Deck 12: Evolution of the Nervous System

Question 1

Which of the following is an example of evolution as a result of selection acting on genetic variation?
(a) Achromatopsia (extreme sensitivity to light and lack of color vision) becomes widespread among individuals on an isolated island because a common ancestor carried a mutant gene.
(b) A random mutation produces a novel odorant receptor that allows a fly to detect a specific fruit, allowing it to monopolize a new food source and produce more offspring.
(c) In a small population of birds, more individuals that produce one song variant reproduce than those that produce a different song variant, just by chance.
(d) Frog eggs that happen to be laid during a severe drought are much less likely to survive than their siblings that developed in a wetter month.
(e) All of the above

Answer 1

(B)
Natural variation occurs as a result of random genetic mutation, and selection depends on that mutation conferring a reproductive advantage relative to other members of the population.

Question 2

In bacterial chemotaxis, diagrammed in Figure Q12-12, the CheZ phosphatase rapidly reverses the phosphorylation of CheY by histidine kinase CheA. Figure Q12-12 This activity by CheZ:
(a) amplifies CheA histidine kinase activity.
(b) promotes swimming in a straight line.
(c) promotes tumbling.
(d) mediates sensory adaptation.
(e) increases receptor sensitivity.

Answer 2

(B)
Phosphorylated CheY changes the direction of the flagellar motors and promotes tumbles. CheZ dephosphorylates CheY, inhibiting its activity. This should inhibit tumbles and promote swimming in a straight line.

Question 3

What are some of the parallels between the yeast mating signal transduction pathway and neuronal signaling?

Answer 3

In both cases, guanine nucleotide exchange of small GTPases is used to alter the dynamics of the actin cytoskeleton and cell polarity. Also, the MAPK pathway is used in synapse-to-nucleus signaling as well as the transcription of genes involved in cell fusion. Finally, GPCRs are used for detection of chemicals in the environment by yeast as well as by chemosensory neurons.

Question 4

Put the following events of the yeast mating pathway in chronological order.
A. Replacement of GDP by GTP in CDC42
B. Polarized growth (shmoo formation)
C. Dissociation of G protein βγ subunits (STE4/STE18) from α subunit
D. Regulation of actin cytoskeleton
E. Cell fusion
F. Binding of mating factor to G-protein-coupled receptor

Question 5

Some dragonfish (Stomiidae) family members have rod opsins tuned to short-wave (blue) wavelengths, which is a very common adaptation found in deep-sea fish. However, other dragonfish species both emit and detect long-wave (red) emissions. Use maximum parsimony analysis along with the phylogenetic tree in Figure Q12-4 in order to deduce when red color vision was lost and/or gained. Figure Q12-4

Question 6

Glial wrapping of axons:
(a) emerged once, prior to the vertebrate-invertebrate split.
(b) reduces membrane resistance.
(c) increases membrane capacitance.
(d) speeds up conduction of action potentials.
(e) All of the above

Question 7

The conserved function of GPCRs in unicellular organisms is:
(a) regulation of cell proliferation.
(b) photoreception.
(c) detection of chemosensory cues.
(d) regulation of cell migration.
(e) None of the above

Question 8

In both insects and mammals, most ORNs express only one specific odorant receptor, and the axons of ORNs that express the same odorant receptor project to the same glomerulus. Is this similarity in organization likely to be due to shared ancestry or to convergent evolution? Briefly explain your reasoning.

Question 9

Which of the following is an example of a metazoan (animal) innovation?
(a) 24TM Na⁺ channels
(b) 24TM Ca²⁺ channels
(c) 6TM K⁺ channels
(d) 2TM K⁺ channels
(e) None of the above

Question 10

Many synapse components, neural fate and patterning genes, and neurotransmitters are found in both cnidarians and ctenophores, which have nerve nets, but not in sponges, which do not. Ctenophores are currently believed to be the outgroup to cnidarians, bilaterians, and sponges, casting doubt on the origins of nervous systems. What are two plausible scenarios consistent with these observations, and how could one experimentally distinguish between them?

