Wijnen - Origin of NS Flashcards
(65 cards)
What are crown eukaryotes, and which major groups are included in them?
Crown eukaryotes are the most evolved lineages of eukaryotes.
Include: plants, fungi, and animals.
Which groups represent the earliest diverging animals?
Sponges (Porifera)
Comb jellies (Ctenophores)
Comb jellies are thought to have diverged earlier than sponges.
Do sponges (Porifera) have a nervous system?
No, sponges do not have neurons or a nervous system.
Do comb jellies (Ctenophores) have a nervous system?
Yes, they have neurons, suggesting nervous systems may have evolved before the split from other animals.
Which groups show the emergence of a central nervous system and brain?
The central nervous system and brain emerge in bilaterians and cnidarians.
Are there nervous systems in unicellular organisms?
No
What are the two main hypotheses about the origin of the nervous system based on comb jellies, sponges, and other animals?
Hypothesis 1: A nervous system evolved before comb jellies diverged, and was later lost in sponges.
Hypothesis 2: The nervous system in comb jellies evolved independently, separate from that in cnidarians and bilaterians.
Which simple animal group lacks a nervous system but is more closely related to humans than comb jellies or sponges?
Placozoans
Despite their simplicity and lack of a nervous system, they’re more closely related to cnidarians and bilaterians.
What behaviours do placozoans display despite lacking a nervous system?
They move (not sessile like sponges).
They can aggregate and forage.
Show coordinated movement, indicating some form of cell-cell communication.
How do simple animals like sponges and placozoans move and feed without neurons?
Use cilia (hair-like structures) to move or generate water flow.
This enables basic movement and environmental interaction.
In more complex animals, muscles evolved to replace cilia for movement.
What genetic evidence suggests placozoans may be related to animals with nervous systems?
Genomic analysis shows they possess some genes shared with nervous systems, even though they lack neurons.
What does the behaviour of placozoans suggest about early nervous system evolution?
Complex behaviour can occur without neurons.
Implies cell-cell communication evolved before the nervous system.
Raises questions about how coordination and movement were managed pre-neuronally.
What are Myxozoa and why are they surprising from an evolutionary perspective?
Myxozoa are unicellular animals.
They are highly derived cnidarians that lost their nervous system.
Initially mistaken for protists due to their size and simplicity.
How was it discovered that Myxozoa are animals?
Genomic sequencing revealed their evolutionary lineage.
Despite their simple, unicellular appearance, their genes link them to cnidarians.
Why did Myxozoa lose their nervous system?
They evolved a parasitic lifestyle, relying on hosts.
This allowed them to shed complex functions like having neurons.
What is the life cycle of Myxozoa like?
Involves two hosts: a fish and an annelid worm.
They undergo both sexual and asexual reproduction.
Produce spores during their cycle.
Where do Myxozoa fall within the animal phylogenetic tree?
Genetically, they sit within Cnidaria, between groups like sea anemones and jellyfish.
Indicates that they once had neurons but lost them secondarily.
Can unicellular organisms sense and respond to their environment?
Yes, they can sense, integrate, and respond to environmental cues.
They exhibit behaviour without a nervous system.
Why don’t unicellular organisms need a nervous system?
Everything—sensing, integration, and response—happens within one cell.
No need for a circuit of multiple cells like in a nervous system.
Do unicellular organisms have precursors to neuronal signalling components?
Yes, they have molecular precursors (e.g. ion channels, signalling proteins) similar to those used in neurons.
How is the basic function of a neuron similar to a unicellular organism?
Both sense their environment, process input, and generate output (behaviour/action).
The key difference: neurons are part of circuits, unicellular organisms operate independently.
What is chemotaxis and which organisms exhibit it?
Chemotaxis is movement towards an attractant or away from a repellent chemical gradient.
Exhibited by both bacteria (e.g. E. coli) and animals (e.g. C. elegans).
How can we experimentally show that chemotaxis is directed behaviour?
Measure the fraction of movement “runs” that are oriented toward an attractant over time.
Longer runs are biased toward attractants, indicating purposeful behaviour.
What example illustrates bacterial chemotaxis?
E. coli moves toward aspartate using biased random walks.
Despite being unicellular, it shows goal-directed behaviour.
