locomotion ### Sensory cells - Sensory cell= flask cell in sponges - May be precursor to nerve cells - Flask cells have junctions similar to synaptic cleft - HYPOTHESIS: sensory flask cells of the sponge evolved into neurons of the ctenophora - Permits diversification of sensory cell types - More complex interpretation of environment - More complex outputs \> neuromuscular junctions