Question 11

Which of the following would be an example of convergent evolution?
(a) Neurons, synapses, and nerve nets emerged independently in the ctenophores (comb jellies) and in the eumetazoan lineage leading to bilaterians and cnidarians.
(b) Neurons, synapses, and nerve nets emerged in the common ancestor of ctenophores (comb jellies), bilaterians, and cnidarians but were lost in the poriferans (sponges).
(c) The centralization of neurons into brains and nerve cords occurred before the divergence of protostomes and deuterostomes.
(d) All modern vertebrates feature a notochord inherited from a common chordate ancestor.
(e) All of the above

Question 12

Type I rhodopsins:
(a) are only found in prokaryotes.
(b) are converted from 11-cis to all-trans retinal isomers by light.
(c) can be used experimentally either to photo-activate or -inactivate mammalian neurons.
(d) are closely related to type II rhodopsins in sequence and structure.
(e) All of the above

Question 13

What evidence best supports the independent gain of gyri and sulci in neocortex multiple times in the mammalian lineage?
(a) Species in all three branches of mammals (monotremes, marsupials, and placental mammals) have gyri and sulci.
(b) Species in all three branches of mammals (monotremes, marsupials, and placental mammals) lack gyri and sulci.
(c) Reptiles, the outgroup of mammals, do not have gyri, sulci, or neocortex.
(d) Larger mammalian brains are more likely to have gyri and sulci.
(e) Smaller mammalian brains are less likely to have gyri and sulci.

Question 14

Which of the following is not a likely outcome of gene duplication?
(a) Expression of one copy of the gene in a different pattern from that of the original gene
(b) Mutations leading to acquisition of a novel function in one copy of the gene
(c) Mutations leading to loss of function of one copy of the gene
(d) Mutations leading to loss of function of both copies of the gene
(e) None of the above

Question 15

Delivery of newly synthesized proteins from the ER to the Golgi apparatus and neurotransmitter release at the synaptic cleft both require:
(a) a Rab GTPase.
(b) a VAMP/synaptobrevin-like v-SNARE.
(c) a syntaxin-like t-SNARE.
(d) an SM (Sec1/Munc18-like) protein.
(e) All of the above

Question 16

Bacteria do not respond directly to a concentration gradient by curving their trajectory toward an attractant; instead, they tumble (randomly switch directions) unless inhibited from doing so. How do larger organisms navigate toward the source of a chemoattractant? With which other sensory system might we expect to see parallels in circuitry organization?

Question 17

Fill in each of the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.
Genetic variation arises from random ____________-changes in DNA that insert, delete, or substitute one or more base pairs, resulting in a new ____________ (version of the gene). If this change results in a phenotype that gives the organism a reproductive advantage, the new allele is likely to ____________ in frequency in the population. The ____________ of an allele can be defined as the ratio of offspring (or grandchildren) produced by individuals of a certain genotype over the highest number produced by any genotype in the population. ____________ selection increases the frequency of alleles with higher fitness. In a ____________ population, allele frequency can also change over generations due to sampling error (because not all individuals produce the same number of offspring). This change in allele frequency by chance is called genetic ____________ and could in principle even lead to the ____________ (frequency of 1) of an allele with relatively low fitness.
allele large
decrease mutation
drift negative
fitness positive
fixation shift
increase small
Evolution of Neuronal Communication

Question 18

State whether each of the following is true for r-opsin, c-opsin, both, or neither.
A. Believed to detect light for circadian rhythm regulation
B. Hyperpolarizes neurons when activated by light
C. Opens an ion channel when activated by light
D. Works with a G protein and second messenger to regulate ion channels
E. Only found in chordates
F. Only found in invertebrates

Question 19

Which of the following issues could lead to erroneous estimates of phylogenetic distance based on molecular clock analysis?
(a) Different lineages have very different basal mutation rates.
(b) DNA sequences that were subject to strong selection are used for the analysis.
(c) The basal rate of mutation varied over time within a lineage being analyzed.
(d) A very distant outgroup (with predominantly random base changes) is used to root the tree.
(e) All of the above

Question 20

Ionotropic receptors:
(a) are expressed in chemosensory neurons in insects, mollusks, and nematodes.
(b) consist of an olfactory receptor plus a co-receptor in insects.
(c) mediate the detection of most odorants by Drosophila.
(d) act as olfactory receptors in mammals.
(e) All of the above

Question 21

Humans that are born blind or deaf depend more on their remaining senses and may even experience rewiring of the neocortex for other functions. This could explain why cochlear implants or hearing aids often do not work well in individuals who have experienced extended hearing loss prior to treatment.
A. What is the experimental evidence in ferrets for adaptive specialization of the neonatal neocortex?
B. Bats that use echolocation to identify prey have expanded areas in the auditory cortex specifically tuned to the ultrasonic frequencies that they emit, while non-echolocating bats do not. Describe an experiment that could demonstrate whether this difference is due to developmental plasticity of the properties of the neocortex.

