8. Evolution of multicellularity Flashcards
Define multicellularity
Multicellularity - a property of organism’ which are composed of more than one cell type
What is the closest living unicellular organism?
Choanoflagellates
Describe choanoflagellates
Choanoflagellates - Salpingoeca rosetta:
- closest living unicellular organism to metazoa
- have nucleus - eukaryotes
- bacteria eating eukaryotes
- has cell adhesion precursors - don’t sue for adhesion - for feeding
- can form colonies with others when convenient - ECM in between the cells
Explain what are metazoa
Metazoa - a major division of animal kingdom - all animals except protozons and sponges
What proteins compose collar and flagellum of choanoflagellates (S. rosetta)? What are collar and flagellum functions?
Collar - protein: actin - function: trap bacteria for feeding
Flagellum - protein: tubulin - function: movement
Do choanoflagellates have an axis of asymmetry? Do chanoflagellates eat other eukaryotes?
Yes, have axis of asymmetry
No, feed on bacteria - prokaryotes
What is the closest true animal to chanoflagellates?
Sponges
How are sponges similar to choanoflagellates anatomically?
Spinges composed of choanocytes - very similar to choanoflagellates
How do choanoflagellates and all animals differ from second closest unicellular ancestor?
Choanoflagellates and animals have lots of tyrosine kinases (TK) which act as receptors - switch proteins on / off => mechanism for cell-cell communication - increased TK complexity in animals compared to choanoflagellates
Are tyrosine kinase (TK) genes decreased in number during transition to multicellularity?
No, TK genes increased a little bit
What multicellular adhesion molecules are present in choanoflagellates?
Cadherins - unusual because choanoflagellates are unicellular - don’t need cell-cell adhesion mechanisms - but lack a domain for binding actin (would allow cell-cell adhesion) -> use cadherins for trapping bacteria - for feeding
Where are cadherins localised in choanoflagellates? Why?
Most cadherins found** in collar on microvilli** - E-cadherin acts as bacterial receptor for feeding through collar
How choanoflagellates come into colonies if they don’t use the produced cadherins?
Cadherins not used for cell-cell adhesion - used for feeding - choanoflagellates come together by sticking by bodies (not collars where cadherins are located)
What is the function of cadherins in choanoflagellates?
Choanoflagellates use E-cadherins as bacterial receptors - bacterial capture proteins
=> cadherins first evolved to bind bacteria before being used for cell-cell adhesion
How many body axes do choanoflagellates have vs animals?
Choanoflagellates - anterior-posterior axis
Bilateral animals - 2/3 axes
Explain Wnt signalling pathway
- Wnt secreted
- Frizzled receptors bind Wnt
- beta-catenin activated - downstream response
- beta-catenin activates TCF (TF) in nucleus - activates genes
When Wnt not bound to frizzled receptors - beta-catenin degraded in proteosomes - TCF inactive - no genes transcribed
Is TCF active when beta-catenin is unbound?
No, TCF (TF) only active when beta-catenin binds
Do choanoflagellates have Wnt signalling pathway?
No, Wnt signalling pathway only present in animals
Explain what is the Wnt expression pattern in sponge embryos
Asymmetric Wnt expression in sponge embryos - establishes an axis of assymetry -> forms body axis
Explain what is the Wnt expression pattern in hydra
Asymmetric Wnt expression - defines axis of asymmetry
What is the function of Wnt in primary radiata?
Wnt establishes the primary axis of radiata (aboral-oral axis)
The higher Wnt - oral end, lower Wnt - aboral end
What is the function of Wnt in all animals?
Wnt establishes the primary axis - anterior-posterior axis
In bilaterally symmetrical - high Wnt establishes the tail end
Is Wnt signalling unique only to bilaterally symmetrical animals?
No, radially use too to establish aboral-oral axis
Which animals were the first to develop nerve cells?
Ctenophores - comb jellyfish
Explain what is the nervous tissue in ctenophora
- the first to develop nerves - ctenophora (comb jellyfish) - nerve net
- sensory-to-motor transformations: sensory input -> muscle locomotion
What are the nerve cell precursors in sponges that evolved into neurons of ctenophora?
Flask cells - evolved into neurons of ctenophora
Explain what is the currently supported hypothesis what are the precursors of nerve cells?
Flask cells in sponges - reach is not far to transmit a signal (no axons)
Simple nerves in ctenophora - axons extend the reach to transmit a signal to other cells - more complex interpretation of the environment (neuro-muscular junctions)