Question 22

Which of the following is not true of human FoxP2?
(a) It encodes a protein with two amino acids unique to humans among extant animals.
(b) It possesses a base substitution in an intron that is likely to regulate its expression.
(c) It had the same amino acid sequence in our closest relatives, the Neanderthals and the Denisovans.
(d) It can substitute for all functions of mouse FoxP2.
(e) None of the above

Question 23

Mice that are homozygous mutant for Small eyes/Pax6 completely lack eyes. If you were to express the fly Eyeless gene in the eye primordia of a mouse embryo during development, you would expect to see mice with:
(a) no eyes.
(b) mouse eyes in the appropriate location.
(c) fly-like compound eyes in the appropriate location.
(d) extra mouse eyes growing in other parts of their bodies.
(e) extra fly-like compound eyes growing in other parts of their bodies.

Question 24

Which of the following are NOT conserved between vertebrates and invertebrates?
(a) Dorsoventral patterning of the nerve cord by TGFβ/BMP signaling
(b) Antagonism of TGFβ/BMP signaling by sog/chordin
(c) Expression of TGFβ/BMP in the dorsal part of the early embryo
(d) Expression of Msh/Msx in the dorsal part of the nerve cord
(e) None of the above

Question 25

The complex organization of the human eye has often been used as an argument for intelligent design. What evidence would you cite to respond to someone who believes that it would have been impossible for the eye to evolve by natural selection?

Question 26

Which of the following are true of color vision in New World monkeys?
(a) Both male and female howler monkeys have trichromatic vision.
(b) Many species maintain multiple alleles of the red/green opsin gene in the population.
(c) Random X-inactivation would allow a female monkey heterozygous for two opsin alleles with differing spectral sensitivities to distinguish red and green.
(d) A shared regulatory region likely ensures that only one opsin gene (red or green) is expressed in each cone cell in howler monkeys.
(e) All of the above

Question 27

Put the following events in the evolution of human color vision in chronological order. A. Duplication of one cone opsin gene to generate the rod opsin
B) Duplication of one red/green cone opsin gene on the X chromosome
C) Divergence of humans from other Old World monkeys and apes
D) Loss of two cone opsin genes
E) Two genome duplication events in the chordate lineage
F) Appearance of red/green cone opsin variants with distinct spectral sensitivities in an ancestral Old World primate population (still hypothetical)

Question 28

How can natural selection explain both the loss and gain of color vision at different points in our evolutionary history?

Question 29

Draw a parallel between gene duplication and cell duplication in the evolution of increasing specialization and complexity, and give an example of the latter.

Question 30

True or False: A significant increase in the number of cortical neurons is sufficient for the formation of gyri and sulci in previously lissencephalic brains.

Question 31

Which mechanism(s) could contribute to larger brains with more cortical neurons?
(a) A longer period of symmetrical cell division, increasing the pool of radial glia
(b) More radial glia divisions, increasing the number of intermediate progenitors
(c) Outer radial glia (oRGs) in the subventricular zone that self-regenerate and produce more intermediate progenitors
(d) Expression of Wnt pathway effector β-catenin
(e) All of the above

Question 32

Hox genes are highly conserved players in anteroposterior patterning (Figure Q12-28) and were named for the homeotic transformations exhibited by their mutants in Drosophila. Figure Q12-28
A. The original Antp mutant has legs in place of antennae. Is this a gain- or loss-of-function mutation?
B. The original Ubx mutant has a second set of wings instead of halteres (flying organs normally found on the third thoracic segment). Is this a gain- or loss-of-function mutation?
C. Although the role of Hox genes in anteroposterior patterning is conserved in vertebrates, HoxA9-13 and D9-13 are used in proximodistal limb patterning. What evolutionary events made acquisition of this novel function possible?

Question 33

In clever experiments, expression of the human L-opsin gene was shown to be sufficient to improve spectral discrimination in both female mice and male squirrel monkeys.
A. Would male mice with the knocked-in human L-opsin gene be expected to perform spectral discrimination tasks as well as their heterozygous female siblings? Briefly explain your reasoning.
B. Would male squirrel monkeys with virally transduced human L-opsin be expected to perform spectral discrimination tasks as well as their heterozygous female siblings? Briefly explain your reasoning.
Evolution of Nervous System Structure and Development